FRUIT MANUFACTURING

FRUIT MANUFACTURING
Scientific Basis, Engineering Properties, and
Deteriorative Reactions of Technological Importance
FOOD ENGINEERING SERIES
Series Editor
Gustavo V. Barbosa-Cánovas, Washington State University
Advisory Board
Jose Miguel Aguilera, Pontifica Universidad Catolica de Chile
Pedro Fito, Universidad Politecnica
Richard W. Hartel, University of Wisconsin
Jozef Kokini, Rutgers University
Michael McCarthy, University of California at Davis
Martin Okos, Purdue University
Micha Peleg, University of Massachusetts
Leo Pyle, University of Reading
Shafiur Rahman, Hort Research
M. Anandha Rao, Cornell University
Yrjö— Roos, University College Cork
Walter L. Spiess, Bundesforschungsanstalt
Jorge Welti-Chanes, Universidad de las Américas-Puebla
Food Engineering Titles
Jose M. Aguilera and David W. Stanley, Microstructural Principles of Food Processing and
Engineering, Second Edition (1999)
Stella M. Alzamora, Marı́a S. Tapia, and Aurelio López-Malo, Minimally Processed Fruits and
Vegetables: Fundamental Aspects and Applications (2000)
Gustavo V. Barbosa-Cánovas and Humberto Vega-Mercado, Dehydration of Foods (1996)
Gustavo V. Barbosa-Cánovas, Pedro Fito, and Enrique Ortega-Rodriguez, Food
Engineering 2000 (1997)
Gustavo V. Barbosa-Cánovas, Enrique Ortega-Rivas, Pablo Juliano, and Hong Yan, Food Powders:
Physical Properties, Processing, and Functionality (2005)
P.J. Fryer, D.L. Pyle, and C.D. Reilly, Chemical Engineering for the Food Industry (1997)
A.G. Abdul Ghani Al-Baali and Mohammed M. Farid, Sterilization of Food in Retort Pouches (2006)
Richard W. Hartel, Crystallization in Foods (2001)
Marc E.G. Hendrickx and Dietrich Knorr, Ultra High Pressure Treatments of Food (2002)
S.D. Holdsworth, Thermal Processing of Packaged Foods (1997)
Lothar Leistner and Grahame Gould, Hurdle Technologies: Combination Treatments for Food
Stability, Safety, and Quality (2002)
Michael J. Lewis and Neil J. Heppell, Continuous Thermal Processing of Foods: Pasteurization
and UHT Sterilization (2000)
Jorge E. Lozano, Fruit Manufacturing: Scientific basis, engineering properties, and deteriorative
reactions of technological importance (2006)
R.B. Miller, Electronic Irradiation of Foods: An Introduction to the Technology (2005)
Rosana G. Moreira, M. Elena Castell-Perez, and Maria A. Barrufet, Deep-Fat Frying: Fundamentals
and Applications (1999)
Rosana G. Moreira, Automatic Control for Food Processing Systems (2001)
M. Anandha Rao, Rheology of Fluid and Semisolid Foods: Principles and Applications (1999)
Javier Raso-Pueyo and Volker Heinz, Pulsed Electric Field Technology for the Food Industry:
Fundamentals and Applications (2006)
George D. Saravacos and Athanasios E. Kostaropoulos, Handbook of Food Processing Equipment
(2002)
FRUIT MANUFACTURING
Scientific Basis, Engineering Properties, and
Deteriorative Reactions of Technological Importance
Jorge E. Lozano
PLAPIQUI (UNS-CONICET)
Bahia Blanca, Argentina
Dr. Jorge E. Lozano
PLAPIQUI (UNS-CONICET)
Camino La Carrindanga KM.7
C.C.: 717
8000 Bahia Blanca
Argentina
[email protected]
Color illustration: Santiago Lozano, Amblagar Studio, www.amblagar.com
Library of Congress Control Number: 2005936521
ISBN-10: 0-387-30614-5
ISBN-13: 978-0387-30614-8
e-ISBN 0-387-30616-1
Printed on acid-free paper.
ß 2006 Springer ScienceþBusiness Media, LLC
All rights reserved. This work may not be translated or copied in whole or in part without the written permission of
the publisher (Springer ScienceþBusiness Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for
brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information
storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known
or hereafter developed is forbidden.
The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not
identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary
rights.
Printed in the United States of America.
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PREFACE
The fruit processing industry is one of the major businesses in the world. While basic
principles of fruit processing have shown only minor changes over the last few years, major
improvements are now continuously occurring, and more efficient equipment capable of
converting huge quantities of fruits into pulp, juice, dehydrated, frozen, refrigerated products,
etc. make possible the preservation of products for year-round consumption. The fruit
processing and storage, even under the most industrially available ‘‘mild conditions,’’ involves
physical and chemical changes that negatively modify the quality. These negative or deteriorative changes include enzymatic and nonenzymatic browning, off-flavor, discoloration,
shrinking, case hardening, and some other chemical, thermophysical, and rheological alterations that modify the nutritive value and original taste, color, and appearance of fruits. The
ability of the industry to provide a nutritious and healthy fruit product to the consumer is
highly dependent on the knowledge of the quality modifications that occur during the
processing.
This book emphasizes the products rather than the processes, procedures, or plant
operations. It presents the influence in fruit products’ quality of the different processing
methods, from freezing to high temperature techniques. Origin of deterioration, kinetics of
negative reactions, and methods for inhibition and control of the same are discussed. Probable changes in thermodynamical, thermophysical, and rheological properties and parameters
during processing of fruits at a wide range of soluble solids, temperatures, and pressure are
also summarized.
This book is intended to provide professionals involved in development and operations
of the fruit industry, with the necessary information for the understanding of the deteriorative
effects on the fruit quality during processing.
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CONTENTS
1.
2.
Overview of the Fruit Processing Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Classification of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 World Production and Commercial Applications of Fruits . . . . . . . . . . . . . . . .
1.4 History of Fruit Products’ Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Harvest of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5.1 Chemical Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6 Postharvest Handling of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6.1 Postharvest Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6.2 Cooling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7 Controlled Atmosphere Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8 Modified Atmosphere Packaging of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8.1 Factors Affecting Fruit Respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8.2 Factors Influencing the Exact Modified Atmosphere Within
a Sealed Pack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.9 Technology of Semiprocessed Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.9.1 Preservation of Semiprocessed Fruit Products . . . . . . . . . . . . . . . . . . . . .
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Processing of Fruits: Ambient and Low Temperature Processing . . . . . . . . . . . . . . . . .
2.1 Fruit Products and Manufacturing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Fruit Juice and Pulp Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Front-End Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1.1 Reception Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1.2 Final Grading, and Inspection and Sorting . . . . . . . . . . . . . . .
2.2.2 Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.1 Citrus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2 Pomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.3 Pressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.4 Other Extraction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Clarification and Fining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3.1 Partial Concentrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 Use of Enzymes in the Fruit Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4.1 Other Enzymes in Juice Production . . . . . . . . . . . . . . . . . . . . . .
2.2.4.2 Pectinase Activity Determination . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4.3 pH Dependence on the Pectic Enzymes Activities . . . . . . . . . .
2.2.4.4 Enzymatic Hydrolysis of Starch in Fruit Juices . . . . . . . . . . . .
2.2.5 Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5.1 Pressure Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5.2 Filter Aid and Precoating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Contents
2.2.5.3 Types of Pressure Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5.4 Vacuum Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Membrane Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6.1 Stationary Permeate Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6.2 Permeate Flux as a Function of Time . . . . . . . . . . . . . . . . . . . .
2.2.6.3 Influence of VCR on the Permeate Flux . . . . . . . . . . . . . . . . .
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51
Processing of Fruits: Elevated Temperature, Nonthermal
and Miscellaneous Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Batch Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 HTST (Short Time) Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 UHT Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.4 Nonthermal Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Sterilization of Food by High Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 High-Pressure Equipment and the System . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Concentration by Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Batch Pan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Rising Film Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Falling Film Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4 Scraped-Surface Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.5 Multiple Effect Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.5.1 Thermocompression (TC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.5.2 Mechanical Vapor Recompression (MVR) . . . . . . . . . . . . . . .
3.4 Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Spray Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Powder Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Miscellaneous Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1 Size Enlargement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1.1 Instantizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1.2 Agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1.3 Agglomeration Process and Equipment . . . . . . . . . . . . . . . . . .
3.5.1.4 Agglomeration Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1.5 Selective Agglomeration (Spherical Agglomeration) . . . . . . . .
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Thermodynamical, Thermophysical, and Rheological Properties of Fruits
and Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Thermophysical Properties’ Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Fruits and Fruit Products’ Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Fruit and Fruit Products’ Properties During Freezing . . . . . . . . . . . . . .
4.3.2 Water Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Experimental Data and Prediction Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1.1 Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1.2 Density Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1.3 Empirical Equations and Theoretical Density Models . . . . . .
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2.2.6
3.
4.
Contents
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4.4.2
4.4.3
4.4.4
4.4.5
4.4.6
5.
Specific Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.2.1 Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.2.2 Prediction Models and Empirical Equations . . . . . . . . . . . . . .
Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.3.1 Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.3.2 Prediction Models and Empirical Equations . . . . . . . . . . . . . .
Thermal Diffusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.4.1 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.4.2 Empirical Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.5.1 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.5.2 Newtonian Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.5.3 Non-Newtonian Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.5.4 Effect of Temperature and Pressure on the Viscosity
of Foodstuffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boiling Point Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Color, Turbidity, and Other Sensorial and Structural Properties of Fruits
and Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Measurement of Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Absorbance Spectrophotometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.1 Spectrophotometer Components . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.2 Improved Spectrophotometers . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.3 Turbidity and Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.4 Reflection Spectrophotometer . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.5 Tristimulus and Special Colorimeters . . . . . . . . . . . . . . . . . . . .
5.2.1.6 CIELAB Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.8 Measurement of Tristimulus Values . . . . . . . . . . . . . . . . . . . . . .
5.2.1.9 Application of Colorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Food Dispersions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 Food Dispersion Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3 Particle Size, Shape, and Size Distribution . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3.1 Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3.2 Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3.3 Photon Correlation Technique . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.4 Cloudy Fruit Juice Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Fruit Aroma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Activity Coefficients of Fruit Juice Aroma . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3 Thermodynamic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3.1 Wilson Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3.2 NRTL Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3.3 UNIQUAC Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3.4 UNIFAC Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.4 Fruit Aroma Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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5.4.5
5.4.6
Fruit Shrinkage During Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
5.4.5.1 Shrinkage coefficient, sb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Structural Damage During Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.
Chemical Composition of Fruits and its Technological Importance . . . . . . . . . . . . . . .
6.1 Proximate Composition of Fruit and Fruit Products . . . . . . . . . . . . . . . . . . . . .
6.1.1 Proteins and Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Organic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3.1 Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3.2 Pectin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.4 Lipids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.5 Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.6 Vitamins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.7 Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.8 Aroma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.9 Color compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anthocyanidins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Flavones and Flavonols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Flavonones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Catechins and Leucoanthocyanidins. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Influence of Processing and Storage on the Composition of Fruits . . . . . . . . .
6.2.1 Vitamin Destruction During Processing and Storage . . . . . . . . . . . . . . .
6.2.2 Effect of Storage on Metal Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2.1 Influence of Storage on Fruit Juice Aroma . . . . . . . . . . . . . . .
6.2.3 Fruit Juice Change in Amino Acid Content During Storage . . . . . . . .
6.2.4 Effect of Storage on Fruit Sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.5 Effect of Processing and Storage on Fruit Pigments . . . . . . . . . . . . . . .
6.2.6 Changes in Organic Acid Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.7 Changes in Phenolic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
133
133
136
136
137
139
140
140
140
141
144
144
147
148
148
148
149
150
150
152
152
153
155
157
157
157
7.
Fruit Products, Deterioration by Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.1 Different Mechanisms of Deterioration . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Enzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.1 Phenolic Compounds and Oxidases . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.2 Kinetics of Enzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.2.1 Effect of the Temperature in the Color Change . . . . . . . . . . .
7.3 Nonenzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1 Maillard Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1.1 Tristimulus Parameters and Absorbance as a Measurement
of Browning in Fruit Juices . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1.2 Kinetics of Nonenzymatic Browning (NEB) . . . . . . . . . . . . . .
7.3.1.3 Effect of Soluble Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1.4 Effect of Reducing to Total Sugars’ Ratio (R/T) . . . . . . . . . .
7.3.1.5 Effect of the Fructose to Glucose Ratio (F/G) . . . . . . . . . . . .
7.3.1.6 Effect of Amino Acids (AA) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
163
163
163
164
164
165
167
167
168
170
170
170
171
171
172
Contents
xi
7.3.1.7 Effect of the Content of Organic Acids . . . . . . . . . . . . . . . . . . .
7.3.1.8 Effect of Other Minor Components . . . . . . . . . . . . . . . . . . . . . .
7.3.1.9 Effect of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-HMF Formation During Storage and Processing
of Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
173
173
173
Inhibition and Control of Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Inhibition and Control of Enzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.1 Thermal Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.1.1 Elevated Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.1.2 Refrigeration Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.2 Chemical Inhibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.3 Effect of the Ascorbic Acid (AA) Content in Color Change . . . . . . . . .
8.2.4 Nonconventional Chemical Inhibition of EB . . . . . . . . . . . . . . . . . . . . . .
8.2.4.1 Honey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.4.2 Aromatic Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.4.3 Proteases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.5 Miscellaneous Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.5.1 Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.5.2 Ultrafiltration (UF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.5.3 High-pressure treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3 Inhibition and Control of Nonenzymatic Browning (NEB) . . . . . . . . . . . . . . . .
8.3.1 Preventive Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.1.1 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.1.2 Process Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.1.3 Ion Exchange Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.2 Restorative Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.2.1 Effect of Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.2.2 Use of PVPP (Polyvinyl Polypyrrolidone) . . . . . . . . . . . . . . . . .
8.3.3 Miscellaneous Methods for Inhibition and Control of
Nonenzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.3.1 Color Reduction by Combined Methods . . . . . . . . . . . . . . . . .
8.3.3.2 Use of Chemical Inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
183
183
183
184
184
188
189
192
193
194
195
195
195
196
196
197
197
198
198
200
203
205
206
209
7.3.2
8.
174
209
209
210
210
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
CHAPTER 1
OVERVIEW OF THE FRUIT
PROCESSING INDUSTRY
1.1. INTRODUCTION
The Latin word fruor, meaning ‘‘I delight in,’’ is the source of the word fruit. Fruits are
essential in the human diet as they contain compounds of nutritional importance, including
vitamins that are not synthesized by the human body. Fruits are defined as the reproductive
organs arising from the development of floral tissues with or without fertilization (Fig. 1.1).
1.2. CLASSIFICATION OF FRUITS
It is common to classify fruits as temperate fruits, subtropical fruits, and tropical fruits
depending on the region where they grow (Kader and Barret, 1996). A more specific classification is given in Table 1.1.
Commonly the term fruit is often restricted to succulent (fleshy) edible fruits of woody
plants like apples, melons, and such small fruits as strawberries and blueberries. As the ovary
matures, its wall develops to form pericarp. Pericarp is divided into three layers: exocarp
(outer), mesocarp (middle), and endocarp (inner). In fleshy fruits the pulpy layer is usually the
mesocarp (as in peaches and grapes). The seed or seeds, which in some cases constitute the
entire edible portion of the fruit, lie immediately within the pericarp (Koning, 1994).
For example, the hard husk of coconut is the pericarp and the edible part inside is the
seed. While in typical cases the fruit is confined to the ripened ovary, in apples it includes
ovary plus receptacle. On the other hand, strawberry is an aggregation of small fruits and
pineapple is a development of the entire inflorescence. Dehiscent and indehiscent dry fruits, and
some other fruits like pumpkin and cucumber are classified as vegetables. Their processing is
specifically not considered in this book. Cereals, sunflower, peanuts, and beans are also beyond
the interest of this work. With this in mind, the simplified classification of fruits shown (Table
1.1) will be of interest when considering different processing methods.
1.3. WORLD PRODUCTION AND COMMERCIAL APPLICATIONS
OF FRUITS
Table 1.2 lists world production and major commercial applications of selected fruits. In 2003
world fruit production reached nearly 380 million metric tons (MT). World fruit production
grew at an average of 0.86% per year for the period 2000–2003 (FAOSTAT, 2005) and rose
1.6% in 2004, according to the latest crop production data collated by the United Nations’ Food
and Agriculture Organization (FAO). This marks the fourth consecutive annual increase in
1
2
Fruit Manufacturing
Strawberry
Cashew
apple
Mangosteen
Fig
Grape
el
Ari
dic
l
Receptacle
Pe
le
nc
du
Pe
Pomegranate seed
arp
Peric
Endodermal
intralocular
tissue
l
e
enta
Plac lar tissu
u
c
o
l
intra tum
Sep
ssu
Mesoc
y ti
sor
ces
Ac
Pineapple
arp
e
Ac
c
tis ess
su or
e y
Outer layer of
the testa
cle
un
d
Pe
Apple
Orange
Tomato
Peach
Figure 1.1. The origin of selected fruits from plant floral tissue. Reprinted, with permission, from the Annual Review
of Plant Physiology, Vol. 27 (copyright) 1976 by Annual Reviews.
international fruit production levels. The FAO data reveal bananas as the most commonly
produced fruit in the world, with output inching upward from 103,000 MT to 70.63 million MT
in 2004. It was followed by grapes, with production rising from over 3 million MT to 65.5
million in 2004. Oranges are the third most widely grown fruit in the world, with output in 2004
rising roughly from 2.4 million MT to just over 63 million MT. Apple production rose from
almost 270,000 MT to just over 59 million MT in 2004.
1.4. HISTORY OF FRUIT PRODUCTS’ DEVELOPMENT
People have been trying to improve the quantity and quality of their food and drink for
centuries. As soon as the first humans decided to settle in one place and grow their own food,
they started to improve its quality and increase its quantity. The following are the remarkable
milestones associated to fruit and fruit processing:
1
.
Overview of the Fruit Processing Industry
3
Table 1.1. Classification of edible fleshy fruits based on: (1) the structure of
the flower where the fruit belongs; (2) the number of ovaries included; and
(3) the number of carpels in each ovary.
One carpel
One single ovary
Drupe
True berry
Single
More than one
carpel
Hesperidium
Pepo
Aggregate of single
fruits
Multiple
Derived from
different parts of
the flower
YEAR
-4000
-2000
-1000
-300
False berries
Peduncle and
accessory tissues
(inflorescence)
Receptacle
Peach
Plum
Coconut
Blueberry
Gooseberry
Grape
Squash
All citrus
fruits
Apple
Squash
Watermelon
Raspberry
Blackberry
Pineapple
Fig
Strawberry
MILESTONE
The Egyptians master viticulture and the art of wine making.
Egyptians and Sumerians learn fermentation. Although people had been eating
naturally fermented foods since the Neolithic Age, the process was never understood.
The Greeks develop grafting techniques, leading to the creation of orchards and
groves.
In the Roman Empire, drying process was used for preservation. In addition,
honey was sometimes used as a preserving agent for fruit.
Pliny the Elder in his Natural History describes 20 varieties of apples
0
1000
1276 The first whiskey distillery was established in Ireland.
1400 Modern candy is created in Europe when cooks dip fruits and berries into melted
sugar.
1650 Frère Jean Oudart and Dom Pierre Pérignon, abbeys of Saint Pierre aux Monts
de Châlons and d’ Hautvillers became the fathers of naturally sparkling wine
(Champagne)
1676 The Compagnie de Limonadiers of Paris was granted a monopoly for the sale of
lemonade soft drinks
1850 Soft drinks are invented by mixing fruit juice with other ingredients such as
sugar, carbonated water and citric acid.
4
Fruit Manufacturing
Table 1.2. World production and commercial applications of selected fruits (based on Hui, 1991).
1998 world production
(106 ton/year)
Fruit
Scientific name
Apples
Pyrus malus
56,060
Apricots
Prunus armeniaca
2,670
Avocados
Bananas
2,325
58,618
Black berries
Persea americana
M. Caveindeishii,
M. Paradisiaca
Rubus alleghaniensis
—
Boysen berries
Rubus
—
Breadfruits
Cherries, sour
Artocarpus altilis
Prunus cerasus
—
1,550
Cherries, sweet
Prunus avium
Coconuts
Livistona chinensis
47,695
Crabapples
Cranberries
—
285
Currants
Malus pumila
Vaccinium
macrocarpum
Ribes vulgare, rubrum
Elder berries
Sambuca nigra
—
Figs
Ficus carica
1,178
Goose berries
Ribes hirtellum
—
Grapefruits
5,072
Grapes
Citrus paradisi,
pommelo
V. rotundifolia
V. vinifera
Vitis labrusca
Guavas
Psidium guajava
2.1
654
—
57,397
Major commercial
applications
Brandy, cakes, cider, citric acid, cookies,
confections, dried, essence, filling, jelly,
juice, marmalade, pectin, preserves,
sauce, and vinegar
Brandy, cakes, citric acid, confections,
dried, essence, filling, jelly, juice,
marmalade, preserves, and purée
Crushed and Purée
Cookies, crushed, purée, dried,
and frozen
Brandy, cakes, canned, cocktail, cookies,
crushed, essence, jam, jelly, juice, nectar, pie,
pie filling, preserves, purée, snack bars,
strained purée, syrup, and wine
Brandy, cakes, canned, cereals, cocktail,
confections, cookies, essence, jam, jelly,
juice, nectar, pie, pie filling, preserves,
strained purée, syrup, and wine
Chunks and purée
Brined, cakes, canned, citric acid, cookies,
diet spread, frozen, jam, preserves, purée,
and wine
Brined, cakes, canned, cocktail, confections,
cookies, frozen, nectar, pie, pie filling,
preserves, and purée
Brandy, cakes, canned, chunks, cocktail,
confections, cookies, crushed, dried, frozen,
jam, juice, nectar, pie, purée, snack bars,
strained purée, and syrup
Jelly, pectin, and pickles
Canned, confections, crushed, glacé, jam, jelly,
juice, purée, sauce, strained purée, and syrup
Canned, diet spread, and purée.
Canned, cookies, crushed, essence, glacé,
jam, and purée
Brandy, cocktail, frozen, glacé, jelly, juice,
nectar, and purée
Cakes, canned, cereals, cocktail, confections,
cookies, crushed, diet spread, dried, frozen,
Glico, leather, preserves, purée,
and snack bars
Canned, cocktail, confections, jelly, juice,
nectar, pie, pie filling, preserves, and wine
Canned, chunks, confections, crushed,
rozen, Glico, juice, and marmalade
Cocktail, jam, jelly, juice, sauce,
Grapes, and wine
Champagne, cocktail, crushed, diet spread,
jam, juice, sauce, vinegar, and wine
Crushed, diet spread, jam, jelly, leather,
purée, and snack bars
(continued )
1
.
Overview of the Fruit Processing Industry
5
Table 1.2. (Continued )
1998 world production
(106 ton/year)
Fruit
Scientific name
Honeydew
melons
Kiwifruits
Kumquats
Lemons
Cucumic melo
—
Actidia chinensis
Fortunella margarita
Citrus lemon
852
Limes
Citrus aurantifolia
Longans
Loquats
Lychees
Mangoes
Nectarines
Euphoria
Eriobotrya japonica
Litchi chinensis
Mangifera indica
Prunum nectarina
—
Olives
Oranges
Olea europala
Citrus quarantium,
sinensis
13,757
66,212
Papayas
Passion fruit
Carica papaya
Passiflora
4,801
—
Peaches
Prunus persica
11,065
Pears
Pyrus cominunis
14,379
Persimmons
D. virginiana
1,960
Persimmons
Pineapples
Diospyros kaki
Ananas cormosus
—
12,100
Plums and
Prunes
Pomegranates
Prunus domestica
8,008
Punica granatum
—
Quinces
Cydonia vulgaris
334
Raspberries
Rubus idueus or
R. stigosus
326
Sapotes
Strawberries
Pouteria sapota
Fragaria chiloensis
—
2,600
Tangerines
Citrus reticulata
6.0
9,335
23,455
—
Major commercial
applications
Crushed, diet spread, jam, jelly, leather,
purée, and snack bars
Canned, cocktail, crushed, and dried
Jam
Citric acid, cocktail, essence, frozen,
Glico, juice, marmalade, and pectin
Citric acid, cocktail, crushed, essence,
frozen, juice, marmalade, and pectin
Citric acid and crushed
Cocktail and purée
Canned and dried
Crushed and purée
Canned, cocktail, confections, cookies,
crushed, diet spread, frozen, jam, and purée
Brined and canned
Cereals, champagne, confections, cookies,
crushed, diet spread, essence, frozen, Glico,
juice, marmalade, pectin, and wine
Crushed, juice, leather, purée, and snack bars
Crushed, juice, nectar, purée, and strained
purée
Brandy, brined, cakes, canned, cereals,
chunks, cocktail, confections, cookies,
crushed, diet spread, dried, essence, frozen,
jam, juice, leather, pickles, pie, pie filling,
preserves, purée, sauce, snack bars, strained
purée, syrup, and wine
Brandy, canned, cereals, chunks, cocktail,
confections, cookies, crushed, diet spread,
dried, and frozen
Brandy, chunks, crushed, jam, juice,
marmalade, pectin, pie, pie filling, preserves,
and syrup
Chunks and crushed
Cakes, canned, cereals, chunks, cocktail,
confections, cookies, crushed, diet spread,
frozen, glacé, jam, leather, preserves, jelly,
pectin, preserves, purée, vinegar, and wine
Cereals, cocktail, confections, crushed,
diet spread, juice, purée, and strained purée
Jam, juice, leather, purée, snack bars, and
strained purée
Jam, jelly, juice, pickles, preserves, purée,
strained purée, and wine
Cakes, canned, cereals, confections, cookies,
crushed, diet spread, essence, frozen, jam,
jelly, uice, nectar, pickles, pie filling, preserves,
snack bars, strained purée, and syrup
Crushed, juice, and purée
Brandy, cakes, cereals, confections, cookies,
crushed, frozen, and juice
Cocktail, frozen, glacé, juice, and purée
6
Fruit Manufacturing
1861
1870
1906
1913
1920
1940
1957
1958
1965
1970
1990
1992
2005
Louis Pasteur develops his technique of pasteurization, in which he protects food
by heating it to kill dangerous microbes, removing the air and sealing it in a
container.
The Navel orange is introduced into the United States from Brazil.
Modern freeze-drying techniques are mastered in France.
Home refrigerators are invented in the United States.
American Charles Birdseye invents the process of deep freezing foods.
Microwave technology is developed, which leads to the invention of the microwave oven.
The first aluminum cans were used.
‘‘Basic Four’’ food guide introduced by USDA: milk, meat, vegetable and fruit,
and bread and cereal groups.
Soft drinks in cans dispensed from vending machines.
Plastic bottles are used for soft drinks.
Irradiation approved by the FDA and USDA for use on selected foods, including
papaya.
USDA releases the new Food Guide Pyramid,
USDA releases the new Food Guide Pyramid, providing a graphics based quantitative guide to food consumption.
1.5. HARVEST OF FRUITS
Harvesting at the correct time is essential to the production of quality fruits (O’Brien et al.,
1983). The correct time to pick fruit depends upon several factors, including variety, location,
weather, ease of removal from the tree, and purpose to which the fruit will be put. Oranges
change with respect to both sugar and acid as they ripen on the tree: sugar increases and acid
decreases. The ratio of sugar to acid determines the taste and acceptability of the fruit and the
juice. For this reason, in some countries there are laws that prohibit picking until a certain
sugar–acid ratio has been reached. These and other measurements indicate when the fruit is
ready for harvesting and subsequent processing.
A large amount of the harvesting of most fruit crops is still done by hand; this labor may
represent about half of the cost of growing the fruit. Mechanical harvesting is currently one of
the most active fields of research for the agricultural engineer. For proper harvesting:
.
.
.
the fruit should be picked by hand and placed carefully in the harvesting basket, in
order to avoid any mechanical damage;
the harvesting basket and the hands of the harvester should be clean;
the fruit should be picked when it is ready to be processed into a quality product.
Moreover, the proximity of the processing plant to the source of supply for fresh raw
materials presents several advantages, including the possibility to pick at the best suitable
moment, reduce losses by handling/transportation, minimize raw material transport costs,
and simplify methods for raw material transport.
After harvesting, the organoleptic and nutritional properties of fruits deteriorate
in different degrees. Causes of deterioration include the growth and activity of microorganisms, the activities of the natural food enzymes, the action of insects and rodents,
changes in temperature and water content, and the effect of oxygen and light. Usual storage
1
Overview of the Fruit Processing Industry
.
7
life of fruits is between 1 and 7 days at 218C if proper measures are not taken (Kader and
Barret, 1996).
Many quality measurements can be made before a fruit crop is picked in order to
determine if proper maturity or degree of ripeness has developed:
.
.
.
.
Color can be checked with instruments (see Chapter 4) or by comparing the color of
fruit on the tree with standard picture charts.
Texture may be measured by compression by hand or by simple type of plungers.
Percentage of soluble solids, which are largely sugars, is generally expressed in degrees
Brix, which relates specific gravity of a solution to an equivalent concentration of pure
sucrose. The concentration of soluble solids in the juice can be estimated with a
refractometer or a hydrometer. The refractometer measures the ability of a solution
to bend or refract a light beam, which is proportional to the solution’s concentration.
A hydrometer is a weighted spindle with a graduated neck, which floats in the juice at a
height related to the juice density.
The acid content of fruit changes with maturity and affects flavor. Acid concentration can be measured by a simple chemical titration on the fruit juice. For many
fruits the tartness and flavor are affected by the ratio of sugar to acid. In describing
the taste of tartness of several fruits and fruit juices, the term sugar to acid ratio or
Brix to acid ratio is commonly used. The higher the Brix the greater the sugar
concentration in the juice, the higher the Brix to acid ratio the sweeter and less tart
is the juice.
Once the fruit is harvested any natural resistance to microorganisms is lost. Fruits are living
tissues and they continue to respire even after they have been harvested. In case of aerobic
respiration, refrigeration is not enough to retard ripening and foods may not develop desired
flavor/texture. Moreover, it may be harmful for tropical or subtropical fruits. To ensure
maximum storage life, fruits should be harvested when mature, but not yet fully ripe or
overripe (Claypool, 1983). Ripe fruit should be avoided because it will continue to ripen in
storage. If harvested before they have matured, fruits will be more susceptible to storage
disorders. Firmness and the level of soluble solids in the fruit are good indicators of maturity
in determining picking time.
Fruits are very susceptible to bruising and other forms of mechanical damage, and
therefore should not be handled more than necessary. Fruits are normally transported and
stored in bulk boxes (bins) kept in the orchard. Bins should not be allowed to sit for extended
periods in direct sunlight, nor for more than a few hours before cooling is started (Hanson,
1976).
1.5.1. Chemical Treatments
Harvested fruits are often treated with chemicals to inhibit storage disorders. Dip or spray
treatments with calcium chloride plus a scald inhibitor mixed with a surfactant and fungicides
are commonly used to prevent scald and a group of disorders such as bitter pit. If necessary, a
surfactant is used to provide complete wetting of the fruit.
Many chemicals destroy microorganisms or stop their growth but most of these are not
permitted in foods; those that are permitted as food preservatives are listed in Table 1.3.
Chemical food preservatives are those substances that are added in very low quantities
8
Fruit Manufacturing
Table 1.3. Selected chemicals permitted as food preservatives.
Agent
Citric acid
Acetic acid
Sodium diacetate
Sodium benzoate
Sodium propionate
Potassium sorbate
Methyl paraben
Sodium nitrite
Sulfur dioxide*
Acceptable daily intake
(mg/kg body weight)
Commonly used
levels (%)
No limit
No limit
15
5
10
25
10
0.2
0.7
No limit
No limit
0.3–0.5
0.03–0.2
0.1–0.3
0.05–0.2
0.05–0.1
0.01–0.02
0.005–0.2
Source: Dauthy, 1995.
*Sulfite containing additives have been used extensively as antibrowning agents to keep vegetables and fruits
fresh looking. Because sulfites have been linked to allergic reactions, the Food and Drug Administration (FDA)
prohibited the use of sulfite preservatives in fresh vegetables and fruits (Langdon, 1987).
(up to 0.2%) and do not alter the organoleptic and physicochemical properties of the foods or
change only very little.
Preservation of food products containing chemical food preservatives is usually based on
the combined or synergistic activity of several additives, intrinsic product parameters (e.g.,
composition, acidity, water activity) and extrinsic factors (e.g., processing temperature,
storage atmosphere, and temperature).
1.6. POSTHARVEST HANDLING OF FRUITS
Fruits continue to live and respire even after they are picked (Biale and Young, 1981).
A major economic loss occurs during transportation and/or storage of fresh fruits due to
the effect of respiration. A conventional attempt to reduce such degradation has been to
refrigerate the fruits, thereby reducing the rate of respiration. Even if fruits are to be stored
for only a short period, it is still very important that the field heat be removed from them as
soon as possible. The higher the holding temperature, the greater the softening and respiration rate, and the sooner the quality becomes unacceptable. Apples, for instance, respire
and degrade twice as fast at 4.58C as at 08C. At 168C they will respire and degrade more
than six times faster. The optimum storage temperature for fruits depends also on the
variety.
On the other hand, fruits require humidity to preserve, which may be reached by adding
water vapor to the air in the storage room with one or more humidifiers. Maintaining the
humidity within this range will also reduce weight loss. Humidity near the saturation point
will promote the growth of bacteria and fungi. Table 1.4 lists the recommended storage
temperature and relative humidity for selected fruits.
If chilled fruits are suddenly transferred into warm air, water vapor in the air will
condense on them. This ‘‘sweating’’ also occurs when the doors of a cold storage room are
opened, allowing warm, moist air to enter. Sweating causes wetting, which facilitates the
growth of microorganisms. If molds are found to be growing in the storage room, the interior
surfaces, refrigeration coils, fans, and ducts must be disinfected.
1
Overview of the Fruit Processing Industry
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9
Table 1.4. Recommended storage temperature and relative humidity for selected fruits.
Fruit
Apples
Apricots
Bananas
Berries (other than cranberries)
Figs
Grapefruits
Grapes, vinifera
Kiwis
Oranges
Passion fruit
Peaches
Pears
Pineapples
Plums and prunes
Temperature(8C)
Relative humidity(%)
Storage life (weeks)
1.0/4.0
0.5/0.0
13/14
2/4
0.5/0.0
10–15
1/0.5
0.5/0.0
3–9
7–10
0.5/0.0
1.5/0.5
7/13
0.5/0.0
90–95
90–95
90–95
90–95
85–90
85–90
90–95
90–95
85–90
85–90
90–95
90–95
85–90
90–95
5–52
1–3
—
1–5
1–2
6–8
4–31
14–25
8–12
3–5
2–4
2–7
2–5
2–4
Source: Hardenburg et al. (1986) and Hanson (1976).
1.6.1. Postharvest Cooling
Proper postharvest cooling is advisable to:
.
.
.
.
suppress enzymatic degradation (softening) and respiratory activity;
slow down or inhibit water loss (wilting);
slow down or inhibit the growth of decay-producing microorganisms (molds and
bacteria);
reduce the production of ethylene (a ripening agent) or minimize the commodity’s
reaction to ethylene.
1.6.2. Cooling Methods
To reduce the cooling load fruits should be harvested as much as possible during the cool
hours of the day. Allowing the fruits to sit outside overnight in bulk boxes will generally not
lower their temperature. Moreover, bulk storage may cause the fruit temperature to increase.
It is recommended to cool the fruits quickly and thoroughly.
There are many methods of cooling fruit products before storage or shipment, including
room cooling, forced-air cooling, vacuum cooling, hydrocooling, package icing, and top icing.
One of the common and least expensive methods for cooling fruits is room cooling (Raghavan
et al., 1996).
.
Room cooling is accomplished by stacking bulk boxes, or bins, inside a refrigerated
room where the heat is allowed to dissipate slowly. Cooling is achieved by moving room
air around the containers. An airflow rate of 5---10 m3 min1 is necessary to cool fruits.
Moreover a high relative humidity (90–95%) is necessary to avoid fruit dehydration.
Although time of cooling may be too long with this method, it requires minimum handling
and labor. The time of cooling may vary from several days to more than 2 weeks for the
fruits to reach approximately the same temperature as the air in a cold storage room. After
cooling is completed, the facility can be used for short-term storage. Bins should be spaced
10
.
Fruit Manufacturing
between the containers and walls must be from 25 to 60 cm, and between the bins and
ceiling, 45 to 60 cm. Normally, more refrigeration is required to cool down fruits than to
maintain fruits at a cool temperature.
Forced-air cooling. The rate of heat transfer from fruits in the middle of the box may
be insufficient to overcome the temperature rise produced by natural respiration. In such a
case, forced-air movement is necessary and adequate space for proper air circulation
between rows of stacked bins should be allowed. Cooling is carried out by exposing the
bulk boxes in a storage room to a higher air pressure on one side than the other, by means
of fans that draw refrigerated air through the container vents (Fraser, 1991).
Pressure difference increases the cooling rate up to 4 times more than room cooling. Relative
humidity needs to be checked to avoid substantial water loss and fruit shrinkage. Water loss
increases with the cooling air velocity. As the key to forced-air cooling is the moving of cold
air through the containers vents, location and size of vents need to be carefully calculated.
While few or small vents slow the flow of cooling air, too many vents may produce container
collapse.
Some of the forced-air cooling alternatives are: (i) cold wall (where cold air is driven from
a false wall, or air plenum, to cold room by fans); (ii) forced-air tunnel (an exhaust fan is
placed at the end of the aisle of two rows of bins; the aisle’s top and ends are covered with
plastic or canvas, creating a tunnel); and (iii) serpentine cooling (a serpentine system, which is
a modification of the cold wall method, is designed for bulk bin cooling). Figure 1.2 shows a
typical cold wall alternative for the forced-air cooling of fruit bins.
Hydrocooling is one of the quickest methods for removing field heat from fruits. This
process can be used on most commodities that are not sensitive to wetting and generally
requires large volumes of chilled water to flood the fruit (Raghavan et al., 1996). Cold
well or stream water may be used as a source of hydrocooling fluid, after it has been checked
for purity. Fruits in boxes are placed on a conveyor that pushes the boxes through a cooling
tunnel. Large quantities of chilled water are sprayed directly on the tops of the boxes.
Water flows down through the product and is collected in a tank or sump underneath
the tunnel.
Cooling units
Cold wall
Fans
Fruit bins
Figure 1.2. Cold wall alternative for the forced-air cooling of fruit bins.
1
Overview of the Fruit Processing Industry
.
.
11
It may be possible to apply fungicides and scald inhibitors during hydrocooling. Water
removes heat about 5 times faster than air, but is less energy efficient. Mechanical
refrigeration is the most efficient method of cooling water; however, ice in water will
also provide a source of coolant. If hydrocooling water is re-circulated, it should be
chlorinated to minimize disease problems.
The temperature of water for hydrocooling must be kept as near to 08C as possible. To
save time and energy, fruits are seldom hydrocooled to lower than 78C. Slower methods, such
as forced-air or room cooling, are usually employed complementarily. The rate at which
fruits may be hydrocooled depends on fruit size. Figure 1.3 shows a compact hydrocooling
equipment using water as the refrigerant.
It can be approximated that the size of the refrigeration systems needed for hydrocooling
is 10 tons of refrigeration capacity for each ton of fruits cooled per hour. Hydrocooling may
prevent recently harvested fruits from wilting, shrinking, and losing flavor.
Top or liquid icing. This may be used on a variety of commodities and is particularly
effective on dense and palletized packages that are difficult to cool with forced air. Because of
its residual effect ice methods work well with high-respiration commodities such as sweet
corn, and are not recommended for fruits.
Alternative cooling methods. Alternatives to the above-mentioned cooling methods,
particularly to smaller volumes of commodities, are:
.
.
.
.
.
Harvest time: Harvest should be made during mornings or, when possible, night time, when
commodities and air temperatures are usually coolest.
Refrigeration with well water: Temperatures are usually lower than 158C.
Altitude: If easily accessible, higher elevations can provide cooling.
Cellars/caves: Generally maintain fairly constant, cooler-than-air temperatures.
Shade: If refrigeration is not available, at least keep commodities from warming up.
Figure 1.3. Hydrocooling system (from Boyette et al. (1990). Published by North Carolina Cooperative Extension
Service, with permission).
12
Fruit Manufacturing
Temperature
Initial fruit
temperature
Still air
Hydrocooling
Forced-air
cooling
Room temperature
Time (relative)
Figure 1.4. Comparison of relative cooling rate of different cooling systems.
Refrigerated trucks are not designed to cool fresh commodities. They can only maintain
the temperature of previously cooled products. While Fig. 1.4 compares the relative cooling
rate of different cooling systems, Table 1.5 lists recommended cooling methods for selected
fruits.
1.7. CONTROLLED ATMOSPHERE STORAGE
Controlled atmosphere (CA) storage prolongs fruit life by lowering the oxygen concentration
and increasing the carbon dioxide concentration in the storage atmosphere. The effects of CA
are based on the often-observed slowing of plant respiration in low O2 environments. There is
about 21% O2 in air (Table 1.6). As the concentration of O2 falls below about 10%, fruit
Table 1.5. Recommended cooling methods for selected fruits.
Commodity
Recommended cooling methods*
Apples
Blueberries
Peaches
Strawberries
Watermelons
Room cooling, forced-air cooling, hydrocooling
Room cooling, forced-air cooling
Forced air cooling, hydrocooling
Room cooling, forced-air cooling
Room cooling
Adapted from Wilson et al. (1999).
Normal storage life
1–12 months
2 weeks
2–4 weeks
5–7 days
2–3 weeks
1
.
Overview of the Fruit Processing Industry
13
Table 1.6. Average air composition.
Air components*
Nitrogen (%)
Oxygen (%)
Argon (%)
Carbon dioxide (ppm)
Neon (ppm)
Helium (ppm)
Methane (ppm)
Krypton (ppm)
Hydrogen (ppm)
Nitrous oxide (ppm)
Xenon (ppm)
Carbon monoxide (ppm)
Ozone (ppm)
Symbol
Volume (dry air)
N2
O2
Ar
CO2
Ne
He
CH4
Kr
H2
N2 O
Xe
CO
O3
78.08
20.95
0.93
350
18.2
5.24
2
1.1
0.5
0.3
0.08
0.05 – 0.2
0.02–0.03
*Dry atmosphere below 80 Km (ppm ¼ parts per million).
respiration starts to slow. This suppression of respiration continues until O2 reaches about
2–4% for most fruits. Depending on product and temperature, if O2 gets lower than 2–4%,
fermentative metabolism replaces normal aerobic metabolism; and off-flavors, off-odors, and
undesirable volatiles are produced. Similarly, as CO2 increases above the 0.03% found in air,
a suppression of respiration results for some commodities. Reduced O2 and elevated CO2
together can reduce respiration more than either alone. These concentrations of oxygen and
carbon dioxide also reduce the ability of the ethylene produced by ripening fruits to further
accelerate fruit ripening (Kader, 1986).
CA storage facilities are specially constructed, airtight cold storage rooms with auxiliary
equipment to monitor and maintain specific gaseous atmospheres. Oxygen, carbon dioxide,
and ethylene levels should be monitored daily and controlled within narrow limits. Recommendations for CA storage conditions change as a result of ongoing research. Optimum
conditions depend on several factors, including variety and growing conditions. In general
CA methods are much too expensive for applying to process fruit. Table 1.7 lists recommended oxygen and carbon dioxide storage condition for various fruits (Kader, 1985;
Raghavan et al., 1996).
It must be considered that higher storage temperatures lead to higher respiration rates,
and gas concentrations recommended in Table 1.7 will not be successful.
1.8. MODIFIED ATMOSPHERE PACKAGING OF FRUITS
Vegetables and fruits differ from other foodstuffs in that they continue to respire even when
placed in a modified atmosphere. Due to the respiration, there is a danger that CO2 will
increase to levels harmful to the packed commodities. On the other hand, respiration consumes oxygen and there is a danger of anaerobiosis. If fruits are packed in a sealed impermeable package, O2 is rapidly used up, CO2 builds up, anaerobic respiration will take place,
and off-flavors and odors will develop. There is also the risk of the growth of anaerobic foodpoisoning organisms such as Clostridium botulinum. On the other hand, if the packaging film
is completely permeable the fruit would not benefit from modified atmosphere. Furthermore,
14
Fruit Manufacturing
Table 1.7. O2 and CO2 condition for several fruits’ storage
(optimal temperatures are listed in Table 1.4).
Fruit
O2 concentration (%)
CO2 concentration (%)
2–3
2–3
2–5
5–10
5–10
3–10
2–5
2
5–10
1–2
2–3
5
1–2
1–2
1–2
2–5
15–20
15–20
5–10
1–3
5
0 –5
5
0 –1
10
0 –5
Apples
Apricots
Bananas
Berries (other than cranberries)
Figs
Grapefruits
Grapes, vinifera
Kiwis
Oranges
Peaches
Pears
Pineapples
Plums and prunes
Adapted from del Valle and Palma (1997).
when fruits are cut, sliced, shredded, or otherwise processed, their respiration rates increase.
This is probably due to the increased surface area exposed to the atmosphere after cutting that
allows oxygen to diffuse into the interior cells more rapidly, thereby increasing metabolic
activity of injured cells.
Fruit packaging has progressed in the past several years. Appropriate packaging materials have been developed for most of the more common fresh-cut products. Technical
challenges still exist in fruit packaging. A number of special packaging materials intended
for vegetables and fruits have been developed such as smart films, microporous films, and
microperforated films.
1.8.1. Factors Affecting Fruit Respiration
The ability of modified atmosphere packaging (MAP) to extend the shelf life of foods has
been recognized for many years. MAP may be defined as the packaging of a perishable
product in an atmosphere, which has been modified so that its composition is different from
that of air. In MAP of respiring foods, e.g., fresh fruits, once the atmosphere has been
changed to the desired level, the respiration rate of the produce should equal the diffusion
of gases across the packaging material in order to achieve an equilibrium atmosphere in the
package.
The potential advantages and disadvantages of MAP have been reviewed by Farber
(1991). The main effects of MAP on fruits are to:
.
.
.
lower the rate of fruit respiration (slow down ripening and senescence);
slow down the rate of ethylene production, which is a natural plant hormone involved
in the control of ripening;
retard the growth of molds, extending the storage/shelf life of the fresh fruit.
If the packaging film is semipermeable O2 and CO2 can diffuse through it, and an
equilibrium concentration of both gases is established when the rate of diffusion through
the package is equal to the rate of respiration.
1
Overview of the Fruit Processing Industry
.
15
The main disadvantages are:
.
.
.
.
.
cost increase
need of temperature control
different gas formulations for each product type
special equipment and personnel training
product safety.
MAP technology is largely used for minimally processed fruits. MAP combined with
low-temperature storage is a common method to improve the storage stability of ready-to-use
products.
The three main gases used commercially in MAP are oxygen, nitrogen, and carbon
dioxide. The gases and their concentrations should be tailored for each individual product.
The required combinations of temperature, oxygen, and carbon dioxide levels vary with fruit
type, variety, origin, and season. Carbon dioxide is important because of its biostatic activity
against many spoilage organisms that grow at refrigeration temperatures. Oxygen inhibits the
growth of anaerobic pathogens, but in many cases does not directly extend shelf life. Nitrogen
is used as a filler gas to prevent pack collapse, which may occur in high CO2 -containing
atmospheres.
Modified atmospheres may be produced naturally by respiration (passive MA) and by
the application of gas flushing techniques (equilibrium MA) (Fig. 1.5).
A sliced fruit is still alive and it continues respiring. Therefore it creates an MA within the
pack with a reduced level of oxygen and an increased level of carbon dioxide. During passive
MA gases’ equilibrium due to package film permeability will be reached at a certain relatively
long time (a week or longer). When the estimated final equilibrium concentration of gases is
% CO2 or O2 (relative)
Active MAP
CO2
Passive MAP
O2
Active MAP
Time after packaging
Figure 1.5. CO2 and O2 concentration evolution during passive and active MAP of packaged fruit products
(adapted from Zagory and Kader, 1988).
16
Fruit Manufacturing
artificially created immediately after packaging (by air purge and replacing) respiration rate is
more quickly controlled.
For respiring products, the permeability characteristics of the film determine the equilibrium gas concentration achieved in the package to a large degree. The actual equilibrium
MA attained within a package will also depend on factors such as the prepared form of the
fruit studied, the rate of respiration at storage temperature, the pack volume and fill weight,
and the surface areas for gas exchange.
1.8.2. Factors Influencing the Exact Modified Atmosphere Within a Sealed Pack
Respiration depends not only on the variety of fruit but also on the stage of maturity of fruit
when harvested. Storage temperature influences respiration rate of the product, affects the
rate of diffusion of O2 and CO2 through the package film, and affects the rate of spoilage. The
temperature must be kept both constant and low #48C.
In the second place, the fitted atmosphere within the pack is influenced by gaseous
environment inside the pack. The initial gas mix must be corrected and tailored to the
individual products. As previously indicated, too high levels of CO2 will still reduce the
respiration rate and inhibit the growth of bacteria, but physiological damage of the product
may take place. Too low levels of O2 may still reduce the respiration rate, but if they are too
low anaerobic respiration will take place.
The package must be made from a suitable material. PVC and LDPE are the most
commonly used films. Among other factors affecting MAP, antifogging agents added to film,
weight of product, volume of gas, and area of film must be considered.
Lipton (1975) proposed a helpful approach to selecting the required ratio of O2 to CO2
permeability of a polymeric film, which was expressed as:
in
Pout
PCO2
O PO2
¼ in 2
,
P O2
PCO2 Pout
CO2
(1:1)
in,out
where PCO2 and PO2 are the permeability coefficients of CO2 and O2 respectively, and P CO2
in,out
and P O2 are the partial pressure of gases inside and outside package, respectively. Table 1.8
gives permeability coefficients of different polymeric films to oxygen and carbon dioxide.
EXAMPLE
Using Eq. (1.1) and Table 1.4 select the appropriate film to create an atmosphere
containing 2% O2 and 5% CO2 .
Partial pressure of atmospheric gases at normal conditions is Pout
CO2 ¼ 0:001 atm and
Pout
O2 ¼ 0:21 atm, respectively. Then, from Eq. (1.1):
PCO2 0:21 0:02
¼ 3:8
¼
0:05
PO2
From Table 1.8, both HDPE and PP have permeability ratio to oxygen and carbon
dioxide close to the required calculated value. It is worth noting that the actual ratio for
a particular film is not constant, but depends on the temperature. In general, the ratio
increases as the temperature decreases.
1
.
Overview of the Fruit Processing Industry
17
Table 1.8. Permeability coefficient of selected polymer films.
Permeability coefficient
[mL (STP) cm cm2 s1 cmHg1 ]
Film material
Low-density polyethylene (LDPE)
High-density polyethylene (HDPE)
Polypropylene (PP)
Polystyrene
Polyethylene terephthalate (PET)
O2
CO2
55
10.6
23
11
0.22
352
35
92
88
1.53
Adapted from Robertson, 1993.
In summary, handling of fruits requires care during harvesting, transportation and
handling operation within the processing plant. Temperature and relative humidity need to
be properly maintained. Refrigeration is required for prolonged storage or transportation for
long distances.
1.9. TECHNOLOGY OF SEMIPROCESSED FRUIT PRODUCTS
The semiprocessed fruit products are manufactured in order to be delivered to industry
processing plant, to be manufactured in finished products such as jams, jellies, syrups, fruits
in syrup, etc. The following categories of semiprocessed fruit are defined:
.
.
.
Fruit pulps: Obtained by mechanical treatment (or, less often, by thermal treatment) of fruit
followed by their preservation. Either whole fruit, halves, or big pieces are used, which
enables easy identification of the species. Pulps can be classified as boiled or nonboiled.
Fruit purées-marks: Obtained by thermal and mechanical treatment operations by which all
nonedible parts (cores, peels, etc.) are removed. Purées-marks are also classified as boiled or
nonboiled.
Semiprocessed juices: Products obtained by cold pressure, or eventually by other treatments
(diffusion, extraction, etc.) followed by preservation.
1.9.1. Preservation of Semiprocessed Fruit Products
Preservation can be achieved by chemical means, freezing, or pasteurization. The choice of
preservation process for each individual case depends on the semiprocessed product type and
the shelf life needed. Chemical preservation may be carried out with sulfur dioxide, sodium
benzoate, formic acid, and, on a small scale, with sorbic acid and sorbates. Preservation with
sulfur dioxide is a common process because of its advantages: universal antiseptic action and
very economic application. The preservation with sulfur dioxide, although linked to allergic
reactions, is mainly used in pulps and purées.
Sodium benzoate is also in use in pulps and purées-marks. Preservation with sodium
benzoate does not firm up the texture and does not modify fruit color. The disadvantages are
that it is not a universal antiseptic, and needs an acidie medium to act. Moreover, sodium
benzoate is difficult to remove. Practical dosage level for 12 months’ preservation was
18
Fruit Manufacturing
recommended in the range 0.18 – 0.20% sodium benzoate, depending on the product to be
preserved. Sodium benzoate is used as a solution in warm water; the dissolution water level
has to be at maximum 10% reported to that of semiprocessed product weight.
Formic acid, an antiseptic effective against yeasts, may be used for semi-processed fruit
juices at a dosage level of 0.2% pure formic acid (100%). Formic acid does not influence color
and is easily removed by boiling. Because of a potential effect of pectic substance degradation,
formic acid is less used in pulps and purées-marks preservation.
Sorbic acid can be used for preservation of semiprocessed fruit products at a dosage level
of 0.1% maximum. Advantages of sorbates are that they are completely harmless and without
any influence on the organoleptic properties of semiprocessed fruit products.
Heat treatment. As fruits have a low pH, preservation of semiprocessed fruit products by
heat treatment step at maximum temperature of 1008C, can be done (pasteurization). This
treatment results in a more hygienic process, thereby assuring long-term preservation. However, air-tight containers are needed and pectic substances could deteriorate if the thermal
treatment is too long.
Freezing. Freezing is applied to semiprocessed fruit products with a very high quality and
cost. This can be done with or without sugar addition. The obvious advantages of this process
are the absence of added substances, a very good preservation of quality of fruit constituents
(pectic substances, vitamins, etc.), and good preservation of organoleptic properties. Freezing
is done at about 20 to 308C and storage at 10 to 188C.
REFERENCES
Biale, J.B. and Young, R.E. (1981). Respiration and ripening in fruits—retrospect and prospect. In Recent Advances
in the Biochemistry of Fruits and Vegetables, Friend, J. and Rhodes, M.J.C. (eds.). Academic Press, NY,
pp. 1–39.
Boyette, M.D., Estes, E.A. and Rubin, A.R. (1990). Hydrocooling. Published by North Carolina Cooperative
Extension Service 10/92.2M. TAH.220544 AG-414–4. www.bae.ncsu.edu/programs/extension/publicat/
postharv.
Claypool, L.L. (1983). Biological and cultural aspects of production and marketing of fruits. In Principles and
Practices for Harvesting and Handling Fruits and Nuts. O’Brien, M., Cargill, B.F. and Friedly, B.B. (eds.).
AVI Publishing Company, Inc., Westport, CT, pp. 15–42.
Coombe, B.G. (1976). The development of fleshy fruits. Ann. Rev. Plant Physiol. 27: 507.
Dauthy, M.E. (1995). Fruit and Vegetable processing. FAO AGRICULTURAL SERVICES BULLETIN No. 119
Food and Agriculture Organization of the United Nations, Rome. In: http://www.fao.org/documents.
del Valle, J.M. and Palma M.T. (1997). Preservación II. Atmósferas controladas y modificadas. In Temas en
Tecnologı́as de Alimentos. Vol. 1. J.M. Aguilera Ed. CYTED-IPN MEXICO.
FAOSTAT Data (2005). FAO Statistical Databases. www.fas.usda.gov/http/Presentations./
Farber, J.M. (1991). Microbiological aspects of modified-atmosphere packaging technology—a review. J. Food
Protection 9: 58–70.
Fraser, H.W. (1991). Forced-air rapid cooling of fresh Ontario fruits and vegetables. Ministry of Agriculture and Food,
Toronto, Ontario, AGDEX 202–736.
Hanson, L.P. (1976). Commercial processing of fruits. Noyes Data Corporation, London, p. 302.
Hardenburg, R.E., Watada, A.E. and Wang, C.Y. (1986). The commercial storage of fruits, vegetables, and florist
and nursery stocks. USDA, Agric. Handbook, 66, p. 130.
Hui, Y.H. (1991). Data sourcebook for Food Scientists and Technologists. VCH Publishers, New York.
Kader, A.A. (1985). Modified atmospheres: an indexed reference list with emphasis on horticultural commodities,
supplement no. 4. University of California, Davis, Postharvest Hort. Series No. 3, 31 pp.
Kader, A.A. (1986). Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits
and vegetables. Food Technol. 40(5): 99–103.
1
.
Overview of the Fruit Processing Industry
19
Kader and Barret (1996). Classification, composition of fruits, and postharvest maintenance of quality. In Processing
Fruits: Science and Technology. V. 1 Biology, Principles, and Applications, Somogyi, L.P., Ramaswamy, H.S.
and Hui, Y.H. (eds.). Technomic Publishing Company, Inc., pp. 1–25.
Koning, R.E. (1994). Plant Physiology Information Website. http://plantphys. info/index. html.
Langdon, T.T. (1987). Prevention of browning in fresh prepared potatoes without the use of sulfiting agents. Food
Technol. 41(5): 64–67.
Lipton, W.J. (1975). Controlled atmospheres for fresh vegetables and fruits- why and when. In Postharvest Biology
and Handling of Fruit and Vegetables, Haard, N.F. and Salunke, D.K. (eds.). AVI Publishing Company, Inc.,
Westport, CT, p. 130.
Nagy, S., Chen C.S. and Shaw, P.E. (Eds.) (1993). Fruit Processing Technology. Agscience, Inc., Auburndale, FL.
O’Brien, M., Cargill, B.F. and Fridley, R.B. (1983). Principles and Practices for Harvesting and Handling of Fruits and
Nuts. AVI Publishing Company, Inc., Westport, CT, 636 pp.
Raghavan, G.S.V., Alvo, P., Gariépy and Vigneault, C. (1996). Refrigerated and controlled modified atmosphere
storage. In Processing Fruits: Science and Technology. V. 1 Biology, Principles, and Applications. Somogyi,
L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomic Publishing Company, Inc., pp. 135–167.
Robertson, G. (1993). Food Packaging. Principles and Practice. Marcel Dekker, Inc., NY, pp. 472–473.
Wilson, L.G., Boyette, M.D. and Estes, E.A. (1999). Postharvest handling and cooling of fresh fruits, vegetables,
and flowers for small farms. Part II. Cooling7/99 HIL-800 NC cooperative extension service publication
AG-414–1, and USDA Agricultural Handbook No. 66.
Zagory, D. and Kader, A.A. (1988). Modified atmosphere packaging of fresh produce. Food Technol. 42(9): 70–74,
76–77.
CHAPTER 2
PROCESSING OF FRUITS:
AMBIENT AND LOW
TEMPERATURE PROCESSING
2.1. FRUIT PRODUCTS AND MANUFACTURING PROCESSES
World trade of fruit and vegetable juice averaged nearly US$4,000 million last decade
(FAOSTAT, 2005). By far the largest volume of processed apples and oranges, the two
most important fruit commodities, is in the form of juices, and a great part of the present
chapter is devoted to describing the processing of these liquid foods. There are however many
other products obtained from fruits, including canned, dried, and frozen fruit; pulps; purées;
and marmalades. Table 2.1 lists final products and processes applied on selected fruits. In
addition, developments in aseptic processing have brought new dimensions and markets to
the juice industry.
Juices are a product for direct consumption and are obtained by the extraction of cellular
juice from fruits; this operation can be done by pressing or diffusion. Fruit juices are
categorized as those without pulp (‘‘clarified’’ or ‘‘not clarified’’) and those with pulp
(‘‘pulps,’’ ‘‘purées,’’ and ‘‘nectars’’). Other classifications include ‘‘natural juice’’ products
obtained from one fruit, and ‘‘mixed juice’’ products obtained from the mixing of two or three
juices of different fruit species or by adding sugar. Juices obtained by removal of a major part
of their water content by vacuum evaporation or fractional freezing are defined as ‘‘concentrated juices.’’
Fruit composition is mainly water (75–90%), which is mainly found in vacuoles, giving
turgor (textural rigidity) to the fruit tissue. Juice is the liquid extracted from the cells of mature
fruits. Fruit cell wall is made of cellulose, hemicellulose, pectic substances, and proteins.
The primary cell wall, composed of crystalline cellulose microfibrils, is made up of
polymers of b-D-glucose linked by b-1-4-glycosidic linkages and cellulose embedded in an
amorphous matrix of pectin and hemicelluloses. The definition of a mature fruit varies with
each type. Typically, sugar and organic acid levels, and their ratio indicate maturity stage.
The extracted liquid is composed of water, soluble solids (sugars and organic acids), aroma
and flavor compounds, vitamins and minerals, pectic substances, pigments, and, to a very
small degree, proteins and fats. The various sugars, such as fructose, glucose, and sucrose,
combined with a large number of organic acids (most important being citric, malic, and
tartaric), help give the fruit its characteristic sweetness and tartness.
During ripening of fruits, a general decrease in acidity and starch as well as an increase in
sugars is seen. Moreover, formation of odors, breakdown of chlorophyll, and hydrolysis of
pectic substances also occur. It must be noted that plant tissues continue to ripen after
harvest. Finally, senescence occurs, at a rate accelerated by the increase in ethylene.
21
22
Fruit Manufacturing
Table 2.1. Principal fruit products and manufacturing processes.
Product
Fruit
Process description
Canned
Apples
Canned apple is the product prepared from fresh apples of one variety, which are not
overripe, and whose fruit is packed with or without any of the following ingredients: water, salt, spices, nutritive sweetening ingredients, and any other ingredients
permissible under regulations. The product is then heat processed to ensure
preservation in hermetically sealed containers
Canned apple sauce is the product prepared from comminuted or chopped apples,
which may or may not be peeled and cored, and to which may have been added
thereto one or more of the optional ingredients specified by regulations. The
product is heated and, in accordance with good manufacturing practices, bruised
apple particles, peel, seed, core material, and other coarse, hard, or extraneous
materials are removed. The product is processed by heat, either before or after
sealing, so as to ensure preservation. The soluble solids’ content is $ 98Brix
Canned cranberry sauce is the jellied or semijellied cranberry product prepared from
clean, sound, matured cranberries, and contains sweetening ingredients and water.
Pectin may be added to compensate for deficiency of the natural pectin content of
the cranberries. The mixture is concentrated and sufficiently processed by heat to
ensure preservation of the product. Final soluble solid is ffi 35–45%
Canned fruits for salad consist of carefully selected apricots, cherries, yellow
clingstone peaches, pears, pineapple, and grapes. The product is packed in a
suitable liquid medium with or without the addition of sweetening ingredients, or
other permissible ingredients. The product is heat processed and is processed to
ensure preservation of the product in hermetically sealed containers
Frozen apples are prepared from fresh apples of one variety, not overripe, which are
peeled, cored, trimmed, sliced, sorted, washed, and properly drained before filling into
containers. Sweetening ingredient and any other ingredient permissible under regulations may be used. The product is frozen in accordance with good commercial
practice and maintained at temperatures necessary for the preservation of the product
Frozen apricots are prepared from fresh fruit of one variety, which are not overripe,
which are sorted, washed, and may be trimmed to ensure a clean and wholesome
product. The apricots are properly drained of excess water before placing into
containers. The addition of sweetening ingredients, including syrup containing
pureed apricots, suitable antioxidant ingredients, and or any other ingredients
permissible under regulations is allowed
Frozen berries are prepared from properly ripened fresh fruit berries, are stemmed and
cleaned, may be packed with or without packing media, and are frozen and stored at
temperatures necessary for the preservation of the product. The same is applicable to
frozen blueberries. Frozen cranberries do not need stemming before freezing
Frozen sweet cherries are prepared from fresh fruit of one variety, which are not
overripe, fruit of any commercial variety of sweet cherries, which are sorted,
washed, and drained. The addition of nutritive sweetening ingredients is allowed.
The product is frozen in accordance with good commercial practice and
maintained at temperatures necessary for the preservation of the product
Frozen grapefruit is prepared from fresh fruit of one variety, which are not overripe.
After the fruit has been washed and peeled, and separated into segments by
removing the core, seeds, and membrane it is packed with or without packing
additives. The product is frozen and stored at temperatures necessary for the
preservation of the product
Frozen lemon concentrate is the product prepared from lemon juice (from fresh,
sound, ripe, and thoroughly cleansed fruit) and lemonade ingredients (sweeteners;
lemon oil, its extracts, or emulsions) and water in sufficient quantities to standardize the product. The product contains $48.08Brix (corrected for acidity). Such
juices may be fresh or frozen, or fresh concentrated or frozen concentrated;
processed in accordance with good commercial practice and is frozen and
maintained at temperatures sufficient for the preservation of the product
Apple sauce
Cranberry
sauce
Fruit salads
Frozen
Apples
Apricots
Berries
Frozen
Cherries
Grapefruit
Lemon
(continued )
2
.
Processing of Fruits
23
Table 2.1. (Continued )
Product
Fruit
Process description
Melon
Melon balls are spheres of melon flesh prepared from balls of suitable varieties of
sound, fresh melons. The balls are prepared and washed in a manner to assure a
clean and wholesome product. The product may be packed with the addition of a
suitable fruit and or vegetable garnish; nutritive or non-nutritive sweetening
ingredients, including syrup and any other ingredient permissible under
regulations. It must be frozen in accordance with good commercial practice and
maintained at temperatures necessary for preservation
Frozen peaches are prepared from fresh peaches of one variety, which are not
overripe, peaches are peeled, pitted, washed, cut, and trimmed to assure a clean and
wholesome product. The peaches may be packed with the addition of a sweetening
ingredient, including syrup and/or syrup containing pureed peaches and any other
permissible ingredients. The product must be frozen in accordance with good
commercial practice and maintained at temperatures necessary for the preservation
Frozen pineapple is prepared from the properly ripened pineapple fruit, which is
peeled, cored, trimmed, and washed; is packed with or without packing media; and
is frozen and stored at temperatures necessary for the preservation of the product
Frozen plums are prepared from clean, sound, fresh fruit of any commercial variety
of plums, which are sorted, washed, drained, and pitted; which may be packed with
or without the addition of a nutritive sweetening ingredient; and which are frozen
in accordance with good commercial practice and maintained at temperatures
necessary for the preservation of the product
Dried apples are prepared from fresh apples of one variety, which are not overripe, by
washing, sorting, trimming, peeling, coring, and cutting into segments. The prepared apple segments are properly dried to remove the greater portion of moisture
to produce a semidry texture. The product may be sulfured sufficiently to retard
discoloration. The sulfur dioxide content of the finished product should not exceed
1,000 parts per million. No other additives are allowed
Dehydrated low-moisture apricots are prepared from fresh fruits of one variety,
which are not overripe, which are cut, chopped, or otherwise prepared into various
sizes and shapes; are prepared to assure a clean, sound, wholesome product; are
processed by dehydration whereby practically all of the moisture is removed to
produce a very dry texture; and are placed in a container, which has low moisture.
The product is packaged to assure dryness retention and should be sulfured at a
level sufficient to retain a characteristic color
Dried figs are prepared from clean and sound fruits and are sorted and thoroughly
cleaned to assure a clean, sound, wholesome product. The figs may or may not be
sulfured, or otherwise bleached
Dried peaches are the halved and pitted fruit from which most of moisture has been
removed. The dried fruit is processed to cleanse and it may be sulfured sufficiently
to retain color
Dried pears are made with the halved fruit, which may or may not be cored, from
which the external stems and calyx cups have been removed. Before packing, the
dried fruit may be sulfured sufficiently to color
Processed raisins are dried grapes of vinifera varieties, such raisins as Thompson
Seedless Sultanian, Muscat of Alexandria, Muscatel Gordo Blanco, Sultana, or
White Corinth. The processed raisins are from fresh fruit, which are not overripe.
Grapes are properly stemmed and cap stemmed, seeded, sorted or cleaned, or both,
and are washed in water to assure a wholesome product
Dehydrated prunes are prepared from clean and sound prunes, which are pitted and
prepared into various sizes and shapes, washed, and processed by dehydration to
produce a very dry texture. The product is then packaged to assure retention of the
dryness characteristic of the product. A safe preservative may be added
Peaches
Pineapple
Plums
Dried
Apples
Apricots
Figs
Peaches
Pears
Raisins
Prunes
(continued )
24
Fruit Manufacturing
Table 2.1. (Continued )
Product
Fruit
Process description
Juices
Grape
Juices
Apple frozen
concentrate
Frozen concentrated sweetened grape juice is prepared from concentrated
unfermented single-strength grape juice from fresh fruit, which are not overripe,
with or without aging, or grape juice depectinization, and is then concentrated.
Single-strength grape juice or natural grape essence, or a combination of singlestrength grape juice and natural grape essence may be mixed to the concentrate
and may or may not be packed with the addition of ingredients like sweeteners,
edible fruit acid, and ascorbic acid. The product is then frozen in accordance
with good commercial practice
Frozen concentrated apple juice is prepared from the concentrated unfermented,
liquid obtained from apple juice during the first pressing of properly prepared,
clean, mature, fresh apples by good commercial processes. The juice is clarified
and concentrated to at least 22.98Brix. The apple juice concentrate so prepared,
with or without the addition of ingredients permissible under regulations, is
packed and frozen in accordance with good commercial practice and maintained
at temperatures necessary for the preservation
Canned apple juice is the unfermented juice obtained from sound, ripe apples, with
or without parts. No water may be added directly to the finished product.
However, concentrated apple juice is allowed. Apple essence may be restored
to a level that provides a natural apple juice flavor
Canned grape juice is the unfermented liquid obtained from the juice of properly
matured fresh grapes. Such grape juice is prepared without concentration, without
dilution, is packed with or without the addition of sweetening ingredients, and is
sufficiently processed by heat to assure preservation of the product in hermetically
sealed containers
Canned lemon juice is the undiluted, unconcentrated, unfermented juice obtained
from sound, mature lemons of one or more of the high-acid varieties. The fruit is
prepared by washing prior to extraction of the juice to assure a clean product. The
product is sufficiently processed with heat to assure preservation in hermetically
sealed containers
The fruit is prepared by sorting and by washing prior to extraction of the juice.
The concentrated lemon juice is prepared and concentrated in accordance with
good commercial practice. It may or may not require processing by heat,
subsequent refrigeration, or freezing to assure preservation of the product.
The finished product may contain added pulp, lemon oil to standardize flavor,
and or permissible chemical preservatives.
Concentrated tangerine juice is the tangerine concentrated product obtained from
sound, mature fruit. The fruit is prepared by sorting and by washing prior to
extraction of the juice. The concentrated tangerine juice is processed in accordance
with good commercial practice, and may or may not require processing by heat or
subsequent refrigeration to assure preservation of the product. Cold-pressed oil to
standardize flavor and permissible chemical preservatives may be added
Orange marmalade is the semisolid or gel-like product prepared from orange fruit
ingredients together with ingredients like sweeteners, food acids, food pectin,
lemon juice, or lemon peel. Soluble solids of finished marmalade is $65%
Canned apple
juice
Canned grape
juice
Lemon singlestrength
Lemon
concentrate
Tangerine
Others
Marmalade
Source: Hui (1991); Nagy et al. (1992); Somogyi et al. (1996).
2.2. FRUIT JUICE AND PULP PROCESSING
Fruit processing plants can vary from a simple facility for single juice extraction and canning,
to a complex manufacturing facility, which has ultrafiltration and reverse osmosis equipment,
cold storage, and waste treatment plant. A simplified characteristic flow diagram of a juice
2
Processing of Fruits
.
25
processing line is shown in Fig. 2.1. Processed products can be either single strength or bulk
concentrate, and are available either as clarified or cloudy juice. Production of fruit juices can
be divided into four basic principal stages:
.
.
Front-end operation
Juice extraction
Fruit
Washing
(brushing, spraying, etc.)
Grape, berries
Steming, destoning, peeling
Blanching
If necessary
Milling, chopping,
crushing
Extraction
Seeds´removing
(berries)
Enzymatic treatment
(maceration, liquefaction)
Puree
Centrifugation
Pressing
Turbid juice
Enzymatic treatment
Heat treatment
Flocculation
Clarification
filtration
UF
Clear juice
Centrifugation
Cloudy juice
Concentration step
Final product
Figure 2.1. Typical fruit juice (clear or cloudy) and purée-processing line steps.
26
Fruit Manufacturing
.
.
Juice clarification and refining
Juice pasteurization and concentration.
Figure 2.2a and b shows descriptive sketches of alternative processing steps for cloudy
and/or clarified apple juice concentrate elaboration.
Reception line
(from silos)
Pressing
Milling/pulping
Enzymatic mashing
Screening
Aroma
stripping
Cloudy
(a)
(Sd)
UF
Supernatant (Sp)
Sediment
(Sd)
Clarification
Vacuum filtration
Centrifugation
(Sp)
(UF)
Ultrafiltration
(UF)
Fining
Storage
Other
(b)
Concentration
Figure 2.2. Typical apple juice processing plant. (a) From fruit to cloudy juice; (b) From cloudy juice to concentrate.
2
Processing of Fruits
.
27
2.2.1. Front-End Operations
This stage includes those operations related with the reception and classification of fruits in
the manufacturing plant:
2.2.1.1. Reception Line
.
.
.
.
Weighing of incoming fruit: Origin and variety are usually recorded in this step.
Unloading of fruits into silos system: Harvesting containers known as bins are commonly used worldwide for transportation of fruits from the orchard to the processing
plant. Standard bins are 1:21 1:21 1:0 m in size. Up to 30 or more bins may be
placed in a single truck. Once in the plant, bin dumping–unloading can be performed at
least in three different ways, depending on the fruit (Fig. 2.3).
Sampling and laboratory testing: Table 2.2 lists the recommended fruit controls at the
reception in processing plant, including assay of soluble solids, yield, Brix-acid ratio,
etc. Other special tests are Magnus–Taylor pressure tester for pears and apples, and
background color for peaches.
Washing of fruit: The harvested fruit is washed to remove soil, microorganisms, and
pesticide residues. Spoiled fruits should be discarded before washing in order to avoid
contaminating the washing tools and/or equipment and the contamination of other
fruits during washing. Washing efficiency can be estimated by the total number of
microorganisms present on fruit surface before and after washing.
Apples require heavy spray applications and rotary brush wash to remove any rot. Many
fruits such as mechanically harvested berries are air cleaned on mesh conveyors or vibrators
• Hinged sides:
Good for cherries,
apricots, and
peaches
Tilt when
partly empty
• Bin tippers: Good
for apples, pears,
grapes, etc.
Bins
•
Flotation unloaders:
Good for fruits with
density < 1 (apples)
Flotation
Bins
Figure 2.3. Unloading of fruits into silos systems. Reprinted from the Encyclopedia of Food Science and Nutrition.
Lozano J.E., Separation and Clarification, pp. 5187–5196 (copyright) 2003, with permission from Elsevier.
28
Fruit Manufacturing
Table 2.2. Recommended fruit controls at reception.
Checks per lot
Checks for every 10 lots
Once during harvest season
Color
Taste
Texture
Flavor
Soluble solids ( Brix)
Variety
Sanitary evaluation
Density
Water content
Total sugars, reducing sugars
Total acidity
Ascorbic acid
Mineral substances
Tannic substances
Pectic substances
passing over an air jet. Washers are conveyor belts or roller conveyors with water sprays, reel
(cylinder) type with internal spray (Fig. 2.4), brushes and/or rubber rolls with or without
studs. Vibratory-type washers are very effective for berries and small fruits. Brushes are
effective in eliminating rotten portions of fruits, thus preventing problems with micotoxins
(patulin in apples). Some usual practices in fruit washing are:
.
.
.
Addition of detergents or 1.5%-HCl solution in washing water to remove traces of
insecticides and fungicides;
Use of warm water (about 508C) in the prewashing phase;
Higher water pressure in spray/shower washers.
Washing must be done before the fruit is cut in order to avoid losing high-nutritive value
soluble substances (vitamins, minerals, sugars, etc.).
2.2.1.2. Final Grading, and Inspection and Sorting
Fruit sorting covers two main separate processing operations:
(1) Removal of damaged fruit and any foreign substance; and
(2) Qualitative sorting based on organoleptic criteria and maturity stage.
The most important initial sorting is performed for variety and maturity. However, for some
fruits and in special processing technologies, it is advisable to carry out a manual dimensional
Fruit in
Water in
Water spray
Figure 2.4. Sketch of a reel washer with internal spray. Reprinted from the Encyclopedia of Food Science and
Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196 (copyright) 2003, with permission from Elsevier.
2
Processing of Fruits
.
29
Table 2.3. Fruit sorting methods.
Sorting method
Description
By size
By weight
By texture firmness
By color
Rollers (cherries), diverging belts, reels with holes
Apples and citrus sorters. Sort into 20 or more weight grades
Bounce system (cranberries)
Citrus color sorter measures green to yellow ratio
sorting (grading). Sorting may be performed by different ways, such as those listed in Table
2.3 (Fellows, 1988):
.
.
.
.
.
.
Aligning: Feeding into some processes (peeling, trimming, etc.) needs the fruit to be
placed in a single line. This may be performed with accelerating belts or water flumes.
Peeling (skin removal): Although manual peeling is still used for certain large vegetables, the method is very expensive. When required, fruits are usually peeled with one
of the methods (Woodroof, 1986; Fellows, 1988) listed in Table 2.4. In general, loss
increases with surface to volume ratio and decreases with fruit size. Mechanical
methods are the worst, with up to 30% loss, while chemical (caustic) methods reduce
loss to #10%.
Trimming: This is usually a manual operation that precedes cutting, in order to
eliminate few defective pieces.
Cutting: Many special cutters are available, including sector cutters for apples, berry
slicers, dicers, etc.
Pitting and coring: This operation usually occurs after sorting and peeling. In peaches,
pitters cut away some flesh. Automatic cherry pitters have also been developed.
Belt conveyor: Transport fruits to juice extractors (citrus), crusher and mills (pomes),
or stem and seed remover (grapes and berries).
Table 2.4. Peeling methods.
Method
Description
Mechanical peeling
.
.
.
.
Steam peeling
.
Chemical peeling
.
Hot gas peeling
.
Freeze–thaw peeling
.
By abrasion: It is used in batch with rotating abrasive base and water wash. This method
is inefficient, with excessive losses.
Abrasive roll peelers: This is a continuous method that combines rolls and brushes.
Blade type: The fruit rotates and mechanized knives separate the peel.
Live knife: Incorporates hydraulic control of the knife pressure. Good for apples
and pears.
Pressure steam peeling make the peel blow off with pressure drop coming out of peeling
chamber. May be combined with dry caustic peeling system.
Caustic peeling is extremely common. The simplest type involves immersion on a
pocketed paddle wheel, with hot NaOH (20%), followed by scrubbing and washing.
Tomatoes, peaches, and apples are peeled by this method. KOH is preferred because
or its tissue penetration and disposal properties.
When hot gas contacts a vegetable on the belt or roller conveyor, the skin is blown
off by the steam formed. It is generally not used in fruits.
Fruit is frozen in a low temperature medium (408C) for few seconds and then warmed
in water (408C). As a result of freezing the immediate subpeel cells are disrupted,
releasing pectinases, which free the peel. Peeling loss is reduced to a minimum.
30
Fruit Manufacturing
2.2.2. Extraction
The method of separating most of water and soluble solids (juicing) depends on the fruit
variety.
2.2.2.1. Citrus
.
There are three main types of extractors manufactured by different companies (Ramaswamy and Abbatemarco, 1996):
(1) The FMC citrus juice extractor, in which juice is extracted from the whole fruit without
first cutting the fruit into half. Outlet streams carry juice peel, center part, and oil
emulsion.
(2) The Brown extractor, in which the fruit is cut into half. Outlet streams are juice of high
yield and quality, and rag and peel.
(3) The Rotary press, in which the fruit is cut in half and the juice extracted in rotary
cylinders.
More than 75% of the world’s processors use FMC technology,this process is described in
more detail here.
When the upper and lower cups start to come closer to each other, the upper and lower
cutters cut two holes in the fruit (Fig. 2.5a). As the upper and lower cups continue to
come together, the peel is separated from the fruit (Fig. 2.5b). The peeled fruit moves into
the strainer tube where the juice is instantaneously separated from the seeds and the rest of the
fruit (Fig. 2.5c).
2.2.2.2. Pomes
There are few problems in reducing the size of fresh ‘‘hard’’ apples or pears. After washing,
pome fruits are milled. The fruit to be milled is continuously fed into the milling device. For
the disintegration fixed positioned or rotating grinding knives may be used. Depending on the
product quality different types of knives need to be selected. The types of fruit mills generally
used are:
Figure 2.5. FMC citrus juice extractor (with permission).
2
.
Processing of Fruits
31
Figure 2.6. Fruit grinding mill.
.
.
.
Fruit grinding mill: The milling tool is a rotating disk with radially arranged grinding
knives. The speed of the disk is variable, permitting to produce the required particle size
(Fig. 2.6).
Rasp or grater mill: It consists of a revolving metal cylinder with adjustable toothed blades,
which rotate toward a set of parallel metal knifes or plates.
Fixed blade hammer mills: The rotor with fixed blades rotates within a perforated screen.
Hammer mills may be horizontal, sloping, or vertical shaft mounting (Fig. 2.7).
Mills must not produce too much fines as these will contribute to pressing and later high pulp
content in the juice. The particles should all be about the same size. Grater mills are found to
be more efficient with firm fruits, while hammer mills are more suited for mature or softer
fruits, provided speed is properly adjusted.
Fruit
Rotating
hammer
Mesh
Pulp
Figure 2.7. Hammer mill.
32
Fruit Manufacturing
2.2.2.3. Pressing
Most systems for extracting juices from apples and similar fruit pulps use some method of
pressing juice through cloth of various thicknesses, in which pomace is retained. These
systems, called filter presses, include (Lozano, 2003): (i) rack and cloth press, (ii) horizontal
pack press, (iii) continuous belt press, and (iv) screw press.
(1) In a rack and cloth press the milled fruit pulp is placed in a nylon, Dacron, or polypropylene cloth to form a ‘‘cheese,’’ with the help of a cheese form. Layers of up to 10-cm
thick pulp cheeses, separated by racks made of hardwood or plastic, are stacked up to 1 m
or more in height depending on maturity of the fruit and size of racks (Fig. 2.8).
Rack and cloth presses are efficient but very labor intensive as they require operation,
cleaning, and repairing. Maximum yield may be obtained by use of a series of two to three
pressure heads located around a central pivot, using pressures up to 200 atm.
(2) Cage presses are horizontal presses with enclosed cages of several cubic meters in which
pressing takes place. Pomace is pumped into the cage without contact with air, thereby
reducing oxidation (Fig. 2.9a). The cage is filled with a complex filter systems consisting
of grooved flexible rods filled with sleeves of press cloth material.
During the pressing step (Fig. 2.9b), the juice passes from the pulp, through press cloth
sleeves, along grooves in the flexible rods, and out to collecting channels at the ends of the
cage and the piston.
The drum may be rotated, thereby breaking up the pulp and adding more water. This
permits a second pressing with more juice extraction. The whole process may be automated.
Although some cleaning labor is saved, rods and sleeves require a considerable amount of
Press
Pulp
cheese
Racks
Expressed juice
Figure 2.8. Sketch of a rack and cloth press. Reprinted from the Encyclopedia of Food Science and Nutrition,
Lozano J.E., Separation and Clarification, pp. 5187–5196 (copyright) 2003, with permission from Elsevier.
2
.
Processing of Fruits
33
Pulp in
Juice
out
Figure 2.9. Hydraulic press: (a) loading, (b) pressing.
maintenance. These presses may slow down the operating cycle for production of stable
cloudy nonoxidized juice.
(3) Continuous belt press: Based on the Ensink design for paper pulp pressing this type of
presses offers a truly continuous operation (Fig. 2.10). In belt presses, a layer of mash
(pulp) is pumped onto the belt entering the machine. The press aid may be added for
improved yield.
(4) Screw presses: A typical screw press consists of a stainless steel cylindrical screen,
enclosing a large bore screw with narrow clearance between screw and cylinder. Adjustable back-pressure is usually provided at the end of the chamber. Breaker bars must be
incorporated to disrupt the compressing mash. Capacity for screw presses of 41-cm
diameter is up to 15,000 kg/h (Bump, 1989).
Pulp
Mesh
Bagasse
Juice
Figure 2.10. Sketch of a typical fruit belt press.
34
Fruit Manufacturing
2.2.2.4. Other Extraction Methods
.
.
Centrifugation: Both cone and basket centrifuges have been used in producing fruit
juice. Both systems have resulted in high levels of suspended solids and a high
investment cost for a given yield. Horizontal decanters are presently used for juice
clarification.
Diffusion extraction: This was adapted from the method used for the extraction of
sugar from sugar beet. Extraction is a typical countercurrent-type process. It is desirable to retain the same driving force DC (concentration of soluble components in solids
versus concentration of soluble components in liquids). In order to maintain a constant
DC throughout the extraction process, it is necessary to carry out a continuous
weighing of ingoing apple slice and control the water flow to the counterflow extractor,
by means of a relatively simple control loop (Fig. 2.11).
The diffusion extraction process is influenced by a number of variables, including temperature, thickness, water, and fruit variety. Slices from extractors pass through a conventional
press system, and the very dilute juice is returned to the extractors. It is seen that the extra
juice yield from diffusion extraction compensates the extra energy cost involved for concentration.
.
Addition of press aids: Hydraulic pressing does not usually require addition of press
aids, unless exceptionally overmature fruit is used. For continuous screw presses
however it is usually necessary to add 1% (w/w) or more of cellulose. Mixing of
cellulose and fruit occurs in the mill and subsequent pumping to press. Pumping is
commonly performed with a Moyno-type moving cavity food pump.
Warm
water
inlet
Fruit
pulp out
Fruit
pulp in
Juice
Figure 2.11. Sketch of fruit juice diffusion extraction process.
2
.
Processing of Fruits
35
2.2.3. Clarification and Fining
The conventional route to concentration is to strip aroma, then depectinize juice with
enzymes, centrifuge to remove heavy sediments and filter through pressure precoat filters
and polish filters (Figure 2.2a).
The juice is then usually concentrated through a multistage vacuum concentrator. This
process involves a slight decrease in concentration of juice during the stripping step (usually
up to 10% volume is removed). Stripping usually precedes depectinization, as pectin methyl
esterase releases significant quantities of methanol, which spoils the essence. The use of
enzymes for clarification is described later in this chapter. When a more concentrated juice
is clarified (ffi20 8Brix) the volume to handle is reduced practically in a half. However,
viscosity increased with concentration, which may slow flocculation and filtration. If a cloudy
product is required, the juice is pasteurized immediately after pressing to denature any
residual enzymes. Centrifugation then removes large pieces of debris, leaving most of the
small particles in suspension.
2.2.3.1. Partial Concentrates
Fruit juices, both clarified or opalescent, may be concentrated up to 4 fold (ffi50 8Brix) with
natural pectin gelling with little effort. At this point in the concentration process little heat
damage is detected. This concentrate can be canned and frozen. For clear juice these suspended
particles have to be removed (McLellan, 1996). It may seem simple merely to filter them out,
but unfortunately some soluble pectin remains in the juice, making it too viscous to filter
quickly. A dose of commercial enzyme is the accepted way of removing unwanted pectin.
Depectinization has two effects: it degrades the viscous soluble pectins and it also causes
the aggregation of cloudy particles. Pectin forms a protective coat around proteins in
suspension. In an acidic environment (apple juice typically has a pH of 3.5) pectin molecules
carry a negative charge. This causes them to repel one another. Pectinolytic enzymes degrade
pectin and expose part of the positively charged protein beneath. As the electrostatic repulsion between cloudy particles is reduced, they clump together. These larger particles will
eventually settle, but to improve the process flocculating agents (fining) such as gelatin,
tannin, or bentonite (a type of clay) can be added. Some fining agents adsorb the enzyme
onto their surface, so it is important not to add them before the enzyme has done its job.
Fining agents (Table 2.5) work either by sticking to particles, thereby making them heavy
enough to sink; or by using charged ions to cause particles to stick to each other, thereby
making them settle to the bottom. Although this method of conventional clarification was
widely used in the clarified juice industry, this technology has been practically replaced by
mechanical processes such as ultrafiltration and centrifugal decanters.
Yeasts and other microbes, which may have contaminated the juice, may also be
precipitated by fining. What is left is a transparent, but by no means, clear juice. A second
centrifugation and a subsequent filtration are needed to produce the clear juice that many
consumers prefer.
Another potential contributor to the haziness of juice is starch. This is particularly so if
unripe apples have been used. Unripe apples may contain up to 15% starch. Although the first
centrifugation—before the juice reaches the clarification tank—removes most of the starch,
about 5% usually remains. This can be broken down using an amylase (amyloglucosidase)
active at the pH of apple juice, added at the same time as the pectinase.
36
Fruit Manufacturing
Table 2.5. Fruit juice clarification agents.
Name
Description
Sparkolloid
Sparkolloid is a natural albuminous protein extracted from kelp and sold as a
very fine powder
It is in general a mixture of gelatins and silicon dioxide, with animal collagen being the
active ingredient
Kieselsol is a liquid in which small silica particles have been suspended. It is usually used in
tandem with gelatin. The dosage is 1 ml/g of gelatins. This fining aids in pulling proteins
out of suspension
Bentonite is sold as a powder and as course granules. It is refined clay. A better way is to
add the same amount to a liter of hot water, stir well, and let stand for 36–48 h. In this
time the clay swells and becomes almost a gelatin
Produced from sturgeon swim bladders, isinglass is sold either as a fine white powder or
as dry hard fragments. It is a protein extracted from the bladders of these fish. This
product is also available as a prepared liquid called ‘‘super-clear’’
With a fine porosity pad, filters are very effective in removing particles
(yeast cells, proteins, etc.)
Almost all fruits contain pectin, some more than others. When added as directed, it
eliminates pectin haze. There is no other way to prevent this condition, and if it is in a
juice, the haze will never clear on its own
Gelatin
Kieselsol
Bentonite
Isinglass
Filters or Polishers
Pectic Enzymes
For juice processing both depectinization and destarching are essential. This is because
most apple juice is concentrated by evaporating up to 75% of the water content before
storage. This makes the juice easier to transport and store, and the concentrate’s high sugar
content acts as a natural preservative.
Unfortunately, heat treatment also drives off the juice’s pleasant aroma, so it is necessary
to gently heat the juice and collect the volatile smell and flavor compounds, so that they can
be put back again when the juice is reconstituted. Heating can cause residual pectin or starch
in the juice to gel or form a haze, hence the necessity of enzyme treatment. Increased haze
formation occurs when fining with gelatin and bentonite is not performed.
Optimization of fining and ultrafiltration steps can help retard or prevent postbottling
haze development.
2.2.4. Use of Enzymes in the Fruit Industry
Commercial pectic enzymes (pectinases) and other enzymes are now an integral part of fruit
juice technology (Grampp, 1976). They are used to help extract, clarify, and modify juices
from many fruits, including berries, stone and citrus fruits, grapes, apples, pears, and even
vegetables. When a cloudy juice or nectar is preferred (for example, with oranges, pineapples,
or apricots) there is no need to clarify the liquid, and enzymes are used to enhance extraction
or perform other modifications. The available commercial pectinase preparations used in fruit
processing generally contain a mixture of pectinesterase (PE), polygalacturonase (PG), and
pectinlyase (PL) enzymes (Dietrich et al., 1991). Enzymatic juice extraction from apples was
introduced 25 years ago, and now some 3–5 million tons of apples are processed into juice
annually throughout the world. The methods employed for apple juice are generally the same
as those for other fruits (Table 2.6).
As previously mentioned, after fruits like apples have been washed and sorted, they are
crushed in a mill. Peels and cores from apple slice or sauce production may also be used
2
Processing of Fruits
.
37
Table 2.6. Application of pectolytic enzymes to fruit and vegetable processing.
Enzymatic process
Examples of application
Clarification of fruit juices
Apple juice, depectinized juices can also be concentrated without gelling
and developing turbidity
Soft fruits, red grapes, citrus, and apples; for better release of juice
(and colored material); enzyme treatment of pulp of olives, palm fruit,
and coconut flesh to increase oil yield
Used to obtain nectar bases and in baby foods
Enzyme treatment of pulp
Maceration of fruit and vegetables
(disintegration by cell separation)
Liquefaction of fruit and vegetables
Special applications to citrus fruits
Used to obtain products with increased soluble solids’ content
(pectinases and cellulases combined)
Used for the preparation of clouding agents from citrus peel, cleaning of
peels before use in candy and marmalade production, recovery of oil
from citrus peel, depectinization of citrus pulp wash
(Source: Rombouts and Pilnik, 1978).
together with whole apples. Although pectinases are often added at this stage, better results
are achieved if the apple pulp is first stirred in a holding tank for 15–20 minutes so that
enzyme inhibitors (polyphenols) are oxidized (by naturally occurring polyphenols oxidized in
the fruit). The pulp is then heated to an appropriate temperature before enzyme treatment.
For apples 308C is the optimal temperature, whereas stone fruits and berries generally require
higher temperatures—around 508C. This compares with 60–658C required if pectinase is not
used (here the juice is liberated by plasmolysis of the plant cells).
Prepress treatment with pectinases takes anything from 15 min to 2 h depending upon
the exact nature of the enzyme and how much is used, the reaction temperature, and the
variety of apple chosen. Some varieties such as Golden Delicious are notoriously difficult to
breakdown. During incubation the pectinase degrades soluble pectin in the pulp, making
the juice flow more freely. The enzyme also helps to breakdown insoluble pectin, which
impede juice extraction.
Enzyme treatment is considered to be complete once the viscosity of the juice has
returned to its original level or less. It is important that the pulp is not broken down too
much as it would then be difficult to press.
Pressing is done using the previously described equipment. Juice yields can be increased
by up to 20%, depending upon the age and variety of fruit used and whether preoxidation was
employed. Enzyme treatment is particularly effective with mature apples and those from cold
storage.
2.2.4.1. Other Enzymes in Juice Production
.
.
Cellulases: The addition of cellulases during extraction at 508C improves the release of
color compounds from the skins of fruit. This is particularly useful for treating
blackcurrants and red grapes. Increasingly cellulases are being used at the time of the
initial pectinase addition to totally liquefy the plant tissue. This makes it possible to
filter juice straight from the pulp without any need for pressing.
Arabanase: The polysaccharide araban (a polymer of the pentose arabinose) may
appear as a haze in fruit juice a few weeks after it has been concentrated. Although
commercial pectinase preparations often contain arabanase, certain fruits (like pears)
38
Fruit Manufacturing
.
are rich in araban and may require the addition of extra arabanase to the clarification
tank.
Glucose oxidase (from the fungi Aspergillus niger or Penicillium spp.): Catalyzes the
breakdown of glucose to produce gluconic acid and hydrogen peroxide. This reaction
utilizes molecular oxygen. Glucose oxidase (coupled with catalase to remove the
hydrogen peroxide) is therefore used to remove the oxygen from the head space
above bottled drinks, thereby reducing the nonenzymatic browning due to oxidation,
which might otherwise occur.
2.2.4.2. Pectinase Activity Determination
Complete pectin breakdown in apple juices can be ensured only if all the three types of
pectinolytic enzymes (PG, PE, PL) are present in the correct proportion. Apple juice processor in general lacks reliable methods for checking the different enzyme activities. Application
and success of a pectinase product also depend on the substrate where they act. The problems
in evaluation of pectinolytic activities are caused by the difficulty in standardizing fruit
substrate. Acidity, pH, and the presence of inhibitors or promoters of the enzymatic reaction
depend upon the variety of apple processed.
Figure 2.12 shows the residual polygalacturonase activity of two commercial enzymes
after 30 min of heating at different temperatures. They start to become inactivated at
temperatures higher than 508C, which is a very well defined breaking point, if the enzyme
1 PG-PECTINOL A1
2 PG-R HAPECT D5S
3 PL-R HAPECT D5S
100
Relative Residual Activities (%)
80
60
2
1
3
40
20
0
40
50
60
70
Temperature (⬚C)
Figure 2.12. Enzymatic residual activities after thermal treatment 30 min at different temperatures) of enzyme
solutions in 0.1 M citrate/0.2 M phosphate buffers at optimum pH). Reprinted from Food Chem. 31(½),
Ceci, L. and Lozano, J.E. Determination of enzymatic activities of commercial pectinases, 237–241.
(copyright) 2003, with permission from Elsevier.
2
.
Processing of Fruits
39
were to rapidly loses its activity. The rate of PG activity decrease could be divided into two
periods (Fig. 2.12). The first period was characterized as a thermolabile fraction. The second
can be defined as the thermoresistant fraction of the enzyme. Sakai et al. (1993) and Liu and
Luh (1978) reported that the optimal temperature for PG activity was in the range 30–508C.
Both authors indicated that for temperatures greater than 508C inactivation was notable
after a short period of heating. Moreover, the optimal temperature is also a function of the
type of substrate to be treated (Ben-Shalom et al., 1986).
Inactivation curve of lyase activity (PL) is also shown in Fig. 2.12. As in the case of PG
activities, 508C can be easily identified as the breaking point where PL rapidly inactivates.
Alkorta et al. (1996) found that PL from Penicillum italicum was active after 1 h at 508C but
resulted in complete inactivation for the same period at 608C. The commercial enzymes
proved to be more heat tolerant than purified fractions (Liu and Luh 1978). This phenomenon was attributable to the heat protective action of impurities.
2.2.4.3. pH Dependence on the Pectic Enzymes Activities
Figure 2.13 shows the behavior of PG and PE activities of RHD5 enzyme versus pH. The
resulting optimum pH was approximately 4.6. However, the curve for lyase activity as a
RÖHAPECT D5S
100
RELATIVE ACTIVITIES (%)
80
60
PL
PG
40
PE
20
0
4.0
6.0
pH
Figure 2.13. Effects of pH on the enzymatic activities of Röhapect D5S (Ceci and Lozano, 1998). Reprinted
from Food Chem. 31(½), Ceci, L. and Lozano, J.E. Determination of enzymatic activities of commercial
pectinases, 237–241. (copyright) 2003, with permission from Elsevier.
40
Fruit Manufacturing
function of pH was much broader, and it was difficult to identify a single optimal value. In
this case, an optimal range of pH 5–6 may be defined.
As a result, as much as 40% of PG and PE inactivation can be expected during the
enzymatic clarification of the relatively acidic Granny Smith juice. It was found that a shift in
the optimal pH toward the acid zone (Spagna et al., 1993) or a broadening of the optimal
activities’ range (Ates and Pekyardimci, 1995) can be obtained after enzyme immobilization
on appropriate supports.
In general pectinolytic enzymes (PG and PE) show a rapid decrease in activity at about
pH 5 and become practically inactivated near neutrality. However, this problem becomes
irrelevant because pH values of fruit juices are lower than pH 5. It was found that a shift in
the optimal pH toward the acid zone (Spagna et al., 1993) or a broadening of the optimal
activities’ range (Ates and Pekyardimci, 1995) can be obtained after enzyme immobilization
on appropriate supports
As fruit juice clarification is usually done at 45–50C, special care must be taken to avoid
excessive inactivation when lyase activity is considered important. It is known (Dietrich et al.,
1991) that commercial enzyme preparations cause a certain degree of side activities other than
those required (Ceci and Lozano, 1998).
2.2.4.4. Enzymatic Hydrolysis of Starch in Fruit Juices
Starch can be a problem for juice processors. Polymeric carbohydrates like starch
and arabans can be difficult to filter and cause postprocess cloudiness. In the case of a
positive starch test, the following problems may occur: slow filtration, membrane fouling,
gelling after concentration, and postconcentration haze. Apple juice is one of the juices
that can contain considerable amounts of starch, particularly at the beginning of the
season. Unripe apples contain as much as 15% starch (Reed, 1975). As the apple matures
on the tree, the starch hydrolyzes into sugars. The starch content of apple juice may be high in
years when there were relatively low temperatures during the growing season. Besides
the generalized application of commercial amylase enzymes in the juice industry, there is a
lack of information on apple starch characteristics and extent of gelatinization during juice
pasteurization.
Starch must be degraded by adding starch-splitting enzymes, together with the pectinase during depectinization of the juice. First starch must be gelatinized, by heating the
juice to 778C. When an aqueous suspension of starch is heated the hydrogen bonds weaken;
water is absorbed; and the starch granules swell, rupture, and gelatinize (Zobel, 1984). The
juice must then be cooled to <508C to avoid enzyme inactivation. Starch is generally
insoluble in water at room temperature. Because of this, it is stored in cells as small
granules.
Starch granules (Fig. 2.14) are quite resistant to penetration by both water and hydrolytic enzymes due to the formation of hydrogen bonds within the same molecule and with
other neighboring molecules. When the starch granule is not broken down completely, a
short-chained dextrin is left. This can lead to a condition known as retrograding. When starch
retrogrades, the short-chained dextrin recrystallizes into a form that is no longer susceptible
to enzyme attack, regardless of heating. Figure 2.15 shows a SEM photomicrograph of haze
sediment obtained from a pasteurized apple juice sample.
2
.
Processing of Fruits
41
Figure 2.14. Scanning electron photomicrograph of an isolated apple starch granule (5 kV 4,400).
The scanning electron micrograph shows how the apple starch granules collapsed after
heat treatment, while dispersion of gel-like starch fragments among the other components of
turbidity (pectin, cellular wall, etc.) can be observed. Similar behavior was found when wheat
starch was gelatinized by heat in excess water (Lineback and Wongsrikasen, 1980).
Figure 2.15. Scanning electron photomicrograph of cloudiness precipitated from a pasteurized (5 min at 908 + 1 C)
apple juice (5 kV 3,600).
42
Fruit Manufacturing
2.2.5. Filtration
Filtration is also a mechanical process designed for clarification by removing insoluble solids
from a high-value liquid food, by the passage of most of the fluid through a porous barrier,
which retains most of the solid particulates contained in the food. Filtration is performed using
a filter medium, which can be a screen, cloth, paper, or bed of solids. Filter acts as a barrier that
lets the liquid pass while most of the solids are retained. The liquid that passes through the filter
medium is called the filtrate. There are several filtration methods and filters (Lozano, 2003)
including:
.
.
.
Driving force: The filtrate is induced to flow through the filter medium by: (a) hydrostatic head (gravity), (b) pressure (upstream filter medium), (c) vacuum (downstream
filter medium), and (d) centrifugal force across the medium.
Filtration mechanism: (a) cake filtration (solids are retained at the surface of a filter
medium and pile upon one another). (b) depth or clarifying filtration (solids are
trapped within the pores or body of the filter medium).
Operating cycle: (a) Intermittent (batch) and (b) Continuous rate.
These methods of classification are not mutually exclusive. Thus filters usually are divided
first into the two groups of cake and clarifying equipment, then into groups of machines using
the same kind of driving force, then further into batch and continuous classes. Some filtering
devices usually employed in the food industry are described here.
2.2.5.1. Pressure Filters
The main advantages of pressure filtration compared to other filtering methods are: Cakes are
obtained with very low moisture content, clean filtrates may be produced by recirculating the
filtrate or by precoating, and the solution can be polished (finished) to a high degree of clarity.
Among disadvantages it must be noted that cloth washing is difficult, and if precoat is
required, the operator cannot see the forming cake and is unable to carry out an inspection
while the filter is in operation, and the internals are difficult to clean, which can be a problem
with food-grade applications.
With the exception of the rotary drum pressure filter, this type of filters has a semicontinuous machine in which wash and cake discharge are performed at the end of the filtration cycle.
Since the operation is in batches, intermediary tanks are required. The collection of filtrate
depends on the operating mode of the filter, which can be at constant flow rate, constant pressure,
or both, with pressure rising and flow rate reducing during filtration. The filtration rate is mainly
influenced by the properties of food (particle size and distribution, presence of gelatinous solids
like pectin, liquid viscosity, etc.) Although continuous pressure filters are available, they are
mechanically complex and expensive, so they are not common in the food industry.
2.2.5.2. Filter Aid and Precoating
Filter aid and precoating are often used in pressing and in connection with pressure
filtration. Filter aid is used when the pulp or turbid liquid food is low in solids’ content
with fine and muddy particles that are difficult to filter. To enhance filtration coarse solids
with large surface area that capture and trap the slow-filtering particles from the suspension
in its interstices and produces a porous cake matrix are used.
2
Processing of Fruits
.
43
On the other hand, precoating is the formation of a defined thick medium of a known
permeability on the filter plates. Precoating prior to filtration serves when the particles that
are to be separated are gelatinous and sticky, forming a barrier that avoids cloth blinding. The
most common filter aids and precoating materials employed in the food industry are:
diatomaceous earth (silicaceous skeletal remains of aquatic unicellular plants), perlite (glassy
crushed and heat-expanded volcanic rock), cellulose (fibrous light weight and ashless paper),
and special groundwood.
2.2.5.3. Types of Pressure Filters
Pressure filters usually found in the food industry are:
.
.
Filterpress, also called Plate and Frame, consists of a head and a follower that contain
in between a pack of vertical rectangular plates that are supported by side or overhead
beams (Fig. 2.16). The head serves as a fixed end to which the feed and filtrate pipes are
connected, while the follower moves along the beams and presses the plates together
during the filtration cycle by a hydraulic or mechanical mechanism. Each plate is
covered with filter cloth on both sides and, once pressed together, they form a series
of chambers that depend on the number of plates. The plates have generally a centered
feed port that passes through the entire length of the filter press, so that all chambers of
the plate pack are connected together.
Vertical and horizontal pressure leaf filters consist of a vessel that is fitted with a stack
of vertical (Fig. 2.17), or horizontal leaves that serve as filter elements. The leaf is
constructed with ribs on both sides to allow free flow of filtrate toward the neck and is
covered with coarse mesh screens that support the finer woven metal screens or filter
cloth that retain the cake. The space between the leaves varies from 30 to 100 mm
depending on the cake formation properties and the ability of the vacuum to hold a
thick and heavy cake to the leaf surface. The filters can be used for polishing fruit juices
Filtrate
out
Feed
Filtrate
Feed
Filtrate
Figure 2.16. Filterpress operation sketch and details of filtering plate. Reprinted from the Encyclopedia of Food
Science and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196. (copyright) 2003, with permission
from Elsevier.
44
Fruit Manufacturing
Feed in
Plate
Filtrate
Figure 2.17. Horizontal plate filter. Reprinted from the Encyclopedia of Food Science and Nutrition, Lozano J.E.,
Separation and Clarification, pp. 5187–5196. (copyright) 2003, with permission from Elsevier.
.
with very low solids or for cake filtration with a solids’ concentration of <20–25%. The
cloth mesh screens that cover the leaves can be more easily accessed on horizontal
tanks than on vertical tanks.
Candle filters are used in applications that require efficient low moisture cake filtration
or high degree of polishing (finishing). Candle filters can contain more than 250
filtering elements. They consist basically of three major components (Fig. 2.18):
(1) The vessel;
(2) The filtering elements (candles) and
(3) The cake discharge outlet.
The vessel configuration has a conical bottom for cake filtration and polishing, or has a
dished bottom for slurry thickening, though it is scarcely used in the food industry.
The filtering element generally consists of the filtrate core and the filtering medium. The
core helps in filtrate passage and supports the filter medium. The core is a bundle of
Filtrate
Bunch of
candles
Vessel
Figure 2.18. Candle filter sketch showing major components. Reprinted from the Encyclopedia of Food Science
and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196. (copyright) 2003, with permission
from Elsevier.
2
Processing of Fruits
.
45
perforated stainless steel tubes. The filter medium can be a porous ceramic, woven mesh
screen, sintered metal tube, or synthetic filter cloth. The advantages of a candle filter are the
excellent cake discharge capacity and its mechanical simplicity.
2.2.5.4. Vacuum Filters
Vacuum filters are simple and reliable machines widely used in the fruit industry. Among the
different types of vacuum filter (drum, disk, horizontal belt, tilting pan, and table filters)
drum filters are most commonly utilized in the food industry. The advantages and disadvantages of vacuum filtration compared to other separation methods are:
.
.
Advantages: Continuous operation, very effective polishing (finishing) of solutions (on
a precoat filter), and easy control of operating parameters such as cake thickness.
Disadvantages: Higher residual moisture in the cake and difficulty in cleaning (as
required mainly for food-grade applications).
Precoat filters are used when liquid foods (e.g., clarified apple juice) require a very high degree
of clarity. To polish the solution the drum deck is precoated with an appropriate medium (See
Filter aids and precoating in this chapter). A scraper blade also called ‘‘Doctor Blade,’’ moves
slowly toward the drum and shaves off a thin layer of the separated solids and precoating
material. This movement exposes continuously a fresh layer of the precoat surface, so that
when the drum submerges into the tank it is ready to polish the solution. Precoat filters are
used to recover juice from the sediments originating in clarification tanks. In precoat filters
the entire drum deck is subjected to vacuum (Fig. 2.19).
2.2.6. Membrane Filtration
Filtration of coarse particles down to several microns is done by conventional dead-end
filtration, where all influent passes through a filter medium that removes contaminants to
produce higher-quality clarified juices. Rough screens, sand filters, multimedia filters, bag
filters, and cartridge filters are examples of filtration products that remove 0.1-micron size
particles or larger. Once the medium becomes loaded, it can be backwashed as with multimedia filters, or discarded and replaced as with cartridge filters. The method of obtaining
clean filtration medium is based on economic and disposal concerns. Particles retained by the
filter in dead-end filtration build up with time as a cake layer, which results in increased
resistance to filtration. This requires frequent cleaning or replacement of filters.
Doctor blade
Vacuum
pump
Receiver
Figure 2.19. Vacuum drum precoat filter.
Feed
46
Fruit Manufacturing
Commercially available coarse filtration devices are effective in separating particles
down to about 20 mm. On the other hand, membrane technology involves the separation of
particles below this range, extending down to dissolved solutes that are as small as several
Angstroms.
Membranes are manufactured with a wide variety of materials, including sintered metals,
ceramics, and polymers (Zeman and Zydney, 1996). In order to reject substances smaller than
0:1 mm using polymeric membrane is by far the most popular filtration method in the fruit
processing industry. Millions of small pores per unit area of membrane allow water and lowmolecular weight substances to pass through it while undesired substances are retained on the
influent side. The problem is solved by operating polymeric membranes in the crossflow
mode. In crossflow membrane filtration, two effluent streams are produced, the permeate
and the concentrate. The permeate is the purified fluid that has passed through the semipermeable membrane. The remaining fluid is the concentrate, which has become enriched
with organics and salts that could not permeate the membrane. By doing so, rejected
contaminants are continuously carried away from the membrane surface, thereby minimizing
contaminant build up, leaving it free to reject incoming material and allow free flow of
purified liquid. The size of the polymer’s pores categorizes the membrane into one of the
following groups: reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. Reverse
osmosis (RO) membranes have the smallest pore size ranging from 5 to 15 angstroms,
nanofiltration (NF) covers separations in the 15–30 angstrom size, ultrafiltration (UF)
removes organics in the 0:002---0:2 mm range, while microfiltration (MF) effects separation
typically in the range of 0:1---10 mm.
Figure 2.20 schematically shows the filtering capacity of these ‘‘crossflow’’ membrane
systems. RO is both a mechanical and chemical filtration procedure by which the membrane’s
surface sieves organic substance and actually repels ions. The dielectric repulsion of ions from
the membrane is influenced by the ion’s charge density. Unlike RO membranes that have salt
retentions of 80–99%, NF membranes reject 30–60% salts. UF and MF membranes have even
larger pores and therefore pass most of the salts. Although membrane cleaning is periodically
Suspended
particles
Macromolecules
Water
Ions
(multivalent)
U
F
N
R
Ions
F (monovalent) O
Figure 2.20. Crossflow-type membrane classification by ‘‘rejection’’ capacity.
2
Processing of Fruits
.
47
required, the self-cleaning nature of crossflow filtration lengthens membrane life enough to
make it economically attractive.
The manufacturing processes result in a number of different membrane structures such
as microporous, asymmetric, composite, etc. Membranes are assembled as modules that are
easily integrated into systems containing hydraulic components. The module allows to
accommodate large filtration areas in a small volume and resist the pressures required in
filtration. Tubular, hollow fiber, spiral, and flat plate are the common modules (Cheryan,
1986).
.
.
Tubular module consists of tubular membranes held inside individual perforated support tubes, assembled onto common headers and permeate into the container to form a
module. When several channels are formed in a porous block of material, the tubular
system is called ‘‘monolithic.’’
Hollow fiber module consists of bundles of hollow fibers (0.5–3 mm internal diameter)
sealed into plastic headers and assembled in permeate casings (Fig. 2.21). Hollow fibers
Figure 2.21. Hollow fiber membrane configuration. (a) Manifold with HF cartridge; (b) SEM micrography of a single
fiber (internal diameter 1 mm); (c) SEM magnification of a single fiber, showing filtration surface and support.
48
Fruit Manufacturing
.
.
are used in low-pressure applications only. These can accommodate moderate levels of
suspended particles.
Spiral modules are made by placing a woven plastic mesh, which acts as the permeate
channel between two membrane layers and seals three sides. The fourth side of this
sandwich is attached to the permeate tube. Another plastic mesh that acts as the feed
channel is laid over it and the assembly is wrapped around the central permeate tube.
Flat-platemodules use multiple flat sheet membranes in a sandwich arrangement consisting of a support plate, membrane, and channel separator. The membranes are
sealed to the plates using gaskets and hydraulically clamped to form a tight fit. Several
of these membranes are stacked together and clamped to form a complete module. The
main advantages of the flat-plate module design are that they have high membranepacking densities and low hold-up volumes. This is due to the small channel height of
the flat-plate modules. The main application for the flat-plate module is in recovering
biological products. The advantages and disadvantages of the different UF configurations (Cheryan, 1986) are listed in Table 2.7.
Over time, the physical backwash will not remove some membrane fouling. Most
membrane systems allow the feed pressure to gradually increase over time to around 30 psi
and then perform a clean-in-place (CIP). CIP frequency might vary from around 10 days to
several months. Another approach CIP practice is to use a chemically enhanced backwash
(CEB), where on a frequent basis (typically every 1–14 days), chemicals are injected with the
backwash water to clean the membrane and maintain system performance at low pressure
without going offline for a CIP.
The application of ultrafiltration (UF) as an alternative to conventional processes for
clarification of apple juice was clearly demonstrated (Heatherbell et al., 1977; Short, 1983;
Wu et al., 1990). However, the acceptance of UF in the fruit processing industry is not yet
complete, because there are problems with the operation and fouling of membranes. During
UF two fluid streams are generated: the ultrafiltered solids’ free juice (permeate), and the
retentate with variable content of insoluble solids, which, in the case of apple juice, are mainly
remains of cellular walls and pectin.
Permeate flux (J) results from the difference between a convective flux from the bulk of
the juice to the membrane and a counterdiffusive flux or outflow by which the solute is
transferred back into the bulk of the fluid. The value of J is strongly dependent on hydrodynamical conditions, membrane properties, and the operating parameters. The main driving
force of UF is the transmembrane pressure (DPTM), which in the case of hollow fiber
ultrafiltration systems (HFUF) can be defined as:
DPTM ¼
(Pi þ Po )
Pext
2
(2:1)
Table 2.7. Advantages and disadvantages of the different UF configurations.
UF MEMBRANE CONFIGURATION
3
Pack density (m2 =m )
Fouling resistance
Cleaning facility
Relative cost
Flat/press
Spiral
Tubular
Hollow fibers
300–500
Good
Good
High
200–800
Moderate
Fair
Low
30–200
Very good
Very good
High
500–9000
Poor
Poor
Low
Source: Cheryan, 1986; Zeman and Zydney, 1996.
2
.
Processing of Fruits
49
where Pi is the pressure at the inlet of the fiber, Po the outlet pressure, and Pext the pressure on
the permeate side. In practice, the J values obtained with apple juice are much less than those
obtained with water only. This phenomenon is attributable to various causes, including
resistance of gel layer, concentration in polarization boundary layer (defined as a localized
increase in concentration of rejected solutes at the membrane surface due to convective
transport of solutes (Porter, 1972)), and plugging of pores due to fouling, where some of
these phenomena are reversible and disappear after cleaning of the UF membranes while
others are definitively irreversible.
2.2.6.1. Stationary Permeate Flux
It is well known (Iritani et al., 1991) that the transmembrane pressure-permeate flux characteristic for ultrafiltration shows a linear dependence of J with DPTM at lower values of
pressure (1st region), while the permeate flux approaches a limiting value (Jlim ) independent of
further increase in pressure at higher pressures (2nd region). The last situation was assumed to
be controlled by mass transfer.
Figures 2.22 and 2.23 show the variation of J with DPTM as a function of VCR or
volume concentration ratio (defined as the initial volume divided by retentate volume at any
time), which is a measurement of the retentate concentration, and recirculating flow rate, Qr,
respectively (Constenla and Lozano, 1996). Pressure independence (2nd region) was observed
to occur at a higher pressure at higher Qr. The point at which the pressure independence is
evident is called optimum transmembrane pressure (DPTMo).
80.00
T = 50ⴗC, PC = 50,000 ds, VCR = 1
J,L/hm2
60.00
40.00
Qr = 10 L/min
Qr = 12.5 L/min
20.00
Qr = 15 L/min
0.00
0.00
0.40
0.80
∆PTM,
1.20
1.60
Kg/cm2
Figure 2.22. Effect of DPTM and Qr on J at 508C. (Constenla and Lozano, 1996) with permission.
50
Fruit Manufacturing
80.00
T = 50ⴗC, PC = 50,000 ds, Qr = 10 L/min
J,L/hm2
60.00
40.00
VCR = 1
20.00
VCR = 2
VCR = 5
0.00
0.00
0.40
0.80
1.20
1.60
DPTM, Kg/cm2
Figure 2.23. Effect of DPTM and VCR on J at 508C (Constenla and Lozano, 1996). Reprinted from Lebensm.
Wiss. u. Technol. 27: 7–14, Constenla, D.T. Lozano, J.E., Predicting stationary permeate flux in the
ultrafilteration of apple juice. (copyright) 1996, with permission from Elsevier.
The reduction of Jlim with Qr may be associated with a reduction in the boundary layer
due to an increase in the turbulence. On the other hand, the optimal DPTM values were
practically independent of VCR at Qr > 10 L/min. A hysteresis effect in the permeate flux,
attributable to the consolidation of the gel layer (Omosaiye et al., 1978), has been observed.
The area enclosed by the hysteresis loop was greater at lower Qr and VCR values. Traditionally, correlations of J with DPTM and VCR were determined by parameter fitting of the
experimental data. Since the polynomial functions have no physical basis, a large number
of experimental data are needed for determination of J. Therefore other theoretical and
semiempirical approaches should be considered (Constenla and Lozano, 1986).
2.2.6.2. Permeate Flux as a Function of Time
Several models are proposed in the literature for representing J, most of them being semiempirical and practical equations (Table 2.8). Membrane fouling mechanisms may be studied
through the classical laws of filtration under constant pressure (Table 2.9). During UF
process (Iritani et al., 1991) J behaves as in cake filtration only at the very beginning,
attributable to the formation of the gel layer with minor counterdiffusion flux.
2
.
Processing of Fruits
51
Table 2.8. Some equations representing
permeate flux as a function of time.
No.
Permeate flux equations
(i)
(ii)
(iii)
(iv)
(v)
J ¼ J0 exp ( Bt)
J ¼ JF þ (J0 JF ) exp ( At)
J ¼ (J02 þ 2K t)1=2
J ¼ J0 B ln(VCR)
J JF ¼ (J0 JF )(1 exp ( ( t=B) )
Subindex: 0, 1, F are zero, initial, and final time, respectively;
Vp ¼ permeate volume; A, B, and K are constants.
Sources: Heatherbell et al. (1977), Probstein et al. (1979),
Mietton-Peuchot et al. (1984), Koltuniewicz (1992), Constenla and Lozano (1996).
As previously indicated, pectin and other large solutes like starch, normally found when
unripen apples are processed, tend to form a fairly viscous and gelatinous-type layer on the
‘‘skin’’ of the asymmetric fiber. Flux decline, due to this phenomenon, can be reduced by
increasing flow velocity on the membrane
Traditionally correlations of J with DPTM and VCR have been determined by parameter
fitting of the experimental data. It was found that the following exponential equation,
proposed in the SRT model (Constenla and Lozano, 1996), fitted appropriately:
J ¼ JF þ (JO JF ) exp ( At)
(2:2)
Jo , JF , and A values can be obtained at different Qr and constant values of VCR and DPTM.
An increase in Qr significantly increases the permeate flux. This behavior was reflected as an
extensive increase in the parameter A.
2.2.6.3. Influence of VCR on the Permeate Flux
Constenla and Lozano (1996) found that in the case of pseudoplastic fluids, as fruit juice
retentates, different operative conditions restrain the VCR up to a maximum of 14. The
permeate flux becomes independent of the solute rejection, characteristic of the hollow fibers
Table 2.9. Classic filtration models (pseudoplastic fluids).
Mechanism
Scheme
Representative equations
(1) Total pore blocking
J0 J ¼ K1 Vp ln (J=J0 ) ¼ K1
(2) Partial pore blocking
ln (J=J0 ) ¼ K2 Vp1 =J 1=J0 ¼ K2 t
.
(3) Blocking Progressive pore
J ¼ J0 (1 K3Vp =2)(3n þ 1)=2nJ
¼ J0 (1 þ ((n þ 1)=n)J0 K3 t) (3n þ 1)=(n þ 1)
(4) Cake filtration
(1=J)n ¼ (1=J0 )n þ K4 Vp : (1=J)nþ1 ¼ (1=J0 )nþ1
þ ((n þ 1)=n)K4 t
K 1 , K 2 , K 3 , K 4 : experimental constants; n: flow behavior index.
Source: Lozano et al. (2000). (with permission)
52
Fruit Manufacturing
after a few minutes of operation. This effect is commonly attributed to the build up of the
concentration polarization/gel layer. During the UF of apple juice in the mass transfer region,
a 60% increase in DPTM was reflected only as a 5% increase in J.
Acceleration of the fruit juice retentate near the membrane surface removes the accumulated macromolecules, thereby reducing the effect of concentration polarization gel layer. Due
to the low diameter of the hollow fibers, high-tangential velocities can be obtained at laminar
rates. Equation iv in Table 2.8, fit reasonably well for these types of membranes:
J ¼ Jo B In (VCR)
(2:3)
where JO is the initial permeate flux, and B is a constant, which depends on the system,
operating conditions, and juice properties. Decrease of flux with concentration is nonlinear,
and changes in the rate of permeation were better followed when plotted against ln VCR
(Fig. 2.24). The rate of flux decrease J could be divided into three periods. The first period,
characterized by a rapid decrease in J, occurred in a few minutes.
During the second period (up to VCR ¼ 3 approximately) the variation of J is unstable,
depending on fiber cut-off. Then J approached a ‘‘linear’’ steady logarithmic decrease with
VCR. This behavior could be explained by considering the resistance to flux as two separate
0
additive resistances in series: (i) the membrane resistance (Rm ); and (ii) the concentration
polarization/gel layer resistance (Rp ). During the first period Rp increases very fast reaching a
0
0
value equivalent to that of Rm . In the second region, the Rm value is still an important
100.00
PC = 30,000 ds
PC = 50,000 ds
PC = 10,000 ds
J, L/hm2
80.00
I
60.00
II
40.00
III
20.00
0.00
1
2
3
45
6
7
8
9 10
VCR
Figure 2.24. Decrease of permeate flux with ln VCR for hollow fibers with different MWCO. Full line represents
Eq. (2.4). Reprinted from Lebensm. Wiss. u. Technol. 27: 7–14, Constenla, D.T. Lozano, J.E., Predicting
stationary permeate flux in the ultrafilteration of apple juice. (copyright) 1996, with permission
from Elsevier.
2
.
Processing of Fruits
53
component of the total resistance and J is not completely independent of the properties of the
fiber. Finally, during the last period Rp is dominant and the cut-off of the hollow fiber
becomes irrelevant.
REFERENCES
Alkorta, I., Garbisu, C., Llama, M.J. and Serra, J.L. (1996). Immobilization of pectin lyase from Penicillium Italicum
by covalent binding to Nylon. Enzyme Microb. Technol. 18: 141–146.
Ates, S. and Pekyardimci, S. (1995). Properties of immobilized Pectinesterase on Nylon. Macromol. Rep. A32:
337–345.
Ben-Shalom, N., Levi, A. and Pinto, R. (1986). Pectolytic enzyme studies for peeling of grapefruit segment membrane. J. Food Sci. 51: 421–423.
Bump, V.L. (1989). Apple pressing and juice extraction. In Processed Apple Products, Downing, D.L. (ed.). AVI
Publishing Company, Van Nostrand Reinhold, New York, pp. 53–82.
Ceci, L. and Lozano, J.E. (1998). Determination of enzymatic activities of commercial pectinases. Food Chem.
31(1/2): 237–241.
Cheryan, M. (1986). Ultrafiltration Handbook. Technomic Publishing Company, Lancaster.
Constenla, D.T. and Lozano, J.E. (1996). Predicting stationary permeate flux in the ultrafiltration of apple juice.
Lebensm. Wiss. Technol. 27: 7–14.
Dietrich, H., Patz, C., Schöpplain, F. and Will, F. (1991). Problems in evaluation and standardization of enzyme
preparations. Fruit Process. 1: 131–134.
FAOSTAT Data (2005). FAO Statistical Databases. www.fas.usda.gov/htp/Presentations/2005.
Felloes, P. (1988). Food Processing Technology: Principles and Practice. Ellis Horwood International Publishers,
Chichester, England, pp. 300–310.
Grampp, E.A. (1976). New process for hot clarification of apple juice for apple juice concentrate. Fluss. Obst. 43:
382–388.
Heatherbell, D.A., Short, J.L. and Stauebi, P. (1977). Apple juice clarification by ultrafiltration. Confructa 22:
157–169.
Hui, Y.H. (1991). Data sourcebook for Food Scientists and Technologists. VCH Publishers, New York.
Iritani, E., Hayashi, T., and Murase, T. (1991). Analysis of filtration mechanism of crossflow upward and downward
ultrafiltration. J. Chem. Eng. Jpn 1: 39–44.
Koltuniewicz, A. (1992). Predicting permeate flux in ultrafiltration on the basis of surface renewal concept. J. Membr.
Sci. 68: 107–118.
Lineback, D.R. and Wongsrikasen, E. (1980). Gelatinization of starch in baked products. J. Food Sci. 45: 71–74.
Liu, Y.K. and Luh, B.S. (1978). Purification and characterization of endo-polygalacturonase from Rhizopus arrhizus.
J. Food Sci. 43: 721–726.
Lozano, J.E. (2003). Separation and clarification. In Encyclopedia of Food Science and Nutrition, Caballero, B.,
Trugo, L. and Finglas, P. (eds.). AP Editorial, Elsevier, London, UK, pp. 5187–5196. ISBN: 0-12-227055-X.
Lozano, J.E., Constenla, D.T. and Carrı́n, M.E. (2000). Ultrafiltration of apple juice. In Trends in Food Engineering,
Lozano, J.E., Añón, C., Parada-Arias, E. and Barbosa-Cánovas, G. (eds.). Food Preservation Technol. Series.
Technomics Publishing Company, Inc., Lancaster, Basel, pp. 117–134.
McLellan, M.R. (1996). Juice processing, Chapter III. In Processing Fruits: Science and Technology. Biology,
Principles and Applications, Vol. 1, Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomic
Publishing Company, Inc., Lancaster, Basel.
Mietton-Peuchot, M., Milisic, V. and Ben Aim, R. (1984). Microfiltration tangentielle des boissons. Le Lait. 64,
121–128.
Nagy, S.; Chen C.S. and Shaw, P.E. (eds.) (1993). Fruit Juice Processing Technology. Agscience, Inc. Auburndale,
FL.
Omosaiye, O., Cheryan, M. and Mathews, M. (1978). Removal of oligosacharides from soybean water extracts by
ultrafiltration. J. Food Sci. 51: 354–358.
Porter, M. (1972). Concentration polarization with membrane ultrafiltration. Ind. Eng. Chem.—Prod. Res. Develop.
11(3): 234–248.
Probstein, R., Leung, W. and Alliance, Y. (1979). Determination of diffusivity and gel concentration in macromolecular solutions by ultrafiltration. J. Phys. Chem. 83(9): 1228–1236.
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Ramaswamy, H.S. and Abbatemarco, C. (1996). Thermal processing of fruits. In Processing Fruits: Science and
Technology, Vol. I, pp. 25–65.
Reed, G. (1975). Enzyme in food processing, 2nd ed. Academic Press, New York.
Rombouts, F.M. and Pilnik, W. (1978). Enzymes in fruit and vegetable juice technology. Process Biochem. 13: 9–13.
Sakai, T., Sakamoto, T., Hallaert, J. and Vandamme, E.J. (1993). Pectin, pectinase, and protopectinase: production,
properties, and applications. Adv. Appl. Microbiol. 39: 213–294.
Short, J.L. (1983). Juice clarification by ultrafiltration. Process Biochem. 18(5): VI.
Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.) (1996). In Processing Fruits: Science and Technology. V.2
Major Processed Products, Technomic Publishing Company, Inc., Lancaster, PA.
Spagna, G., Pifferi, P.G. and Martino, A. (1993). Pectinlyase immobilization on epoxy supports for application in the
food processing industry. J. Chem. Tech. Biotechnol. 57: 379–385.
Toribio, J.L. and Lozano, J.E. (1984). Non enzymatic browning in apple juice concentrate during storage. J. Food Sci.
49: 889–893.
Woodroof, J.G. and Luh., B.S. (1986). Commercial fruit processing, 2nd ed. AVI Publishing Company, Westport,
CT.
Wu, M.L., Zall, R.R. and Tzeng, W.C. (1990). Microfiltration and ultrafiltration comparison for apple juice
clarification. J. Food Sci. 55(4): 1162–1163.
Zeman, L.J. and Zydney, A.L. (1996). Microfiltration and Ultrafiltration: Principles and Applications. Marcel Dekker,
Inc., New York, NY.
Zobel, H.F. (1984). Starch gelatinization and mechanical properties. In Starch: Chemistry and Technology, 2nd ed.,
Whstler, R.L., BeMiller, J.N. and Paschall, E.F. (eds.), Academic Press, Orlando, FL, pp. 300–302.
CHAPTER 3
PROCESSING OF FRUITS:
ELEVATED TEMPERATURE,
NONTHERMAL AND
MISCELLANEOUS PROCESSING
3.1. PASTEURIZATION
The process of ‘‘pasteurization’’ pioneered by Louis Pasteur was aimed at the destruction of
bacteria, molds, spores, etc. by exposing them to a certain minimum temperature for a certain
minimum time; the higher the temperature, the shorter the time required. The term ‘‘pasteurized’’ can be used to refer to products with reduced bacteria. Products with no bacteria are
referred to as ‘‘sterile’’ or ‘‘ultrapasteurized.’’
Some products are ‘‘sterilized’’ before they are sold to the public. Most of the fruit juice
sold on store shelves is produced this way. These products have relatively unlimited shelf life
even without refrigeration. However, the time/temperature combination required to kill 100%
of bacteria also destroys some of the flavor components in the juice. There is some dispute
over how much flavor degradation actually occurs, and since this is a subjective opinion on
the part of the consumer, no definitive data are available. The following methods are
commonly accepted for pasteurization.
3.1.1. Batch Pasteurization
This is a typical pasteurization process, by heating the product in a batch pan to about 638C
for relatively long periods (Table 3.1). This method destroys very common pathogenic
bacteria. However, as production demands grow, simply adding more number of pans is
usually not feasible.
3.1.2. HTST (Short Time) Pasteurization
High-Temperature, Short-Time pasteurization is typically conducted at 728C for 15 s. A hold
time of 15 s can easily be achieved in a continuous process by installing a holding tube. The
product is then cooled for storage. This method provides the convenience of continuous
processing, at a temperature low enough to prevent taste and aroma deterioration.
3.1.3. UHT Pasteurization
In UHT pasteurization, the product is brought to over the boiling point (under pressure) for
only a fraction of a second. This results in a sterile product that does not require refrigeration
55
56
Fruit Manufacturing
Table 3.1. Typical pasteurization methods and conditions.
Method
Batch
HTST (High temperature/short time)
UHT (Ultrahigh temperature)
Temperature (8C)
Hold time
63
72
>121
30 min
15 s
0.1 s
Source: Nickerson and Sinskey, 1972; US Department of Health and Human Services, 2004.
later. However, after being brought to this temperature, a slight ‘‘cooked’’ taste is sometimes
said to be detectable.
Most apple juice producers are relatively familiar with both batch and UHT pasteurization. While smaller producers use batch method, UHT systems are commonly employed by
large processors.
3.1.4. Nonthermal Pasteurization
Much has been written about ‘‘new’’ pasteurization and sterilization technologies such as
irradiation, microwave sterilization, and high-pressure processes that have long been available, but, for various reasons, scarcely applied in food processing. Meanwhile, new or
improved thermal and nonthermal technologies have emerged that are available now for
pasteurizing, sterilizing, or otherwise reducing microbiological contamination of foods.
Moreover, some new nonthermal pulsed technologies have been cleared by the FDA for
antimicrobial applications (Bolando-Rodriguez et al., 2000).
Traditional thermal sources, such as steam, are being engineered into new and improved
processes to destroy pathogens with minimal heat, as for example the continuous steamfusion cookers. This technique involves a variable-speed agitator, which rotates on an axis
parallel to product flow through a vertical tube. A row of steam injectors along each side of
the tube provides rapid heating, while turbulence created by the agitator fuses steam to evenly
heat the product as it flows through the tube. The product goes up to cooking temperature in
just a few seconds, and steam is combined with the product to avoid overcooking. Cooker
surfaces are kept at practically the same temperature as the product.
Pressurized juice should be preserved under chilled conditions to retain its fresh flavor
and taste. Low temperature also helps to reduce the development of precipitates, since lowtemperature storage keeps pectin esterase activity low; thus, pectin esterase cannot participate
in the formation of a precipitate.
New nonthermal technologies with potential for processing juices to retain flavor and
extend shelf life include ultrahigh pressure (UHP), pulsed electric fields (PEF), ultraviolet
(UV) light, electric pulse, and carbon dioxide (CO2 ) (Ohlsson and Bengtsson, 2002). Over the
past several years, intense R&D efforts have aimed at validating and commercializing these
technologies. The low-temperature storage is important to other nonthermally treated fruit
products.
Pulsed electric-fields’ method is a cold-pasteurization process for antimicrobial treatment
of liquids and pumpable foods. It is based on the application of short-duration, high-intensity
electric-field pulses to kill both spoilage and pathogenic organisms without affecting product
taste or color.
The use of high-intensity pulsed light to control microorganisms on food surfaces applies
nonionizing, high-intensity flashes of broad-spectrum light to reduce microbial populations
3
.
Processing of Fruits
57
on foods and packaging materials. Each flash is approximately 20,000 times the intensity
of sunlight, with wavelengths ranging from ultraviolet to near infrared (Barbosa-Canovas
et al., 1997).
Radio frequency (RF) energy has been investigated as a nonthermal alternative
to thermal pasteurization (Geveke et al., 2002). Electric-field strengths of 14–30 kV/cm
generated with RF power supply systems at frequencies in the range of 20 kHz–27 MHz
were applied to suspensions of Saccharomyces cerevisiae in water over a temperature range of
28–558C. The population of S. cerevisiae was reduced by >5 log following 30 exposures to a
100-kHz, 25-kV/cm field at 288C.
3.2. STERILIZATION OF FOOD BY HIGH PRESSURE
The basis for applying high pressure to food is to compress the water surrounding the food.
A decrease in volume of water with increasing pressure is very minimal compared to gases.
The volume decreased for water is approximately 4% at 100 MPa, 7% at 200 MPa, and 11.5%
at 400 MPa at 228C. Above 1,000 MPa and at room temperature, however, water changes
to a solid (type VI ice), whose compressibility is very small. Usually irreversible effects
on biological materials are observed at pressure >100 MPa. Therefore, pressure of
100 –1,000 MPa could be useful in food treatment. For reversible effects, pressure up to
200 MPa may be used.
Microbial death at higher pressure is considered to be due to changes in permeability of
cell membranes (Farr, 1990). Bacteria, yeasts, and molds in foods, such as meat, fish, and
agricultural products, are sterilized by high-pressure treatment at 400–600 MPa. The pressurization of mandarin or orange juices at 300–400 MPa for 10 min is enough to sterilize vegetative microorganic cells, although spores of Bacillus sp. are not killed. This retains good taste
and flavor of the juice, and allows to store it at room temperature for 5 months. When pressure
was applied at 458C, the results were considerably better than that at the room temperature.
The major use of the high-pressure sterilization is in partially prepared foods or ovenready foods. Pressure treatment preserves flavor, taste, and natural nutrients, but bacterial
spores are not killed. Hence, these foods require chilled transportation.
3.2.1. High-Pressure Equipment and the System
Test equipment for foods have been developed by several equipment industries and are
available on the market. A typical equipment has 500-ml capacity, is made of stainless
steel, and works at a maximum pressure of 700 MPa. It takes only 90 s to attain the
maximum pressure. Temperature of the inside water, used as the pressure-transducing medium, is regulated by an electric heater outside the pressure vessel. Thus, the hydrostatic
pressure is directly applied to foods placed in the pressure vessel at high speed under regulated
temperature without any harmful contaminants. Several food companies and government
institutions in Japan have been equipped with high-pressure test machines in recent years and
are performing research and development of new food products based on the high-pressure
processing (Barbosa-Canovas et al., 2004)
Industrial equipment for high-pressure processing of foods are operational in several
food industries: a batchwise system of 10–50 l capacity and a semicontinuous system of
1– 4 ton/h treatment. The former is used for the processing and sterilization of packed
foods and the latter for the treatment of liquid foods. These machines are as small as an
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Fruit Manufacturing
industrial machine, but a pressure vessel of 50 l is similar to a heating vessel of 200 l in
capacity. The cycle time for operating the pressure machine is short, generally being 15 min
for food sterilization or food processing, while a large pan takes about 1 h for heating and
cooling in conventional processing (Cheftel, 1995; Cole, 1997).
An industrial system for the high-pressure processing of foods is similar to the conventional heat processing: Raw materials are pretreated, filled in plastic bags, sealed in vacuum,
and pressurized. Final products are obtained after drying the bags. Liquid food may be placed
directly in the pressure vessel in a semicontinuous way. It was also indicated that the use of
pressure processing may save energy and improve sanitary conditions in the use of hightemperature processing.
Selection of packaging materials is important for high-pressure food processing. While
metal and glass are not suitable for high-temperature processing, plastic films are generally
acceptable. Ochiai and Nakagawa (1991) pointed the importance of head space in plastic cups
and suggested the use of plastic as a package material because of its heat sealability, hygiene,
and safety. Packaging materials, which prevent oxygen permeability and light exposure,
should be developed especially for retaining fresh color and flavor of foods.
In brief, high pressure similar to high temperature is useful for the purpose of cooking,
processing, sterilizing, and preserving food. The advantage of high pressure lies in the fact
that it avoids destruction of the covalent bonds and retains natural flavor, taste, color, and
nutrients. Thus, high-pressure technology is of great importance to the fruit industry.
3.3. CONCENTRATION BY EVAPORATION
Evaporation refers to the process of heating the liquid to boiling point to remove water as
vapor. Because fruit products, in particular fruit juices, are heat sensitive, heat damage can be
minimized by evaporation under vacuum to reduce the boiling point (Heldman and Singh,
1981). The basic components of this process consist of: (i) heat exchanger, (ii) vacuum system,
(iii) vapor separator, and (iv) condenser.
The heat exchanger transfers heat from the heating medium, usually low-pressure steam,
to the product via indirect contact surfaces. The vacuum system reduces the product temperature. The vapor separator removes juice from the vapors, driving juice back to the heat
exchanger and vapors out to the condenser, which condenses the vapors from inside the
heat exchanger and may act as the vacuum source.
The driving force for heat transfer is the difference in temperature between steam and
juice. The steam is produced in large boilers, generally tube and chest heat exchangers. The
steam temperature is a function of the steam pressure. Water boils at 1008C at 1 atm., but at
other pressures the boiling point changes. At boiling point, the steam condenses in the coils
and gives out latent heat. If the steam temperature is too high, burn-on/fouling increases, so
there are limits to how high steam temperatures can go. The juice is also at its boiling point,
with an increase of solids’ concentration. The most important types of single effect evaporators are described by Minton (1986) and Perry and Chilton (1973).
3.3.1. Batch Pan
It consists of spherical-shaped, steam-jacketed vessels (Fig. 3.1). The heat transfer per unit
volume is small, requiring long residence times. The heating is due to natural convection.
Heat transfer characteristics are poor.
3
.
Processing of Fruits
59
Vapor
Steam
Condensate
Concentrate
Figure 3.1. Batch pan or calandria.
3.3.2. Rising Film Evaporator
This type of evaporator consists of a heat exchanger isolated from the vapor separator
(Fig. 3.2). The heat exchanger consists of 10 –15 m long tubes in a tube chest, which is heated
with steam. The liquid rises by percolation from the vapors formed near the bottom of the
heating tubes. The thin liquid film moves upward rapidly. The product may be recycled if
necessary to arrive at the desired final concentration.
Vaccum
Feed
Steam
Feed
Concentrate
Condensate
Figure 3.2. Rising film evaporator (single stage).
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Fruit Manufacturing
3.3.3. Falling Film Evaporator
The falling film evaporators are the most widely used type of evaporators in the food
industry. It has similar components to the rising-film type except that the thin liquid film
moves downward under gravity in the tubes (Fig. 3.3). Specially designed nozzles or spray
distributors at the feed inlet permit it to handle products that are more viscous. The residence
time is 20–30 s as opposed to 3– 4 min in the rising film type.
The vapor separator is at the bottom, which decreases the product holdup during
shutdown. The tubes are 8–12 m long and 30–50 mm in diameter.
3.3.4. Scraped-Surface Evaporator
Scraped-surface evaporators are designed for the evaporation of highly viscous and sticky
products, which cannot be otherwise evaporated. This type of evaporator has been specially
designed to provide a high degree of agitation as well as scraping the walls of the evaporator
to prevent deposition and subsequent charring of the product. Scraped-surface heat evaporators consist of a cylinder that has an inner tube (heat transfer surface area) and an outer
tube.
Between the two tubes, there is annular space, where the heating or cooling media flows
countercurrent to the product. Inside the inner tube, a bladed shaft rotates and removes the
product from the heat transfer wall areas (Fig. 3.4). Scraped-surface heaters improve cooking
by allowing better heat transfer to the batch and preventing burn-on. Typical scraped film
evaporator application includes processing of fruit purées, mashes, pulps, concentrates, and
pastes.
Feed
Steam
Vacuum
Condensate
Concentrate
Figure 3.3. Falling film evaporator (single stage).
3
Processing of Fruits
.
Steam
61
Rotor
Fruit
product
Figure 3.4. Cross section of a scraped-surface evaporator.
3.3.5. Multiple Effect Evaporator
Two or more evaporator units can be run in sequence to produce a multiple effect evaporator.
Each effect would consist of a heat transfer surface, a vapor separator, as well as a vacuum
source and a condenser. The vapors from the preceding effect are used as the heat source in
the following effect. There are two advantages to multiple effect evaporators:
.
.
Economy: They evaporate more water per kilogram steam by reusing vapors as heat
sources in subsequent effects
Improvement in heat transfer: This is due to the viscous effects of the products as they
become more concentrated.
Each effect operates at a lower pressure and temperature than the effect preceding it so as to
maintain a temperature difference and continue the evaporation procedure. The vapors are
removed from the preceding effect at the boiling temperature of the product, so that no
temperature difference would exist if the vacuum were not increased. The operating costs of
evaporation are relative to the number of effects and the temperature at which they operate.
As evaporation is a very energy-consuming process, the availability and the relative cost
of energy determine the design of the evaporation plant. Normally an evaporation plant is
designed to use energy as efficiently as possible by using more than one effect. Therefore the
following technical solutions are used in order to keep the temperature of the steam high
enough to run the process:
.
.
.
Thermal vapor recompression (TVR),
Mechanical vapor recompression (MVR), or
Combination of both.
3.3.5.1. Thermocompression (TC)
This includes the use of a steam-jet booster to recompress part of the exit vapors from the first
effect. Through recompression, the pressure and the temperature of the vapors are increased.
As the vapors exit from the first effect, they are mixed with very high-pressure steam. The
steam entering the first effect is at a slightly less pressure than the supply steam. There are
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Fruit Manufacturing
usually more vapors from the first effect than can be used by the second effect; usually only
the first effect is coupled with multiple-effect evaporators.
3.3.5.2. Mechanical Vapor Recompression (MVR)
Whereas only part of the vapor is recompressed using TC, the entire vapor is recompressed in
an MVR evaporator. Vapor is mechanically compressed by radial compressors or by simple
electrical fans. There are several variations: in single effect, all the vapors are recompressed,
therefore no condensing water is needed; in multiple effect, MVR is possible on first effect,
followed by two or more traditional effects, or recompress vapors from all effects.
3.4. DEHYDRATION
Dehydration refers to the nearly complete removal of water from foods to a level of less than
5%. Dehydrated foods are protected from spoilage by lowering the water activity:
aw ¼ pv =po
(3:1)
where pv is the vapor pressure of water in the product; and po the vapor pressure of saturated
water at the same temperature. In dry fruits at aw ¼ 0:85 or above, some other form of
preservation such as SO2 or potassium sorbate may be used. Some definitions are of interest.
.
.
.
Dried: Refers to all products with reduced moisture content regardless of the method.
Evaporated: Refers to use of sun and forced air driers to evaporate moisture to a fairly
stable product. Sun drying in general will not reduce moisture below 15%. Many
evaporated fruits will have up to 25% water level. These products have short storage
life even when a high level of preservative is added. Evaporated fruits need refrigeration.
Dehydrated: Refers to fruits whose moisture has been reduced to 1–5% under carefully
controlled conditions. Dehydrated fruits have more than 2 years’ storage life, particularly if stored in modified atmosphere or low-temperature conditions.
Drying rate of fruits depends on particle size and mode of heat transfer. Sliced and diced
products dry faster. None of the mechanisms involved in drying of fruits are as simple as they
might seem, as foods do not usually deal with a single uniform phase. Two important aspects
of mass transfer in dehydration are: (a) movement of water to the surface of material being
dried, and (b) removal of water from the surface through the (probably) thin immobile
boundary layer. Considering some possible modes of heat transfer that can be applied in
fruit drying we find:
Phases involved in fruit drying
Examples
Gas–solid
Gas–liquid
Liquid–gas
Liquid–solid
Solid–gas
Solid–liquid
Solid–solid
Conventional dehydration of fruits
Concentration of syrup in cascade-type drier
Internally during moist fruits’ drying
Frying
Internally during moist fruits’ drying
Drum drying
Drum drying of fruit purée
3
.
Processing of Fruits
63
Drying Rate
Constant
rate period
Falling rate
period
Xe
TIME, t
Xcr
Figure 3.5. Characteristic drying rate curve.
Drying curve relates to the amount of water removal with time (Fig. 3.5) and can be
divided into different regions (Crapiste and Rotstein, 1997):
(1) An initial period in which evaporation occurs on the surface and temperature wet
bulb value. This is the constant-rate period. However, due to water conditions on
product surface and shrinkage during drying a pseudoconstant-rate period may be
observed.
(2) At a point usually called the critical moisture content (Xcr ) the falling rate period starts.
When surface moisture is lost, the rate falls as water must diffuse from inside to surface
before evaporation can take place. Dominant factor is availability of water at evaporation
surface. The process of dehydration slows greatly at this period, until moisture content
asymptotically reaches Xe , the equilibrium value at the relative humidity and temperature
of the air. Drying rate is affected by drier loading on tray or belt driers. This can be
overcome by fluidizing fruit particles.
During the initial constant-rate stage hotter is better until falling rate phase is reached.
At Xcr , temperature must be reduced to avoid deteriorative reactions. Table 3.2 lists different
dehydration systems’ characteristics.
Other driers used in the fruit industry are bin driers, simple devices consisting of bins with
perforated bottoms, and fluidized bed driers, used for drying food powders. In fluidized bed
driers, air is blown up through a wire mesh belt on porous plate that supports and conveys the
product. A slight vibration motion is imparted to the food particles. Fluidization occurs when
the air velocity is increased to the point where it just exceeds the velocity of free fall of the
particles. The fluidization provides intimate contact of each particle with the air. With products
that are particularly difficult to fluidize, a vibrating motion of the drier itself is used to aid
fluidization—this is called vibrofluidizer. The fluidized solid particles then behave in a manner
analogous to a liquid, i.e., they can be conveyed. Air velocities will vary with particle size and
density but are in the range of 0.3–0.75 m/s. They can be used not only for drying but also for
cooling. If the velocity is too high, the particles will be carried away in the gas stream, therefore,
gravitational forces need to be only slightly exceeded. Minimum air velocity to fluidize 10-mm
pulp is about 115 m/min. It can be used to dry uniform-sized products.
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Fruit Manufacturing
Table 3.2. Description and schematic diagram of different driers.
Type of drier
Description
Drier sketch
Sun or solar
Simplest systems, consisting of trays laid flat on ground
or supported slightly above it. Used for drying apricots,
raisins, etc. Stacked tray system for very low humidity
areas improves dehydration due to natural air
movement
Wind
Cabinet
drier
It is a typical batch operation in which air is heated
and forced to circulate between trays or through the
product by using perforated trays. These driers can
process from pulps to solid pieces of fruits
Hot
air
Tunnel
drier
Tunnel driers have been the most widely used form of
fruit dehydrators. They are set up with parallel flow
air during the constant-rate period, arranged in such
a way that incoming fruit encounters the hottest,
driest air, then counterflows so that the outgoing
product encounters the driest air. Air velocities of
200 – 400 m/min are commonly used. Initial
temperatures of 1008C may be used. Final
temperatures are about 708C or less, depending
on fruit products
Continuous
belt or
conveyor
driers
Product moving through these driers is exposed to
the same successive sets of drying conditions.
A single continuous belt or series of small belts
may be used. Conveyor may consist of mesh belts
or perforated metal plates. Temperature is reduced
in the direction of the outlet of the system
Belt driers
Highly efficient device units, occupying a relatively
small plant area per ton of product. In these driers
high-velocity air (not enough to fluidize the product)
passes through the belt. Adequate for dehydro
freezing lines. Drying fruits to low moisture is
usually attained by a complementary system
Exhaust
air
Hot
air
Trucks’ direction
Hot air
Feeder
Air temperature decrease
downstream
Drum driers
Originally used for dry milk. Can be operated at
atmospheric pressure or under vacuum. In the fruit
industry it is used for making apple sauce flakes.
Almost any purée can be dried if the fiber content
is adequate. Control factors include sheet thickness,
temperature, drum speed, and air flow over drums
Source: Karel et al., 1975; Crapiste and Rotstein, 1997.
Feeder
3
Processing of Fruits
.
65
Microwave drying (Van Arsdel et al., 1973), osmotic dehydration (Shi and Fito, 1993),
explosion puffing (Saca and Lozano, 1992), and freeze drying (Mujumdar and Menon, 1995)
have been also applied for fruit dehydration.
3.4.1. Spray Drying
Spray driers are one of the most widely used types of air convection drier. Spray drying
involves transforming a pumpable food, i.e., juices, low-viscosity pastes, and purées into a
dry-powdered or particle form. This is achieved by atomizing the fluid into a drying chamber,
where the liquid droplets are passed through a hot-air stream (Heldman and Singh, 1981;
Green and Maloney, 1999). The objective is to produce a spray of high surface-to-mass ratio
droplets and then to uniformly and quickly evaporate the water. Evaporation keeps product
temperature to a minimum, so little high-temperature deterioration occurs. In its simplest
form, spray drying consists of four separate process stages:
.
.
.
.
Atomization of the liquid food feed,
Spray-air contact,
Drying,
Separation of the dried food product from the drying air.
Atomization is generally accomplished by: (i) a single-fluid (or pressure) nozzle, (ii) a
two-fluid nozzle, or (iii) a rotary atomizer, also known as a spinning disk or a wheel. The
single-fluid nozzle allows more versatility in terms of positioning with the spray chamber, so
the spray angle and spray direction can be varied.
A typical drying-chamber design used for fruit pulps and juices is the cylindrical flatbottomed drier. A pneumatic powder discharger removes the product, while an air broom
cools chamber walls to prevent sticking. This design allows easier access for cleaning. Drying
occurs in two phases, and air-temperature control is vital to their control. The first phase is a
constant-rate step, in which the moisture rapidly evaporates from the surface, and capillary
action takes out the moisture from within the particle. Then, during the ‘‘falling-rate’’ period,
diffusion of water to the surface controls the drying rate. As moisture content drops, diffusion
rate also decreases. Removing moisture to required values in a single-stage drier is responsible
for most of the residence time in the drier.
As a rule, the residence time of the air and the particle in a single-stage cocurrent drier is
about the same. Since the moisture level is still decreasing toward the end of the process, the
outlet temperature must be high enough to continue the drying process. This can be avoided
by adding a fluid bed after the drier.
The final stage of spray drying is the removing of the dried product from the air.
Depending on drier design, the dried product can be separated at the base (as in a flatbottomed drier). While the heavier product is removed by gravity, the smallest particles are
pulled together in some type of collection equipment. Otherwise, the entire product and air
can be moved to equipment designed to separate particles from air. Fine particles are removed
with cyclones, bag filters, electrostatic precipitators, or scrubbers. Fines are bagged or
returned to an agglomeration process; air is returned to the system.
Drying takes place within a matter of seconds at temperatures approximately 2008C.
Although evaporative cooling maintains low product temperatures, rapid removal of the
product is still necessary.
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Fruit Manufacturing
Air blower
Feed
Nozzle
Air outlet
Dry product
Figure 3.6. Typical spray drier configuration.
The liquid food is generally preconcentrated by evaporation. The concentrate is then
introduced as a fine spray into a tower or chamber with heated air. As the small droplets are
put in contact with the heated air, they flash off their moisture and drop to the bottom of the
tower and are removed. Principal spray components are: (1) a high-pressure pump, for
introducing liquid into the tower, (2) a nozzle for atomizing the feed stream, (3) a blower
with a source of hot air, and (4) a system for removing the dried food (Fig. 3.6).
Several billion particles per liter ensure a large surface area for exposure to drying forces.
Particle size must be reduced in three ways: (1) using a smaller orifice, (2) increasing
atomization pressure, or (3) reducing product viscosity, by increasing feed temperature or
redilution. The exit air temperature is an important control parameter, which can be used to
adjust feed flow rate and inlet temperature.
3.4.2. Powder Recovery
Three systems are available (Walas, 1976; Green and Maloney, 1999) for powder recovery
from the air stream:
(1) Bag filters: Although very efficient they are not very popular due to labor costs and
sanitation problems. They are not recommended for hygroscopic particles.
(2) Cyclone collector: In this type of powder collector, air enters at tangent at high velocity into
a cylinder or cone, which has a much larger cross section. Air velocity is decreased in the
cone, permitting settling of solids by gravity. Several cyclones can be placed in series. High
air velocity is needed to separate small diameter and light materials, while centrifugal force
is important in removing particles from the air stream. To increase centrifugal force cyclone
diameter may be reduced. A rotary airlock is used to remove powder from the cyclone.
(3) Wet scrubber: Wet scrubbers are the most economical outlet air cleaner. The principle of a
wet scrubber is to dissolve any dust powder left in the airstream into either water or the
feed stream by spraying the wash stream through the air. Wet scrubbers also recover
approximately 90% of the potential drying energy normally lost in exit air. Cyclone
separators are hygienic and easy to operate. However, high losses may occur. Either
feed stream or water can be used as scrubbing liquor.
3
.
Processing of Fruits
67
Two- and Three-Stage Drying Processes
In single-stage spray drying, the rate of evaporation is particularly high in the first part of the
process, and it gradually decreases because of the falling water content of the particle
surfaces. Therefore a relatively high outlet temperature is required during the final drying
phase. The two-stage drying process was introduced to reduce temperatures and cost of
production, and increase product quality.
The two-stage drier consists of a spray drier with an external vibrating fluid bed placed
below the drying chamber. The product can be removed from the drying chamber with a
higher moisture content, and the final drying takes place in the external fluid bed where the
residence time of the product is longer and the temperature of the drying air is lower than in
the spray drier.
This principle forms the basis of the development of the three-stage drier. The second
stage is a fluid bed built into the cone of the spray drying chamber. This fluid bed is called the
integrated fluid bed. The inlet air temperature can be raised, resulting in improved efficiency
in the drying process. The exhaust heat from the chamber is used to preheat the feed stream.
The third stage is again the external fluid bed, for final drying and/or cooling the powder. As a
result, a higher quality powder with much better rehydrating properties is obtained. Moreover, lower energy consumption and smaller space requirements are obtained.
3.5. MISCELLANEOUS PROCESSING
3.5.1. Size Enlargement
Size enlargement operations are used in the food processing industry for improving handling
and flowability, producing structural useful forms, enhancing appearance, etc. Food size
enlargement operations are known as compaction, granulation, tabletting (palletizing), encapsulation, sintering, and agglomeration. The main objective of agglomeration is to control
porosity and density of materials in order to manipulate properties like dispersibility and
solubility, known as instantizing, because rehydration is an important functional property in
food processes. If size enlargement is used with the objective of obtaining definite shapes,
extrusion is the selected process to shape and cook at the same time.
3.5.1.1. Instantizing
Powdered fruit juice with particle size less than about 10 mm is considered. Powdered fruit
juice is the product obtained from fruit juice of one or more kinds by the physical removal of
virtually all the water content. The resulting product will be in the powder form and will
require the addition of water before use.
This product tends to form lumps when dissolved in water and require strong mechanical stirring for obtaining a homogeneous dispersion. It was proposed that under those
conditions water penetrates into the narrow spaces between the particles by capillarity, and
the powder starts to dissolve, forming a thick, gel-like mass, which resists further penetration of
water. Therefore, fruit particle agglomeration, with a dried core is formed. Moreover, if enough
air is locked into these lumps, they will float on the water surface, resisting further dispersion.
To avoid this problem, agglomerated powder needs an open structure, allowing water to
penetrate before a tightly packed gel layer is formed. To do this the specific surface of the
powder has to be reduced and the liquid needs to penetrate more evenly around the particles.
In this way the powder can disperse into the bulk of the liquid, and the following steps
represent a complete dissolution (Ortega-Rivas, 2005):
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Fruit Manufacturing
(1) Granular juice particles are wetted, and water penetrates into the pores of the granule
structure.
(2) The wetted particles sink into the water and granules disintegrate into their original
smallest particles.
(3) The small, dispersed particles dissolve in the water.
The total time required for all these steps should be the criteria used to evaluate a product’s
instant properties.
3.5.1.2. Agglomeration
Agglomeration can be defined as the process of size enlargement by which particles are joined
or bind each other in a random way, finishing with an aggregate of porous structure much
larger in size than the original material. Agglomeration is used in food processes mainly to
improve properties related to handling and reconstitution, that is to produce a more uniform
particle size, increase or decrease particle bulk density, improve solubility and dispersibility,
and reduce caking. Dispersibility is defined as the time to dissolve a given weight of material
in water. The term agglomeration includes varied unit operations and processing techniques
aimed at agglomerating particles (Green and Maloney, 1999).
The main attractive forces involved in the agglomeration of food particles are studied by
Rumpf (1962) and Rumpf and Schubert (1978):
(1) Solid bridges, liquid bridges, and capillary forces. The force exerted by a liquid bridge
at rest depends on both the surface tension of the interface and the capillary effects
due to the curvature of the bridge. Solvents’ evaporation and treatment of food
particles at elevated temperatures form solid bridges, by formation of salt bridges,
and partial sintering or melting at the intra-agglomerate contact points. Alternatively
chemical bonding may occur with the use of organic binders, like hydrocolloid for
hydrophilic materials. If the material has some hydrophobicity, the binder may have
to have a wetting agent, e.g., lecithin. Figure 3.7 shows a simplified agglomeration by
liquid bridges’ development. After drying soluble solids crystallize out of the liquid
bridge, forming solid bridges.
(2) Van der Waals’ Forces. These are commonly known as dispersion forces, and are
quantum mechanical in origin. These forces of attraction exist between molecules of
any kind and constitute a general property of matter. Such attractions will cause
particles to stick to each other when they come within a few nm of each other. While
3
.
Processing of Fruits
Wetting
69
Collision
Liquid bridge
formation
Figure 3.7. Agglomeration by liquid bridge formation.
van der Waals’ attractions can be strong at short distances (<10 nm), the attraction
becomes negligible for particles that are far apart.
(3) Electrostatic forces. These tend to be the weakest force between particles. Particles in
close proximity will tend to be held together by the difference in electrostatic
potential, U. To enable calculation of the strength of the agglomerate and the
manner of the bonding, the number of bonds and the mechanics of the breakup
need to be considered (Rumpf, 1962).
Figure 3.8 summarizes the mechanisms usually involved during food particles’ agglomeration, like partial melting, liquid bridges, molecular, interlocking bonds, and electrostatic
and capillary forces (Pietsch, 1991). The agglomeration process causes an increase in the
amount of air incorporated between powder particles. More incorporated air is replaced with
more water when the powder is reconstituted, which immediately wets the powder particles.
Figure 3.8 represents in fact a three-dimensional structure containing a large number of
particles. Particles in agglomerates could be quite numerous. The points of interaction can be
characterized by contact, or by a distance small enough for the development of binder
bridges. Alternatively, sufficiently high attraction forces can be caused by one of the shortrange force fields. The number of all interaction sites of one particle within the agglomerate
Partial
melting
Capillary
force
+
+
+
Interlocking
bonds
−
−
−
Van der Waal and
electrostatic forces
Liquid
bridge
Figure 3.8. Mechanisms usually involved during food particles’ agglomeration.
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Fruit Manufacturing
structure is called the coordination number (Green and Maloney, 1999; Pietsch, 1983; OrtegaRivas, 2005). Indirect measurement of the coordination number can be made as a function of
other properties of the agglomerate, namely porosity.
3.5.1.3. Agglomeration Process and Equipment
Two particles can be made to agglomerate if they are brought into contact. Food particles are
then brought into a sticky state, by wetting with the application of a finely dispersed liquid or
steam, by heating (thermoplastic materials), or by the addition of binder media (a food
adhesive). Use of binder agents is considered a nonagglomeration method. The steam condensation method usually cannot provide enough wetting without adversely heating the material
and is used less frequently on newer systems. The food particles’ surface must be uniformly
wetted and held wet over a selected period of time to give moisture stability to the clusters
formed. The clusters are dried to the desired moisture content and then cooled. Dried clusters
are screened and sized to reduce excessively large particles and remove excessively small ones.
Subsequently, the particles are placed under such conditions where they can form
structures. Successful formation of stable agglomerate structures depends on product solubility and surface tension, as well as on the conditions that can be generated in the process
equipment.
For most products, combinations of moisture and temperature can be established.
Generally agglomeration temperature decreases when particle moisture increases.
3.5.1.4. Agglomeration Equipment
Agglomeration is a complicated process. However, it is possible to agglomerate powder foods
by means of comparatively simple equipment (Pietsch, 1991), which involves the use of a
fluidized bed for rewetting and particle contact phase, followed by a belt or a fluid bed for
moisture removal (Fig. 3.9). The process must be strictly controlled to avoid deposit formation
in the chamber by overwetting, and weak agglomerates due to insufficient liquid or vapor rate.
Water/vapor
Air outlet
Powder
in
Hot air
Agglomerated
product
Figure 3.9. Typical agglomeration system.
3
.
Processing of Fruits
71
Figure 3.10. Gear pelletizer.
Deposit formation will always be a concern in agglomeration equipment. Proper implementation of a fluid bed agglomeration system requires detailed knowledge of the fluidization
technology, including fluidization velocities, bed heights, air-flow patterns, and residence
time distribution.
These processes have allowed the manufacturing of powders with better reconstitution
properties, such as fruit juice powder. During agglomeration, the powder is wetted, uniformly
but not excessively, with water or steam.
Other agglomeration methods are compaction, extrusion, melt forming, mixing, tumbling, and sintering.
During pressure agglomeration particles with only slight amounts of moisture are formed
into tablets, and briquettes into stamp presses, tablet presses, and roller presses. The principal
binding force is van der Waals’ attraction. Figure 3.10 shows a typical gear pelletizer.
3.5.1.5. Selective Agglomeration (Spherical Agglomeration)
In the latest agglomeration process, a second immiscible phase is added to the suspension.
This wets the solid phase and binds the particles together by means of capillary forces. As a
result, rounded flocks or agglomerates form with diameters up to 5 mm. Selective agglomeration can be achieved for mixtures of solids.
REFERENCES
Barbosa-Canovas, G.V., Palou, E., Pothakamury, U.R. and Swanson, B.G. (1997). Application of light pulses in the
sterilization of foods and packaging materials. Nonthermal Preservation of Foods. Marcel Dekker, New
York, Chapter 6, pp. 139–161.
Barbosa-Cánovas, G.V., Maria Tapia and Pilar Cano, M. (eds.) (2004). Novel Food Processing Technologies. CRC
Press, Boca Raton, FL.
72
Fruit Manufacturing
Bolando-Rodrı́guez, S., Góngora-Nieto, M.M., Pothakamury, G.V., Barbosa-Cánovas, G. and Swanson, B.G.
(2000). A review of nonthermal technology. In Trends in Food Engineering. Aspen Publishers, Inc., Maryland,
EEUU, pp. 117–134.
Cheftel, J.C. (1995). High-pressure, microbial inactivation and food preservation. Food Sci. Technol. Int. 1: 75–90.
Cole, R. (1997). High pressure processing: a technology of the future. Food Manuf. 72: 21–26.
Crapiste, G.H. and Rotstein, E. (1997). Design and performance evaluation of driers. In Handbook of Food
Engineering Practice, Valentas, K.J., Rotstein, E. and Singh, R.P. (eds.). CRC Press, Boca Raton, Chapter
4, pp. 125–165.
Farr, D. (1990). High pressure technology in the food industry. Trends Food Sci. Technol. 1: 14–16.
Geveke, D., Kozempel, M., Scullen, O.J. and Brunkhorst, C. (2002). Radio frequency energy effects on microorganisms in foods. Innov. Food Sci. Emerg. Technol. 3: 133–138.
Green, D.W. and Maloney, J.O. (1999). Perry’s Chemical Engineers’ Handbook. McGraw-Hill, New York.
Heldman, D.R. and Singh, R.P. (1981). Food Processing Engineering, 2nd ed. AVI Publishing Company, Inc.,
Westport, USA.
Karel, M., Fennema, O.R. and Lund, D.B. (1975). Principles of Food Science. Part. 2. Physical Principles of Food
Dehydration. O.R. Fennema Ed. M. Dekker Inc. NY.
Minton, P.E. (1986). Evaporator types and applications. In Handbook of Evaporation Technology. William Andrew
Publishing/Noyes.
Mujumdar, A.S. and Menon, A.S. (1995). Drying of solids: principles, classification and selection of driers. In
Handbook of Industrial Drying, Mujumdar, A.S. (ed.). Marcel Dekker, Inc., New York, Chapter 1, pp. 1–40.
Nickerson and Sinskey (1972). Microbiology of Foods and Food Processing. American Elsevier Publishing Company,
NY.
Ochiai S, Nakagawa, Y. (1991). High Pressure Science for Food. Hayashi Edition, Kyoto, Japan.
Ohlsson, T. and Bengtsson, N. (2002). Minimal processing of foods with non-thermal methods. In Minimal Processing Technologies in the Food Industry, Thomas Ohlsson and Nils Bengtsson (eds.). CHIPS, Texas, USA.
Ortega-Rivas, E. (2005). Handling and processing of food powders and particulates. In Encapsulated and Powdered
Foods, Onwulata, C.I. and Konstance, R.P. (eds.). Marcel Dekker, New York, in press.
Perry, R.H. and Chilton, C.H. (1973). Chemical Engineers’ Handbook, 5th ed. McGraw-Hill Book Company, New
York, pp. 11–27.
Pietsch, W. (1983). Low-energy production of granular NPK fertilizers by compaction-granulation. Proceedings of
Fertilizer’83. British Sulphur Corp., London, UK, pp. 467–479.
Pietsch, W. (1991). Size enlargement by agglomeration. John Wiley & Sons Ltd., Chichester, England.
Rumpf, H. (1962).The strength of granules and agglomerates. In Agglomeration, Knepper, W.A. (ed.). Interscience,
New York, pp. 379–418.
Rumpf, H., Schubert, H. (1978). Adhesion forces in agglomeration processes. In Onada & Hench: Ceramic processing
before firing. J. Wiley and Sons, Inc., London.
Saca, A. and Lozano, J.E. (1992). Explosion puffing of bananas. Int. J. Food Sci. Technol. 27: 419–423.
Shi, X.Q. and Fito, P. (1993). Vacuum osmotic dehydration of fruits. Drying Technol. 11: 1429–1442.
US Department of Health and Human Services (2004). Juice HACCP Hazards and Controls Guidance. Guidance for
Industry, 1st ed. Food and Drug Administration, Center for Food Safety and Applied Nutrition (CFSAN).
Van Arsdel, W.B., Copley, M.J. and Morgan Jr., A.I. (1973). Food Dehydration, Vols. 1 and 2. AVI Publishing
Company, Inc., Westport, CN.
Walas, S.M. (1976) Spray driers. In Encyclopedia of Chemical Processing and Design. Vol. 53. J.J. McKetta Ed.
Marcel Dekker Inc. NY. pp. 22–44.
CHAPTER 4
THERMODYNAMICAL,
THERMOPHYSICAL, AND
RHEOLOGICAL PROPERTIES OF
FRUITS AND FRUIT PRODUCTS
4.1. INTRODUCTION
Most processed and many freshly consumed fruits receive some type of heating or cooling
during handling or manufacturing. Design and operation of processes involving heat transfer
needs special attention due to heat sensitivity of fruits. Both theoretical and empirical
relationships used when designing, or operating, heat processes need knowledge of the
thermal properties of the foods under consideration. Food thermal properties can be defined
as those properties controlling the transfer of heat in a specified food. These properties are
usually classified (Perry and Green, 1973) into thermodynamical properties, viz, specific
volume, specific heat, and enthalpy; and heat transport properties, namely, thermal conductivity and thermal diffusivity.
When considering the heating or cooling of foods, some other physical properties must
be considered because of their intrinsic relationship with the ‘‘pure’’ thermal properties
mentioned, such as density and viscosity. Therefore, a group of thermal and related properties, known as thermophysical properties, provide a powerful tool for design and prediction
of heat transfer operation during handling, processing, canning, and distribution of foods
(Fig. 4.1). Abundant information on thermophysical properties of food (Polley et al., 1980;
Wallapapan et al., 1983; Choi and Okos, 1986; Rahman, 1995) is available to the design
engineer. However, finding relevant data is usually the controlling step in the design of a given
food operation, and the best solution may be the experimental determination.
This chapter provides data and information for thermal process calculation for fruits
and fruit products, including a brief description of more commonly used methods for
measurement and determination of thermophysical properties.
4.2. THERMOPHYSICAL PROPERTIES’ IDENTIFICATION
Thermophysical properties include different types of parameters associated to the heat
transfer operations present during fruit processing. It is well known that heat can be transferred by three ways: radiation, conduction, and convection.
Radiation is the transfer of heat by electromagnetic waves. The range of wavelength
0.8–400 mm is known as thermal radiation, since this infrared radiation is most readily
73
74
Fruit Manufacturing
Thermodynamical
properties
“Pure”
Thermophysical
properties
Specific volume
n (m3/kg)
Specific heat
cp (kJ/kg−1/ ⬚ C−1)
Enthalpy
∆H (kJ/kg−1)
Thermal conductivity
k (W/m−1/K−1)
Thermal diffusivity
a (m2/ s−1)
Density
r (kg/m−3)
Porosity
e
Viscosity
m (Pa s)
Heat transport properties
Physical properties
Figure 4.1. Thermophysical properties associated to fruit processing.
absorbed and converted to heat energy. A body emitting or absorbing the maximum possible
amount of radiant energy is known as a ‘‘black body.’’ Energy emitted by a black body is
given by the Stefan–Boltzmann law:
Q ¼ sAT 4
(4:1)
where s is the Stefan–Boltzmann constant; A the area of transfer, and T the absolute
temperature. For no ‘‘perfect’’ black bodies, as real bodies are, Eq. (4.1) is corrected by as
factor known a emissivity («):
Q ¼ s«AT 4
(4:2)
Emissivity values of foods are in the range 0.5–0.97 (Karel et al., 1975).
Conduction is the movement of heat by direct transfer of molecular energy within solids
(for example, heating of a fruit pulp by direct fire through metal containers).
Convection is the transfer of heat by groups of molecules that move as a result of a
gradient of density or agitation (for example, the stirring of tomato purée).
Heat transfer may take place: (i) in steady-state way by keeping constant the temperature
difference between two materials or (ii) under unsteady-state way when the temperature is
constantly changing. Calculation of heat transfer under these conditions is extremely complicated but is simplified by making a number of assumptions or giving approximate solutions
from prepared graphic or tabulated information.
Table 4.1 shows some common simplified equations used for the calculation of heat
transfer. Most of the thermophysical properties are required to solve the heat transfer
equations by conduction and convection. During processing, the temperature within a fruit,
changes continuously depending on the temperature of the heating medium, and two properties of the fruit: the thermal conductivity (k) and the specific heat Cp .
On the other hand thermal diffusivity is related to k and Cp :
a ¼ k=rcp
(4:3)
4
.
Thermodynamical, Thermophysical, and Rheological Properties
75
Table 4.1. Simplified equations used for heat conduction and convection calculations
(Lozano 2005, with permission).
8
< Steady state:
Conduction (unidirectional )
:
Unsteady state:
8
Natural
>
>
<
>
>
:
Convection
Forced
Q ¼ kA(u1 u2 )=x
2
2
8 du=dt ¼ ad u=dx
< Q ¼ hs A(DT)
Gr ¼ r2 gbl 3 DT=m2
:
8 Nu ¼ K(GrPr)
< Nu ¼ f (Re, Pr) ¼ hl=k
Pr ¼ cm=k
:
Re ¼ lnr=m
Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’
Edited by Gustaro V. Barbosa-Cánovas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd. Gr, Nu, Pr, and Re are the
Grashoff, Nusselt, Prand, and Reynolds’ numbers, respectively.
When a solid piece of food is heated or cooled by a fluid, the resistance to heat transfer,
which are the surface heat transfer coefficient and k may be related as follows:
Bi ¼ hl=k
2
(4:4)
1
where h (W=m =K ) is the heat transfer coefficient, f the characteristic half-dimension,
and k (W=m1 =K1 ) the thermal conductivity. At small Bi (<0:2) the surface film is the
predominant resistance, while for Bi > 0:2 it is the thermal conductivity, which limits the rate
of heat transfer (Urbicain and Lozano, 1997).
4.3. FRUITS AND FRUIT PRODUCTS’ PROPERTIES
4.3.1. Fruit and Fruit Products’ Properties During Freezing
It must be considered that thermophysical properties of foods change dramatically during the
freezing process. One of the characteristics of food freezing is that temperature changes
gradually with the phase change, which implies that the fraction of water frozen always
changes continuously with temperature below the freezing point. Depression of the initial
freezing temperature can be predicted from (Heldman and Singh, 1981):
l 1
1
(4:5)
¼ ln XA
R TAo TA
where l is the molal latent heat of fusion, R the universal gas constant, TAo the freezing
temperature of water, TA the freezing temperature of food product, and XA the mole fraction
of water. In most cases Eq. (4.5) is used to predict the unfrozen water fraction with the initial
freezing temperature, through the determination of apparent molecular weights for the mass
fraction of total solids in the product.
4.3.2. Water Content
Though water content (Xw ) is not a thermophysical property, it significantly influences all
thermophysical properties (Lozano 2005). As food is a living commodity, its water content
changes with maturity, cultivar, growth, and harvest and storage conditions. Values of most
of the thermophysical properties can be calculated directly from the water content. Xw is
usually expressed as water mass fraction [kg water per kg food; wet basis].
76
Fruit Manufacturing
4.4. EXPERIMENTAL DATA AND PREDICTION MODELS
Fruits and fruit products show extended variability in composition and structure, which must
be kept in consideration when modeling their thermal properties. Fruits are generally nonhomogeneous, varying in composition and structure not only between products but also
within a single product.
Some thermophysical properties for fruits were modeled only as a function of the water
content (Alvarado, 1991; Gupta, 1990). However, the presence of proteins, fats, and carbohydrates, as major components besides water, differs from one fruit to another. These
compounds have variable effects on the properties of the complex fruit structure (Sweat,
1995). As a result some proposed thermophysical properties models applied to fruit and fruit
products include a combination of the properties of water, fats, proteins, carbohydrates, and/
or ash (Oguntunde and Akintoye, 1991; Rahman, 1995).
Literature provides a large volume of experimental thermophysical food properties data
(Dickerson, 1968; Mohsenin, 1980; Jowitt et al., 1983; Rahman, 1995; Urbicain and Lozano,
1997). However, as the amount of thermal properties data required for describing any
foodstuff under the varied handling, processing, and storage condition is practically infinite,
modeling and prediction of such properties is a must.
Thermophysical properties of any material control the thermal energy transport and/or
storage within it, as well as the transformations undergone by the material under the action of
heat. Fruits are no exception; they are dependent on the temperature and the material’s
chemical composition, and physical structure. Since fruits are complex materials, relevant
information on their properties is given as average or effective value. For this reason the
generation of predictive models requires a physical representation of the material under
study. Fruits show three different levels of complexity.
First, microscopically, fruits may look as a continuous and homogeneous single phase.
However, they are composed of different chemical compounds including proteins, carbohydrates, fats, fiber, water, and other minor components. For this reason models proposed for
the prediction of a given property must consider the individual contribution of such compounds. It may be done by ‘‘weighing factors’’ accounting for the proportion in which they
are present.
In the second place, some fruits or fruit products can be considered as a solid matrix of
the continuous described above and a disperse phase of air or water, respectively. This
description corresponds typically to porous fruits and fruit powders. In this case both the
volumetric fraction and the spatial distribution of each phase are to be considered, which is
done by means of distribution factors adequately described.
Finally, a third level of complexity is achieved when different food materials, including
fruits, are processed together to give composite food. This group includes all kinds of canned
and packed foods, pastries, confectioneries, and a wide variety of prepared foods. Once more,
modeling requires the information of the mean or effective values of the components together
with the representation of the physical structure.
As previously mentioned, the value of the thermophysical property will be a function of
the temperature, through the dependence of the components, and porosity or water content,
for porous or composite foodstuffs. Since water can be either liquid or solid, particular
attention is paid to frozen fruit products.
Available information may be contradictory, due to the different conditions at which
thermophysical properties were gathered, as well as to the differences among fruits of
different origin, composition, and structure.
4
.
Thermodynamical, Thermophysical, and Rheological Properties
77
4.4.1. Density*
Density (r) is the unit mass per unit volume. SI unit for density is [kg=m3 ]. In particular,
when the fruit or fruit product is a porous solid, density plays an important role in
heat transfers intrinsically or through the definition of porosity. A few definitions are
necessary (Lozano, 2005):
.
.
.
Substance density: rs (or true density), is the density measured when the substance has been
broken, milled, or mashed to ensure that no pores remain.
Particle density: rp , is the density of a sample that has not been structurally modified. In the
case of pores not externally connected to the surrounding atmosphere, particle density will
include these close pores.
Bulk density: rb (or apparent density), is the density measured so as to include the volume
of the solid and liquid materials, and all pores, closed or open to the surrounding atmosphere.
As other authors (Maroulis and Saravacos, 1990; Farkas and Singh, 1991) have used different
terms for the same condition, it is recommended to verify the definition of density before
using density data.
4.4.1.1. Porosity*
Porosity indicates the volume fraction of air (or void space). On the basis of the given
densities, the following definitions of porosity have been proposed (Lozano et al., 1980;
Lozano, 2005):
.
.
Total porosity («t ) is the ratio of air space volume to total volume:
«t ¼ (rs rb )=rs
(4:6)
Open pore porosity («a ) is the ratio of the volume of pores connected to the outside to the
total volume:
«a ¼ (rp rb )=rp
(4:7)
As in the case of density, it is recommended to verify definitions before using porosity data.
4.4.1.2. Density Measurement Methods
Techniques developed for density measurement are basically methods for the measurement of
volume, weight being easily measured with different types of precision balances. The principal
measurement techniques applied for volume (and density) determination in fruit and fruit
products are:
.
Hydrometric method: The bulk density is calculated from the apparent weight of the sample
and the buoyant force E.
(4:8)
rb ¼ rliq (Wair =E)
Fruit tissue must be coated to avoid mass loss by dilution or pore inundation (Lozano et al.,
1980).
* Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering
Properties of Foods’’. Edited by Gustaro V. Barbosa-Cánovas. Paris UNESCO Publishing. ISBN 92-3-1039997pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright)
EOLLS Publishers Co.Ltd.
78
.
Fruit Manufacturing
Geometric method: The volume is calculated from the dimensions (L) of the food
sample.
rb ¼ L3 =W
(4:9)
It is not suitable for soft and irregular solid foods.
.
Pycnometry: Liquid pycnometry. The pycnometer is a calibrated flask that allows the
weighing of an exactly known volume of liquid, which in turn gives the density. The weight
is determined by difference with the empty flask. To set the experimental temperature, the
pycnometer is immersed in a constant temperature bath and filled with the sample.
Gas pycnometry. Using perfect gas law it is possible to determine the volume of open
pores (Vpore ) in food by determining the gas volume in a chamber (Vch ) with and without the
sample:
Vpore
Pi Vi ¼ mRT
¼ Vch (P1 P2 )=P2
(4:10)
P1 and P2 are pressure in empty space and in sample chamber,respectively (Mohsenin, 1980;
Lozano et al., 1980).
Table 4.2 lists bulk density values of two fruits (apple and pear) and fruit tissue
components. These values cover a relatively wide range of density, illustrating the influence
of water and air (porosity) in this property.
4.4.1.3. Empirical Equations and Theoretical Density Models
Some empirical equations for the calculation of bulk density of selected fruits, in terms of
temperature and water content, are listed in Table 4.3.
Constenla et al. (1989) proposed a theoretical approach by considering the thermodynamic expression for the specific volume of a multicomponent solution (reciprocal of
density) in terms of partial specific volumes:
X
V ¼ 1=r ¼
w i vi
(4:11)
where wi and vi ; represent the mass fraction and the partial specific volume of the i-component
in solution, respectively. Sugars, organic acids, and different macromolecules interact with a
substantial number of water molecules, resulting in a nonideal solution behavior. Therefore,
specific volume is not necessarily equal to the specific volume of the pure component.
Table 4.2. Bulk density of selected fruits and fruit products or components.
Fruit/component
Apple (GS; Xw ¼ 0:86)
Pear
Cellulose
Fat
Glucose (solid)
Protein
Water
Temperature/range (8C)
Bulk density (kg=m3 )
25
25
—
—
—
—
4
837
990
1,550
900/950
1,560
1,400
1,000
Reference
Lozano et al. (1980)
Lozano et al. (1983)
Kirk–Othmer (1964)
Lewis (1987)
Kirk–Othmer (1964)
Lewis (1987)
Perry and Green (1973)
4
.
Thermodynamical, Thermophysical, and Rheological Properties
79
Table 4.3. Empirical equations to calculate bulk density of selected fruit products or fruit
components.
Application range
Equation
Xw (kg=kg)
rb ¼ a þ T þ cT 2
rb ¼ a þ bXw þ þcXw2
rb
rb
rb
rb
Parameter
Product
¼ a þ b ln Xw
¼ a bxr þ c 106 e(1:33xX =Xo )
¼ a þ þbe(0:01 BrixþcT)
¼ 0:852 0:462e0:66X( Brix)
n. a.
Pistachio
Coconut
(shredded)
Orange juice
Apple (G. Smith)
Pear
Apple juice
Apple
T (8C)
0.05/0.40
—
—
—
0.09/0.65
0.8/6.6
0.15/7.0
0.25/1.0
21
Ambient
Ambient
10/90
Ambient
a
b
439
236
5.003
440
0.994
0.636
1.251
0.828
0.852
0.307
0.102
0.153
0.3471
0.462
c
—
—
0.282
—
0.107
5:4794
From Lozano et al. (1979, 1983, 2002, 2005), Constenla et al. (1989), Moresi and Spinosi (1980), Jindal and Murakami (1984),
Hsu et al. (1991).
In the dilute limit, vw (water) has contributions mainly from structured free-solvent regions,
while vs (solute) is affected by hydration and water–solute interactions. In the concentrated
limit, vw is defined by water–solute aggregations, i.e., hydrogen bonded to hydroxyl groups.
For these reasons, in sugar solutions both vw and vs are functions of concentration and
temperature (Taylor and Rowlinson, 1955; Maxwell et al., 1984).
Constenla et al. (1989) also suggested that the thermal effect on density could be
significantly reduced by referring the specific volume to that of pure water vwo , so Eq.
(4.11) can be written as:
V =vwo ¼ Vw =IVwo þ ws (vs vw )=vwo
(4:12)
Although according to the above discussion the partial specific volumes depend on
concentration, from a practical point of view a linear relationship as suggested in Eq. (4.11)
can be used to correlate density data, as Constenla et al. have found for apple juice:
r ¼ rw =(0:992417 3:7391 103 X )
(4:13)
r2 ¼ 0:9989
Predictions of this equation were also extrapolated to temperatures in the range 10–908C.
Perez and Calvelo (1984) proposed the following semiempirical equation for the bulk
density calculation of beef muscle, during cooking:
rb ¼
h
(1 Xw ) 1 rbo (1 Xwo )
rbo
rw
v
wo )
þ rrbo (1X
(1Xw ) þ u(Xwo Xw )
i
(4:14)
b
where u and v are empirical parameters.
Changes in food density by freezing were predicted by Hsieh et al. (1977) as follows:
1
1
1
1
¼ Mu
þ Ms
þ MI
(4:15)
r
ru
rs
rI
where M is the mass fraction of unfrozen water (u ), ice (I ) and solids (s ). The most significant
change in density occurs immediately below the initial freezing temperature.
80
Fruit Manufacturing
4.4.2. Specific Heat
Specific heat is the amount of heat required to increase the temperature of unit mass by unit
degree at a given temperature. SI unit for Cp is [kJ/kg/K]. Specific heat of solids and liquids
depends upon temperature but is not sensitive to pressure, as it is incompressible to practical
purposes. It is common to use the constant pressure specific heat, Cp , which thermodynamically represents the change in enthalpy for a given change in temperature when it occurs at
constant pressure:
Cp ¼ (dH=dP)p
(4:16)
where H is enthalpy.
4.4.2.1. Measurement Methods
Several methods have been used for the specific heat measurement (Rahman, 1995). Both
differential scanning calorimetry (DSC) and the method of mixtures are commonly used
techniques (Table 4.4). The advantages of DSC are that measurement is rapid, and a very
small sample can yield accurate results for homogeneous products (Wang and Kolbe, 1991).
Specific heat of selected fruits is listed in Table 4.5.
4.4.2.2. Prediction Models and Empirical Equations
Mohsenin (1980) proposed an equation valid for the calculation of Cp of meats, fruits,
vegetables, and other foods, which equals the sum of the specific heat of water (Cpw ) and
solid matter (Cpsm ):
Cp ¼ Cpsm þ (Cpw Cpsm )xW
(4:17)
Table 4.4. Methods for specific heat measurement (Lozano 2005, with permission).
Method
Principle of operation
Mixture
A sample of known mass
(Ws ) and temperature (Ts )
is dropped into a calorimeter
of known specific heat, containing
a liquid (usually water) of known
mass (Wref ) and temperature(Tref ).
Temperature of the mixture is recorded
until equilibrium (Teq )
This method is based on the
determination of the amount of
heat required to raise the temperature
of sample of known mass (Ws ), at a
given rate within a given interval.
The measurement requires an
external standard of known
mass (Wref ) and specific heat (Cpref )
DSC
Governing equation
Additional comments
Cp ¼
Cpref Wref (Tref Teq )
Ws (Teq TS )
Numerous calorimeters
were developed to reduce
heat loss, heat generation
by mixture and mixing
problems
Cp ¼
Cpref Wref d
Ws dref
Reduced sample size and
escape of water vapor
during heating are
important limitations of
this technique
Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’
Edited by Gustaro V. Barbosa-Cánovas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.
4
.
Thermodynamical, Thermophysical, and Rheological Properties
81
Table 4.5. Specific heat of selected fruits and fruit products and components.
Product
Apple juice
Apple, RD whole
Apple sauce
Banana
Orange juice
Sugar
1
Xw
DT (8C)
Cp (kJ=kg1 =K )
758Brix
758Brix
108Brix
0.75/0.85
—
74.8
0.105
0.133
30
90
30
—
—
—
20/40
59.1
2.805
2.973
3.894
3.95
3.73
3.35
1.85
1.256
From Polley et al. (1980), Constenla et al. (1989), Gupta (1990), and Rahman (1995).
A theoretical approach can also be used to predict the specific heat of a solution in terms of
partial specific heats of individual components as follows:
X
Cp ¼
Cpi wi
(4:18)
A linear relationship of Cp with concentration as suggested by Eq. (4.18) is the basis for most
of the existing correlations to evaluate specific heat of liquid foods (Choi and Okos, 1983).
However, due to water–solute interactions, Cpi differs from the specific heat of pure component and usually changes with the concentration of soluble solids.
Actually, the resulting values of Cps for sugar solutions are significantly higher than those
corresponding to crystalline sugar at the same temperature (Taylor and Rowlinson, 1955;
Pancoast and Junk, 1980), while at high water contents Cpw approximates to the heat capacity
of pure water Cpwo . In addition, while Cpwo remains almost constant with temperature, the
specific heat of the solution increases with this variable following the same pattern as that of
crystalline sugar. This behavior was also observed in apple juice (Constenla et al., 1989), so no
improvement in the correlation ability of Eq. (4.18) may be obtained by using the ratio
Cp =Cpwo .
Heldman (1975) proposed an expression for heat-capacity calculation of foods, based on
the composition:
Cp ¼ 4:180 (0:34Xca þ 0:37Xp þ 0:4XFA þ 0:2Xas þ 1:0Xw
(4:19)
Although widely used, Eq. (4.19) shows deviation when compared with experimental values,
due to, among other conditions, the variation of Cp between bound and free water, and the
excess specific heat due to the interaction of the component phases. Rahman (1993) proposed
corrections reducing some of the limitations of Eq. (4.19). Below freezing point the calculation is more difficult. Rahman (1995) cited Van Beek equation, which is considered to be
applicable to all foods below the freezing temperature:
Cp ¼ Cpso (1 Xwo ) þ Cpw Xwo (Tfreezing =T) þ Cpice Xwo (1 Tfreezing =T)
LXwo (Tfreezing =T 2 )
(4:20)
Schwartzberg (1976), Chang and Tao (1981), and Mannapperuma and Singh (1989),
among others, presented more complete equations for the prediction of heat capacity of foods
below freezing point. Some other models applicable for different foods and conditions are
listed in Table 4.6.
82
Fruit Manufacturing
Table 4.6. Models for predicting heat capacity of fruit products and components.
Application range
Equation
Parameter
Fruit/component
Xw (Kg=kg)
Cp ¼ a þ bT þ cT 2
Cp ¼ a þ bXW
Cp ¼ a þ bXw þ T
Cp ¼ 1:56e0:945X w
Sugars
Ash
Fiber
Fats
Ice
Proteins
Water
Fruits and
vegetables
Food in general
Fruit pulps
T (8C)
a
b
c 106
40/150
1.548
1.093
1.846
1.984
2.063
2.008
4.176
1.670
1:962:103
1:890:103
1:831:103
1:473:103
6:077:103
1:209:103
9:086:105
2.500
5.934
3.682
4.651
4.801
—
1.313
5.473
—
2.477
—
2.356
—
40/150
>0.5
0.001/0.80
0.012/0.945
30/60
20/40
3.791
—
* Adapted from Dickerson (1969), Mohsenin (1980), Gupta (1990), Alvarado (1991), Choi and Okos (1986), Lozano (2005).
4.4.3. Thermal Conductivity
Thermal conductivity (k) is an intrinsic property of material and represents the quantity of
heat that flows in unit time through a plate of unit thickness and unit area having unit
temperature difference between faces. SI unit for k is [W/m/K]. Figure 4.2 shows thermal
conductivity values measured in selected fruits at ambient temperature, while Fig. 4.3 shows
the influence of temperature on k.
4.4.3.1. Measurement Methods
Techniques for measurement of thermal conductivity can be classified as: (i) steady state, (ii)
quasisteady state, and (iii) transient.
0.7
0.6
78%
87% 88%
85%
89%
0.5
k (W/m K)
100%
86%
0.4
0.3
30%
0.2
0.1
Water
Strawberry
Orange juice
Pear
Apple
Pineapple
Applesauce
Plum, dried
0
Figure 4.2. Measured k of selected fruits and fruit products at ambient temperature.
4
.
Thermodynamical, Thermophysical, and Rheological Properties
83
2
Sucrose (Xw=75)
Apple (GS)
k, W/m K
1.5
1
0.5
0
60
40
20
0
20
40
60
80
Temperature, ⴗC
Figure 4.3. Influence of temperature in the thermal conductivity of selected foods (Keppler and Boose, 1970;
Kent et al., 1984; Constenla et al., 1989).
These techniques were extensively reviewed and compared by Rahman (1995). Table 4.7
describes different methods for k determination. Steady-state techniques (SST) are wellestablished methods based on the determination of constant heat flux under constant temperature gradient and the solution of the unidirectional steady-state heat conduction equation
(Fourier’s equation) for k calculation.
Although SST are mathematically simple, highly experiment controlled, and precise, they
are practically not applicable in foods due to long equilibrium time (biological active samples,
e.g., fruits, deteriorate), the need of geometrically defined samples (most foods are amorphous
and/or soft), hence convection must be avoided (this excludes liquids or high-moisture foods).
4.4.3.2. Prediction Models and Empirical Equations
Several empirical, semiempirical, and theoretical models and equations have been developed
to predict thermal conductivity of composed material in general, and foodstuffs, in particular.
Tables 4.8 and 4.9 list the most commonly used predictive models and empirical equations,
respectively, valid for fruits and fruit products.
4.4.4. Thermal Diffusivity
Thermal diffusivity (a) is a combination of three thermophysical properties that result from the
derivation of the Laplace equation of heat conduction (Fourier equation in three dimensions):
a ¼ k=rcp
(4:21)
Physically it represents the change in temperature produced in a unit volume of unit surface
and unit thickness, containing r[kg] of matter, by heat flowing in the unit time through the
unit face under unit temperature difference between opposite faces. Figure 4.4 shows thermal
diffusivity values measured in selected fruits at ambient temperature.
>
>
>
>
>
>
>
>
>
:
Thermal
comparator
8
>
Line source
>
>
>
>
>
>
>
>
<
8
>
>
<Fitch (1935) and
further
>
>
: modifications
Method
8
>
Guarded hot
>
>
>
>
plate
>
>
>
>
>
>
>
>
>
<
Concentric
>
>
cylinders
>
>
>
>
>
>
>
>
>
>
>
>
: Heat flux
Zuritz et al. (1987) and Rahman (1991)
modified Fitch’s method for small
individual food particles and frozen foods
Thermal conductivity probe is a test body
made basically of a line source, providing a
constant amount of heat and a
temperature-measuring device. Alternative
designs of the instrument have been
discussed by Sweat (1974) and Hayashi et al.
(1974)
A probe is equilibrated to a higher
temperature than the sample. Then the
probe is placed in contact with food and it
changes temperature at a different rate
increasing emf, which is related by
calibration to k
T and T1 , and t and t1 , are
temperatures and times
corresponding to final and initial
time, Q is heat produced per unit
length of probe
Calibration is required with a
number of materials of known
thermal conductivity
emf ffi k1=2
T i , Ts , mco , and cpco are initial
temperature, source
temperature, copper mass, and
copper Cp , respectively.
T T1 ¼ (Q=4kK) ln (t=t1 )
To do this, time and
temperature are correlated by
the model equation:
k ¼ Q=4ks
ln [(T i Ts )=(T Ts )]
¼ kAt=lmco cpco
k is calculated from the
slope of the plot of
ln [(T i Ts )=(T Ts )]vs:t
k ¼ Wl=(Ti To )
W is the heat flux (W=m2 ).
To calculate k, inner and outer
temperatures (Ti ,To ), sample
thickness (l) and heat quantity
(Q) must be measured
Same as previous
Q ¼ kA(Ti To )=l
Heat source surrounded by sample, and in
turn surrounded by a heat sink. Insulation
is located at ends to avoid heat loss and
ensure unidirectional heat conduction
Usually heat source is the outer cylinder and
heat sink the inner cylinder. Heat absorbed
by coolant is the same as the heat
conducted through sample.
It is based on temperature-gradient
determination across sample.
k is evaluated at (Ti To )=2
Sample is ‘‘sandwiched’’ in between a
constant temperature heat source and a
copper plug as heat sinks insulated on all
faces but one
Q ¼ kAo [(To Ti )=
ro ln (ri =ro )]
Additional comments
Governing equation
Principle of operation
Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’ Edited by Gustaro V. Barbosa-Cánovas. Paris UNESCO
Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.
Transient
Quasisteady
state
Steady state
Technique
Table 4.7. Description of different methods for k determination (Lozano 2005, with permission).
4
.
Thermodynamical, Thermophysical, and Rheological Properties
85
Table 4.8. Predictive models for thermal conductivity estimation applicable to fruit products
(Lozano 2005, with permission).
Model
Description
Equation
Series
In series distribution, layers of components
are thermally in series with respect of heat
flow. It is applied in food gels.
Layers are considered as thermally in parallel
with respect to the direction of heat flow.
It is Proposed for liquid and powder foods
Based on random distribution of noninteractive
spherical particles in a continuous medium
The simplest is based on the weighed geometric
mean of components, using the volume fraction
as the weighing factor
A model of statistical nature that considers a
heterogeneous medium as represented by a virtual
homogeneous one with the same properties. kp is
the thermal conductivity of pores
1=ke ¼
Parallel
Maxwell
Random
Effective
medium theory
ke ¼
P
P
(fi =ki )
(fi ki )
km ¼ kc
kd þ 2kc 2fa (kc ka )
kd þ 2kc þ 2fd (kc kd )
k ¼ Pfi
i
P
fi [(k ki )=ki 2k)] ¼ 0
For porousp
materials:
ffiffi
k ¼ kp [b (b2 þ 2m)]
b ¼ 3e 1 þ [3(1 e) 1]
Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’
Edited by Gustaro V. Barbosa-Cánovas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.
4.4.4.1. Measurement Techniques
The estimation of thermal diffusivity of foods can be done by: (i) direct measurement or
(ii) indirect calculation using Eq. (4.21). Several direct methods for a determination were proposed (Rahman, 1995) based on the solution of one-dimensional unsteady state
heat transport equation with the appropriate boundary conditions for different geometries.
Fourier equation has been solved for numerous conditions, and graphical solutions are
also available (Bird et al., 2002). Fourier equation is limited to temperatures above freezing
and restricted to homogeneous, isotropic substances. However, analytical and numerical
solutions of the one-dimensional heat conduction equation have been used to determine the
thermal behavior of foods. These techniques are similar to those used for k determinations, in
particular the thermal probe. Table 4.10 lists some of the proposed direct techniques for the
determination of a.
Indirect method for determination of r, k, and Cp values needs more time and
instrumentation. It was indicated that indirect determination yielded statistically more
accurate values of a (Drouzas et al., 1991). Andrieu et al. (1989) found roughly 3%
difference when determining thermal diffusivity in potato using both pulse and hot wire
(probe) methods.
4.4.4.2. Empirical Equations
Several empirical equations have been developed to predict thermal conductivity of fruits,
and fruit products and components. Table 4.11 lists some empirical equations.
4.4.5. Viscosity
Fluid and semisolid foods exhibit a variety of rheological behaviors ranging from Newtonian
to time dependent and viscoelastic. Whereas an ideal elastic solid produces an elastic
86
Fruit Manufacturing
Table 4.9: Empirical equations for calculating thermal conductivity of fruits and fruit components.
Application range
Equation*
k ¼ a þ bT
þcT 2
Fruits or fruit
component
Xw (kg=kg)
40/150
Carbohydrates
Ash
Fiber
Fats
Ice
Proteins
Water
Apple (GD)
k ¼ a þ bXw
þcXw2
k ¼ a þ bXw þcT
k ¼ kwo r=rw
[a bXw ( Brix)]
¼ 1:821:66
[esp(0:85) XXwow ]
keff ¼ 0:58Xw
þ0:155Xpr
þ0:25Xca þ
þ0:135Xas
þ0:16XFA
kef
ko
T (8C)
40/150
25/0
0/25
30/70
Starch (gelatinized)
Starch
Pears
Ambient
Tomato paste
Apple juice
Apple
0.54/0.71
0/758Brix
Fresh
Apple juice
0/758Brix
30
10/80
T > Tfreezing
T Tfreezing
T < Tfreezing
10/80
Sucrose
Glucose
Fructose
Fruits and meats
0/75
0/75
0/75
0.05/0.88
10/80
10/80
10/80
20/25
Liquid and solid
foods (more
than 400 data)
—
Parameters
a
0.2014
0.3296
0.1833
0.1807
2.2196
0.1788
0.5712
1.290
0.394
0.210
0.478
0.4875
0.029
0.2793
0.994
10.3
0.378
2.5289
b 103
1.387
1.401
1.250
0.276
6.249
1.196
1.763
9.50
2.120
0.410
6.90
0.057 Xw 2
0.793
3.572 (8Brix)
15.9
0.133
1.376
1.0052
c 106
4.331
2.907
3.168
0.177
101.5
2.718
1.015
—
—
—
—
0.0227
( ln Xw )Xw2
1.135 (K)
2.500
10.300
0.930 T 2
—
2.6122
2.5617
2.4153
—
1.0381
0.9725
0.9807
—
—
—
—
—
—
—
—
*Equations must be checked for application ranges and influence of porosity.
Adapted from Ramaswamy and Tung (1981, 1984), Drouzas and Saravacos (1985), Mattea et al. (1986), Sweat (1986), Constenla et al.
(1989), Marouilis et al. (1991), Rahman (1991), Renaud et al. (1992), Choi and Okos (1996).
displacement when shear stress is applied, a fluid produces viscous flow. If a liquid is held
between two parallel infinite plates and the top plate moves at a velocity U (length/time)
relative to the bottom plate, the force required to maintain this motion will produce a viscous
flow and a velocity gradient, which is equivalent to the shear rate (g ¼ dU=dx) developed.
Under this condition viscosity can be defined as:
g ¼ t=ma
(4:22)
where ma is called the apparent viscosity and is constant only for this one value of g. If ma is a
constant at different values, then:
t¼m
where m is the Newtonian viscosity of the fluid.
(4:23)
4
Thermodynamical, Thermophysical, and Rheological Properties
.
87
Thermal diffusivity (107 m2/s)
1.7
1.6
Water
1.5
Sucrose
1.4
Strawberry
1.3
Apple
Peach
1.2
Banana
1.1
0.7
0.75
0.8
0.85
0.9
0.95
1
Xw
Figure 4.4. Thermal diffusivity of selected fruits and fruit components.
Newtonian behaviors indicate that the viscosity of the food is shear-independent. Other
than water, Newtonian flow is exhibited by sugar solutions and vegetable oils also. Viscosity
of Newtonian foods has the unit Pas in the International System. Table 4.12 lists definitions
of interest in the rheological study of foodstuffs.
Most of the foods show more complicated relationships between shear rate and shear
stress. It is no longer feasible to talk in terms of viscosity, since m varies with the rate of shear.
Table 4.13 shows types of non-Newtonian fluids:
(1) Those whose properties are independent of time of duration of shear,
(2) Those whose properties are dependent on time of shear, and
(3) Those exhibiting characteristics of a solid.
4.4.5.1. Measurement Techniques
Methods of viscometry (measurement of apparent viscosity) are described in Table 4.14.
Viscometers are based on the measurement of either the resistance to flow in a capillary tube,
Table 4.10. Principle of operation and governing equation for the two most commonly used methods
for the determination of thermal diffusivity in fruit products.
Method
Principle of operation
Analytical solution
of Eq. (4.9)
Sample is located in a cylinder (L=D > 4)
immersed in a water bath at constant
temperature. Thermocouples located
at the center of the sample (axis) and
surface of cylinder measure are T vs. t.
After transition, both temperature
gradients are time independent
Similar to thermal conductivity probe,
with additional thermocouple placed at
a known distance in the sample
Probe method
Governing equation
a ¼ VR2 =[4(Ts Tc )]
T ¼ (l=2pk)[ 0:58=2 ln g þ g 2
pffiffiffi
=2:1 g 4 =4:2! þ . . . ]g ¼ r=(2 at)
Dickerson (1965), Hayakawa (1973), Uno and Hayakawa (1980), Singh (1982), Gordon, Lozano (2002) and Thorne (1990); Nix et al.
(1967).
88
Fruit Manufacturing
Table 4.11. Empirical equations for the calculation of the thermal diffusivity of selected fruits
and fruit products.
Application range
Equation
Product/component
a ¼ a þ T þ cT 2
Ash
Carbohydrates
Fats
Fiber
Ice
Proteins
Water
Apple
a ¼ a þ bXw þ cXw2
a ¼ a þ bXw þ cT
a ¼ 0:88:107 þ aw Xw
0:88:107 :Xw
Corn
Multiple regression
Multiple regression
Parameter
T ( C)
a 10
b 104
c 106
40/160
40/160
40/160
40/160
40/160
40/160
40/160
25=10
10=Tfreezing
T > Tfreezing
20/90
8/23
—
0/80
12.46
8.08
9.88
7.39
1.17
6.87
13.17
1:22:105
4:37:105
1:39:105
1:51:105
9:56:105
0:786:105
—
3.73
5.30
1.26
5.19
6.08
4.76
6.25
0:187:103
0:437:103
0:278:105
1:34:106
1:83:102
0:574:103
—
1.22
2.32
0.039
2.22
95.0
1.46
2.40
—
—
—
—
0.44
0:29:103
—
2
Adapted from Lozano (2002), Riedel (1980), Ramaswamy and Tung (1981), Choi and Okos (1986), and Rahman (1995).
or the torque produced by the movement of an element through the fluid. There are three
main categories of commercially available viscometers applicable to foodstuffs: Capillary,
falling-ball, and rotational viscometers. Of late, food scientists and technologists use rheometers available at a relatively low cost, which can measure over wide ranges of shear
behavior and perform complete rheograms, including thixotropic recovery, stress relaxation,
or oscillatory experiment at programmed temperature sweep.
For Newtonian liquid foods it is sufficient to measure m as the ratio t=g. Besides the ratio
of shear stress and rate of shear, the properties required to describe a non-Newtonian material
can be measured by:
(a)
(b)
(c)
(d)
compression (force–deformation relationship),
creep test (stress versus strain as a function of time),
stress relaxation (stress required to maintain a constant strain), and
dynamic test (deformation by a time variable stress, generally oscillatory stress).
Table 4.12. Definition of different types of viscosities.
Name
Equation
Comments
Kinematic viscosity
Relative vis The reference Dickerson
(1969) is not given in the reference list.
Please check.cosity
Specific viscosity
Reduced viscosity
Intrinsic viscosity
y ¼ m=r
mr ¼ m=mo
In (cm2 =s stoke); where r is density
It is the ratio of solute to solvent viscosity at
equal temperatures
msp ¼ mr 1
mred ¼ msp =c
[m] ¼ ( ln mr =c)[c!o]
Where c is the concentration of solute
Also called limiting viscosity number, which is
usually correlated with molecular weight
4
.
Thermodynamical, Thermophysical, and Rheological Properties
89
Table 4.13. Types and examples of non-Newtonian foods (Lozano 2005, with permission).
Rheological
classification
Description
Descriptive diagram
Non-Newtonian time-independent foods
Bingham plastic
foods (ideal plastic
material)
Linear relationship between g and
t does not go through the origin.
The t value at g ¼ 0 is the yield
value or yield stress (ty )
Shear-thinning
behavior
(pseudoplastic
foods)
When g increases with t. When a
food is pseudoplastic above yield
stress is also known as
mixed-type plastic
Shear-thickening
behavior
(dilatant foods)
Their rheological behavior is
opposite to the pseudoplastic in
which g decreases as t increases
Examples
Most foods
t
Cocoa butter
ty
.
γ
t
.
γ
t
.
γ
Most of non-Newtonian
foods (fruit purées,
condensed milk,
ketchup, etc.)
Starch suspensions and
some chocolate syrups
exhibit dilatant flow
Time-dependent foods
Thixotropic foods
Rheopectic materials
Semisolid foods
Rate the shear stress value decrease
with time at constant shear,
while the structure collapses
Include those few materials that are
able to build up (or set up) while
submitted to a shear stress at
constant.
These foods show both solid
(elasticity) and fluid (viscosity)
behavior when they are subjected
to a sudden, instantaneous,
constant shear stress; sufficient
time is allowed for the test; and
the stress is large enough to
prevent the food showing pure
elasticity. During flow, normal
stresses s are built up
ma
Some fruit pulps
t
ma
t
The viscoelastic
behavior of foodstuffs
is commonly
explained by two
basic tests: stress
relaxation and creep
(increase of strain
with time)
Whey protein polymers
have strong
rheopectic properties
Cheeses
Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’
Edited by Gustaro V. Barbosa-Cánovas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.
4.4.5.2. Newtonian Fruit Products
Liquid foods, such as clarified fruit juice, exhibit Newtonian behavior. As an early approximation viscosity of Newtonian foods can be estimated as the viscosity of water (mw ) and that
of the prevalent soluble solids. Different empirical equations relating liquid food viscosity
with both soluble solids and temperature were published (Rao, 1977). The viscosity of water,
90
Fruit Manufacturing
Table 4.14. Methods for determination of viscosity.
Method
Description
Governing equation*
Capillary tube
viscometers
The time for a standard volume of fluid
to pass through a length of capillary tube
is measured. The driving force is gravity,
gas pressure, a descending piston, or partial
vacuum at the exit. The force used to create
flow is usually gravity as seen in Ostwald and
Cannon–Fenske viscometers
m ¼ pDPr4 t=8Vl
Falling-ball
viscometers
Rotational
viscometers
A standardized tube is filled with the product,
and the time under the influence of gravity for
a ball to pass between two specified points is
measured. The falling ball reaches a limiting
velocity when the acceleration is exactly compensated
by the friction of the fluid on the ball. The rolling-ball
instrument uses a tube inclined at a 10-degree angle,
which allows the ball to remain in contact with the
inner tube surface. The drawn-ball type uses a ball
mechanically pulled upward through the tube.
These viscometers accurately measure low- to
medium-viscosity Newtonian fluids. The drawn ball
may allow the measurement of opaque fluids
These instruments can determine the viscosity
of Newtonian and non-Newtonian fluids
contained between two coaxial cylinders
(bob and cup), or different geometries as the
cone and plate geometry, by measuring the drag
of the fluid on a mobile member (cylinder or cone)
while the other member (cylinder or plate) remains
stationary. They produce precise measurements of
absolute viscosity for a wide range of viscosities.
Because the shear rate can be varied, it is possible
to plot the flow curves of non-Newtonian fluids.
Time effects can be studied either manually or
automatically with computerized controls
where
DP is the driving force (pressure),
r the capillary radius,
V the volume, and
l the capillary length.
For Newtonian liquid foods
n ¼ Kt where
K is a constant
m ¼ K(rball rs )=n
where
K ¼ (0:374gD(D þ d) sin u, and
d is the diameter of the ball,
D the diameter of the tube,
g the acceleration of gravity, and
u the angle of tilting of the tube
Coaxial cylindersþ :
m ¼ GT (1=R21 1=R22 )=4plv
Cone plate:
m ¼ 3GT u=(2pR3 v)
where GT is the torque of the bob,
v the angular velocity,
R1 the radius of the bob,
Ro the radius of the cup, and
R the radius of the plate
u the cone angle,
v the angular velocity,
l the cylinder length.
*Adapted from Slattery (1961), Van Waser et al. (1963), Johnson et al. (1975), and Bourne (1982).
and salt and sucrose solutions, major solutes in foodstuffs, can be calculated by Eqs. (4.24),
(4.25), and (4.26), respectively (Kubota et al., 1980):
ln mw ¼ 0:266 2:02 102 T þ 4:4 105 T 2
(4:24)
where T is the temperature in 8C.
m ¼ a exp (b=T n )
(283:2K < T < 323:2K)
(0 < X < 40 Brix)
where log a ¼ 0:00458X 1:15 3:05
and b ¼ 9:90 104 X 1:51 þ 6:1 107
(4:25)
4
.
Thermodynamical, Thermophysical, and Rheological Properties
m ¼ aeb=T
91
n
(283:2K < T < 323:2K)
(0 wt% < X < 24 wt%)
(4:26)
where log a ¼ 3:59103 X 1:33 2:0
and b ¼ 3:09105 X 1:59 þ 6:1107
In another approach Rao et al. (1984) reported that the effect of concentration on
viscosity of fruit juices at constant temperature may be represented by an exponential-type
relationship. Constenla et al. (1989) modified the Mooney (1951) equation in order to express
the concentration on a weight basis (X), as 8Brix, and to take into account the effect of
temperature:
ln
m
A(T)X
¼
mw 100 B(T)X
(4:27)
where coefficients A and B are temperature dependent (Table 4.15).
As Fig. 4.5 shows, viscosity of apple juice expressed as reduced viscosity (m=msucrose ), lies
between the corresponding curves for sucrose and reducing sugars. The same figure also
includes the predictions of Eq. (4.27), for a model system made with sugars in the same
proportions present in apple juice. Although Eq. (4.27) gives a reasonable estimate of the
behavior of apple juice, differences are not insignificant and were associated with the presence
of nonsugar organic components, which usually tend to increase the viscosity.
Some discrepancies were attributable to the effect of malic acid on the refractometric
measurement of soluble solids (Millies and Burkin, 1984). It was observed that viscosity data
of clarified pear juice (Ibarz et al., 1987) also lie between those of sucrose and reducing sugars.
On the other hand, grape and orange juices containing some suspended colloids, mainly
pectin, tartrates, and citrates, were more viscous than sucrose solutions; and cloudy apple,
grape, and orange juices, which contained a significant amount of pulp and suspended
particles, were pseudoplastic (Saravacos, 1970; Moresi and Spinosi, 1980, 1984; Rao et al.,
1984). Thus, it appears that Eq. (4.27) and sugar solutions’ data can be applied to estimate
viscosity only in the case of clarified fruit juices. The presence of suspended material not only
increased the viscosity but also changed the rheological behavior of the product, so a different
approach must be used in that case.
4.4.5.3. Non-Newtonian Fruit Products
Viscolastic and semisolid foods have been extensively studied during the last few decades.
Rheological characterizations of non-Newtonian foods have been in the form of t versus g
curves, dynamic characteristic, time effect on h at constant, g, etc. Values for these parameters were compiled by different authors (Rao, 1977; Kokini, 1992). The following creep
Table 4.15. Parameters of Eq. (4.27) valid for the determination of viscosity
at 208C.
Coefficient
A
B
Glucose
Fructose
Sucrose
2.562
0.972
2.415
0.981
2.612
1.038
Adapted from Constenla et al., 1989.
92
Fruit Manufacturing
1.1
Reduced viscosity (m/m sucrose)
1
0.9
0.8
0.7
Apple juice
0.6
Fructose
Eq.(4.27)
0.5
Glucose
0.4
0.3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Concentration (w/w)
Figure 4.5. Effect of concentration on the reduced viscosity of apple juice at 208C (Constenla et al., 1989)
with permission.
compliance vs. time equation (Sherman and Sherman, 1966), proposed for the description of
the rheological behavior of ice cream, is a representative example of a model for description of
the viscoelastic behavior of semisolid foods:
J(t) ¼ J0 þ J1 (1 ett ) þ J2 (1 ett2 ) þ t=h
(4:28)
where J ¼ g=t is the creep compliance, J0 is the instantaneous elastic compliance, J1 and J2
are the compliances associated with retarded elastic behavior, t 1 and t 2 are retardation times,
associated with retarded elasticity, and h is the viscosity associated with Newtonian flow.
In Table 4.16 selected experimental data of viscosity and values of power law, and other
rheometric parameters for fruit and tomato products are listed.
4.4.5.4. Effect of Temperature and Pressure on the Viscosity of Foodstuffs
Viscosity–temperature dependence is frequently represented by the Arrhenius–type equation:
ln m ¼ k0 þ DEa =RT
(4:29)
where k0 is a pre-exponential factor, DEa is the activation energy of flow, and R is the gas
constant. Ultracki (1974) presented the following empirical equation:
ln m ¼ k0 þ A=(T T0 )
(4:30)
where k, A, and T0 are constants.
During extrusion and other food processing operations relatively high pressures are
applied. In such a case, the calculation of the viscosity at pressures different from those
published may be necessary:
m ¼ m0 eaP
(4:31)
4
.
Thermodynamical, Thermophysical, and Rheological Properties
93
Table 4.16. Viscosity, power-law (n and K) parameters, and yield stress (t y ) of
selected foods at ambient temperature.
Fruit product
m or ma , mPa/s
Apple sauce
Pear juice (708Brix)
Pectin, 0.5 (wt%)
Tomato concentrate (30 wt%)
Tomato paste (308Brix)
n
K Pa=sn
tyPa
0.15–0.24
40.6–76.9
18.4–50.7
0.40
0.28
187
139–252
78–212
1.15
4.5 (500 s1 )
Serum:4.3–140
Adapted from Qiu and Rao (1888), Da Silva et al. (1992), and Harper and El Sahrigi (1965).
where m0 is the viscosity at reference pressure and a is a parameter for the sample food. For
non-Newtonian foods, either t or g must be specified as a parameter.
The composition of apple juices from different sources has been reported by Mattick and
Moyer (1983). Fructose, glucose, and sucrose are the most important constituents of clarified
apple juice, accounting for more than 90% of soluble solids, so thermophysical properties
should be largely determined by the type and concentration of sugars.
The resulting coefficients of the above equations for different sugars at 20–258C, as they
compare with those obtained for apple juice, are presented in Table 4.17. Thermophysical
properties’ data for sugar solutions were obtained from Riedel (1949), Honig (1953), Taylor
and Rowlinson (1955), Pancoast and Junk (1980), and Weast (1985). The properties of
fructose, glucose, and sucrose solutions appear to be very similar.
Thermophysical properties of clarified apple juice
A difference at the 1% level between density data of apple juice and sugar solutions was
observed. At the same concentration expressed as mass fraction, the juice has a density a little
higher than the sugar solutions, which means an average specific volume of solids smaller
than those corresponding to sugars. Thus, it seems that Eq. (4.11) slightly underpredicts the
density of apple juice. If the effect of the minor components is taken into account, a decrease
in density should be expected since a typical value of vs for proteins, the most important
minor component in apple juice, is about 0:73 cm3 =g (Kunz and Kauzmann, 1974).
The observed behavior was explained by considering the influence of organic acids in the
refractometric reading of soluble solids. Millies and Burkin (1984) reported that a reduction
up to 3.5% in the refractometric value was found in concentrated apple juice with a malic acid
content similar to that of the juice samples used in this work. Therefore, the reported value of
Table 4.17. Values of coefficients for evaluating thermophysical properties with some proposed models
at 208C (Constenla et al., 1989) with permission.
Equation number
4.11
4.27
Parallel model
4.18
Coefficient
Sucrose
Glucose
Fructose
Apple juice
ns
nw
A
B
ks
kw
Cps
Cpw
0.6261
0.9956
2.6122
1.0381
0.2090
0.9790
0.4400
0.9971
0.6323
0.9962
2.5617
0.9725
0.2142
0.9810
0.4425
0.9989
0.6277
0.9950
2:4 53
0.9807
–
–
–
–
0.6145
0.9921
2.5289
1.0052
0.2070
0.9789
0.5242
0.9773
94
Fruit Manufacturing
density for a 68.58Brix could be comparable to a sugar solution of 70.8% soluble solids. No
significant differences between densities of grape juice, orange juice, and sucrose solution at
208C were found by Moresi and Spinosi (1980, 1984), the8Brix reading being corrected by
acidity. Thus, it appears that density of fruit juices can be predicted by Eq. (4.11) using their
main sugar composition if the correction by acidity in the refractometric reading is taken into
account.
While Constenla et al. (1989) found the specific heat of apple juice decreased less rapidly
with soluble solids than those of pure sugar solutions, Moresi and Spinosi (1980, 1984)
reported the opposite behavior for orange and grape juices at 208C and concluded that the
minor components were responsible for the reduction in Cp values. However, using suggested
values for Cpi (Kunz and Kauzmann, 1974; Choi and Okos, 1983) in Eq. (4.18), it can be
estimated that the influence of these components on Cp of fruit juices is practically negligible.
Obviously more work is needed in this area to explain the above discrepancies.
4.4.6. Boiling Point Rise
Concentration of fruit products by evaporation is conducted by boiling off the water, which
occurs at the boiling point of the solution. It is well known that the presence of solute results
in depression of partial pressure of the solvent below its vapor pressure. Depression of vapor
pressure and freezing point, and elevation of boiling point and osmotic pressure belong to the
group of colligative properties, depending on the number on molecules in solutions and not
on the concentration of these species by weight (Fig. 4.6).
8
700 mbar
7
473 mbar
311 mbar
6
Boiling Point Rise, ⴗC
199 mbar
123 mbar
5
73 mbar
4
Sacrose, 700 mbar
Reducing sugar, 700 mbar
3
2
1
0
0
20
40
60
80
Concentration, ⴗBrix
Figure 4.6. Effect of concentration on the rise of boiling point of clarified apple juice at different pressures
(Crapiste and Lozano, 1988 with permission).
4
.
Thermodynamical, Thermophysical, and Rheological Properties
95
Two different methods can be used to describe the boiling point elevation of sugar
solutions, applicable to fruit juices as a function of pressure (or boiling point of water) and
concentration of soluble solids. In the first approach, empirical correlations are presented to
fit the experimental data on vapor–liquid equilibrium of solutions.
The use of expressions suitable for describing the vapor pressure of pure water can be
extended to aqueous solutions. A correlation derived from the Clausius–Clapeyron equation
can be expressed in the form
ln P ¼ A(W ) B(W )=T
(4:32)
or the Antoine equation, which can be written as:
ln P ¼ A(W ) B(W )=(T þ C(W ))
(4:33)
where P is the pressure (mbar), T is the boiling temperature (K), and W is the
mass concentration of soluble solids (% by weight or8Brix). More complex and accurate
correlations have been proposed. However, coefficients like A, B, and C in Eqs. (4.31) and
(4.32) result in complex functions of concentration, and only in particular cases can the
boiling point be explicitly obtained from those expressions (Moresi and Spinosi, 1984).
In addition, best representation can be obtained if the boiling point rises instead the temperature of ebullition is used in fitting the data. For the above reasons an empirical equation
of the form (Crapiste and Lozano, 1998):
DTr ¼ aW b exp (gW )Pd
(4:34)
was proposed, where DTr is the boiling point rise (8C) and the parameters a, b, g, and d are
evaluated from experimental information. Parameters of Eq. (4.34) are listed in Table 4.18.
Since the sugars are the most important component of fruit juices DTr was determined largely
by the type and concentration of sugars.
Thermophysical properties of foods are well documented, and the measurement of most
of them is a matter of routine. However, despite the fairly large amount of data collected on
some particular foods, they are sometime contradictory due to the different conditions at
which they are gathered, as well as to the differences among the same foods of different origin,
composition, and structure.
Considerable progress is being made toward explaining the influence of individual
components on effective properties. Moreover, as the amount of thermal property data
required to describe any foodstuff under the varied handling, processing, and storage conditions is practically infinite, modeling and prediction of such properties is a must.
Table 4.18. Value of parameters for evaluating rise of boiling point of apple juice and related sugar
solutions with Eq. (4.34) (Crapiste and Lozano, 1988 with permission)
Sucrose
Reducing sugars
Apple juice
a 102
b
g 102
d
r2
s
3.0612
2.2271
1.3602
0.0942
0.5878
0.7489
5.329
3.593
3.390
0.1356
0.1186
0.1054
0.999
0.997
0.998
0.083
0.078
0.062
r2 is the multiple correlation coefficient (squared); s is the standard error.
96
Fruit Manufacturing
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CHAPTER 5
COLOR, TURBIDITY, AND OTHER
SENSORIAL AND STRUCTURAL
PROPERTIES OF FRUITS AND
FRUIT PRODUCTS
5.1. INTRODUCTION
Measurements of color and turbidity are analytical problems confronting the food technologists working with fruit and fruit products. The appearance of fruit products plays an
important role in determining whether or not a consumer will purchase them (Francis and
Clydesdale, 1975). In the case of fruit juices, opacity, color, and homogeneity contribute to
the overall appearance. Moreover, the kinetics of deterioration can be followed through color
measurement, which is a simple and effective way for studying the phenomenon. The complexity of the reactions and the various compounds involved (Hodge, 1953; Spark, 1969)
make it difficult to study the reactions by means of a simple analytical chemical method.
Two major approaches have been used to evaluate color changes in fruits and fruit
products (Francis and Clydesdale, 1975; Pomeranz and Meloan, 1994):
.
.
Measurements based on absorbance spectrophotometry: The absorption of light depends
on the type and concentration of the chromophores present. A variety of different
types of spectrophotometers have been developed to measure transmission of light
from liquids as a function of wavelength both in the UV and visible regions.
Measurements based on tristimulus colorimetry: The basis of these methods is that
colors can be simulated by combining red (R), green (G), and blue (B), in the
appropriate ratios and intensities.
Other methods used the absorbency measurement of soluble pigments by spectrophotometry at 360 –500 nm, often near 400 nm. The susceptibility of apples to browning was
adequately determined by simultaneous measurement of soluble (Abs400 ) and insoluble (L )
brown pigments (Amiot et al., 1992). It was indicated that for some berry products color
deterioration cannot be characterized by changes in total anthocyanin (TA) alone. Most of
the anthocyanin was polymerized, rather than lost during storage (Ochoa et al., 1999).
Percentage of polymeric color (%PC) is a measure of the pigment resistance to bleaching,
and indicates, in some degree, the anthocyanin polymerization. Increases in %PC values
followed a near-zero reaction kinetics throughout the storage period. As pointed out by
Labuza and Riboh (1982) most of the quality-related reaction rates are either 0 or firstorder reactions, and statistical difference between both types may be small.
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5.2. MEASUREMENT OF COLOR
Color and opacity are the result of interaction between light and food. Incident light may be
transmitted, reflected, or scattered before being detected by the human eye. The interaction of
light with matter is fundamental for qualitative and quantitative analyses of fruit juices in
particular. Color can be quantified in several ways. Measurement of the color of foods can be
by visual systems (by comparison with colored references), spectrophotometry, tristimulus
colorimetry, or specialized instrumentation for particular foodstuffs. (Hutchings, 1994;
McClements, 1999)
As Fig. 5.1 shows, incident electromagnetic waves may be partly reflected and partly
transmitted (or refracted). Refractive index, angle of incidence, and surface topography
determine the relative importance of these phenomena. In Fig. 5.1, Io is the radiant power
arriving at the cuvette, I is the radiant power leaving the cuvette, and b is the path length.
Reflection of light may be specular (angle of incidence, fin , is equal to angle of reflection,
fref ), or diffuse (light reflected over many different angles).
While the former is the predominant form of reflection of smooth surfaces, the latter is
most important in the case of rough surfaces. Particles in suspension may be responsible for
light scattering, depending on the particle size and the wavelength of the incident radiation.
5.2.1. Absorbance Spectrophotometry
Spectrophotometry is based on the fact that substances of interest selectively absorb or emit
electromagnetic energy at different wavelengths, in the range of the ultraviolet (200–400 nm),
the visible (400–700 nm), or the near infrared (700–800 nm). The basic principle of spectrophotometry is that the energy-absorption properties of a substance can be used to measure the
concentration of the substance.
In most cases, the sample is the product of reactions between the original substance and
reagents, and absorbs light selectively according to Beer’s law, which states that equal
thickness of an absorbing material will absorb a constant fraction of the energy incident
upon it. Similarly, Lambert’s law stated that light absorbance is proportional to path length.
These relationships are described in Table 5.1.
A cuvette is designed to keep b as nearly constant as possible. Therefore changes in I
should reflect changes in the concentration (C ) of the absorbing substance in the sample.
Since I and b are kept constant, the absorbance varies with C. The concentration of an
b
fin
Scattered
I
Io
Transmitted
Incident
fref
Reflected
(specular)
Figure 5.1. Simplified scheme for light reflection and transmission through a cuvette filled with a turbid liquid.
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Color, Turbidity, and Other Sensorial and Structural Properties
101
Table 5.1. Light absorbance relationships.
Law name
Formula
Nomenclature
Beer
A/C
C is the concentration of the absorbing
substance, mole/L or g/L
Io is the radiant power arriving at the
cuvette, I the radiant power leaving
the cuvette
A is absorbance
%T ¼ I=Io 100
Lambert
Beer–Lambert
A ¼ log (Io =I)
¼ log (100=%T) ¼ 2 log (%T)
A/b
A ¼ «cb or A ¼ ab
b is path length, cm
« is molar extinction coefficient
¼ a molecular mass
a ¼ absorptivity
unknown can be determined by determining the absorbance (Abs) of a standard with known
concentration (Cs ) of the substance of interest. The concentration of the unknown substance
(Cu ) can be calculated from the following relationship:
Cu ¼ Cs (Absu =Abss )
(5:1)
If the relationship holds over the possible range of concentrations of the substance of interest,
then the determination is said to obey Beer’s law. If the relationship does not hold due to
absorption by the solvent or reflections of the cuvette, then a relatively large number of
standards with known concentration values must be used to compute a calibration curve of
concentration versus absorbance. The absorbance, also called extinction or optical density, is
linearly correlated to concentration. The law is valid only monochromatic light and for
diluted solution.
5.2.1.1. Spectrophotometer Components
Different types of spectrophotometers have been developed to measure the transmission and
reflection of light from objects as a function of wavelength (Clydesdale, 1969; Francis and
Clydesdale, 1975; Hutchings, 1994). A typical visible light spectrophotometer is shown in
Fig. 5.2. A good light source should emit a continuous, featureless spectrum, with no
exaggerated change of intensity as the wavelength is varied.
Reference
photo tube
Light
source
Collimator
Lens
Lens
Filter
Sample
Diffraction
grid
Measurement
photo tube
Figure 5.2. Schematic representation of a visible light spectrophotometer.
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Fruit Manufacturing
For ultraviolet (UV) spectrophotometry deuterium lamp is used. In this lamp electrical
discharge causes D2 gas to emit continuous spectrum in the UV region (200 –350 nm). For
visible light spectrophotometry tungsten filament lamp (350 –800 nm range) or tungsten–
halogen lamp (tungsten filament embedded in a quartz–iodide matrix) is used.
Two classes of devices, filters (glass and interference) and monochromators, are used to
select those portions of the power spectrum produced by the power source that are to be used
to analyze the sample.
Some instruments related to spectrophotometer use filters to select wavelength. Devices
that use filters as their wavelength selectors are called colorimeters or photometers. Colorimeter uses colored glass filters. It is low cost and has a wide band of wavelengths (often
>70 –100 nm deviation from Beer’s law). Glass filters function by absorbing certain wavelengths (e.g., red region) and transmitting others (e.g., blue-green region, the customary
bandwidth of glass filters, is 50 nm). Peak transmittance decreases as bandwidth decreases.
At a bandwidth of 30 nm, the peak transmittance is about 10%, which is too low for most
applications.
Some photometric instruments use interference filters. Interference filters used selectively
spaced reflecting surfaces to reinforce the wavelength of interest and cancel others. Harmonic
frequencies can be eliminated by glass-cutoff filters. Glass filters are used in applications in
which only modest accuracy is required, while interference filters are used in many spectrophotometers.
Monochromators are tunable wavelength selectors. Monochromators use prisms
and diffraction gratings, which disperse incident light into its component spectrum to
provide very narrow bandwidths by dispersing the input beam spatially as a function of
wavelength. A mechanical device then allows wavelengths in the band of interest to pass
through a slit. In diffraction grating wavelength is selected by constructive interference
(reinforcement) at chosen wavelength, while the other wavelengths are cancelled by destructive interference.
Sample cells: Rectangular cuvettes of 10 mm path length are most common (1– 40 mm
sizes are available). Cylindrical tubes (cf. test tubes) are common for some instruments.
However, positioning a cylindrical tube in light beam is very critical.
Table 5.2 lists materials and principal characteristics of cuvettes. While plastic and glass
cuvettes are used only in the visible range (> 340 nm), silica cuvettes have an extended
wavelength range (200 –1000 nm).
Detectors: Spectrophotometers use photoelectric devices that convert radiant energy
into an electrical signal. These detectors use: (a) silicon photodiode (solid-state detector for
general purpose photometer), (b) vacuum phototube, or (c) photomultiplier tube—this is a very
Table 5.2. Different spectrophotometer cuvettes.
Plastic
Glass
Quartz (silica)
Cheap (disposable), not breakable,
easily scratched, aqueous, any pH
Medium price
Expensive
Breakable on impact
More resistant to scratching
Aqueous or organic solvents
(concentrated alkalis must be avoided)
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Color, Turbidity, and Other Sensorial and Structural Properties
103
sensitive detector and is used if the photometric instrument needs to measure very low-light
intensities.
Finally a Readout Device (usually a digital voltmeter or analog-to-digital data acquisition
system) is used to quantify amplified signal from detector. Modern instruments usually
incorporate time averaging or damping to counteract instrument noise.
5.2.1.2. Improved Spectrophotometers
Most spectrophotometers use two lamps: a deuterium lamp with high UV output, and
a tungsten lamp for high visible output (double-beam spectrophotometer). Photodiode
array technology positions multiple detectors side by side on a silicon crystal, with a
capacitor to convert light to electric discharge. Polychromic light from the grating can now
be detected in the same time it takes to measure a single wavelength with a conventional
spectrophotometer.
5.2.1.3. Turbidity and Scattering
An object that allows all the light to pass through it, is referred to as being transparent, whereas
an object that scatters or absorbs all the light is referred to as being opaque (Clydesdale, 1975).
Many dilute suspensions fall somewhere between these two extremes and are therefore referred
to as being translucent. The opacity of most food suspensions is determined mainly by the
scattering of light from the particles: the greater the scattering, the greater the opacity
(Hernandez and Baker, 1991; Dickinson, 1994). When a light wave impinges on a food
suspension, like a cloudy fruit juice, all the different wavelengths are scattered by the particles,
and so the light cannot penetrate very far into the juice. As a consequence, the juice appears to
be optically opaque (Farinato and Rowell, 1983).
The extent of light scattering by a suspension is determined mainly by the relationship
between the droplet size and wavelength. Scattering due to turbidity in a cuvette gives
apparent absorbency, which is proportional to the density of suspended particles. For fruit
juices spectrophotometry samples must be centrifuged or filtered to remove suspended
particles.
Light-scattering techniques may be used to determine the size distribution of the
particles in a food suspension. Knowledge of the particle size distribution enables one to
predict the influence of particles on light scattering and therefore on the turbidity of a
suspension.
Two analytical methods must be differentiated: (i) turbidimetry, for the measuring
of apparent absorbency and (ii) nephelometry, for the measuring of scattered light. A nephelometer measures the intensity of light that is scattered at an angle of 908 to the incident
beam. The intensity of light scattered by a sample is compared with that scattered by a
standard material of known scattering characteristics (Hernandez et al., 1991). Because small
particles scatter light more strongly at wide angles than large particles, the nephelometer is
more sensitive to the presence of small particles than turbidity measurements.
5.2.1.4. Reflection Spectrophotometer
The reflectance (R) of a material was defined as the ratio of the intensity of the light reflected
from the sample (Rs ) to the intensity of the light reflected from a reference material of known
reflectance (RR ): R ¼ RS =RR (Francis and Clydesdale, 1975). For specular reflection, the
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Fruit Manufacturing
intensity of the reflected light is usually measured at an angle of 908 to the incident wave;
whereas for diffuse reflection, the sum of the intensity of the reflected light over all angles is
measured using an integrating sphere. A reflectance spectrum is obtained by carrying out this
procedure across the whole range of wavelengths in the visible region.
The transmittance and reflectance spectra obtained from a sample can be used to
calculate the relative magnitudes of the absorption and scattering of light by an emulsion
as a function of wavelength. Alternatively, the color of a product can be specified in terms of
trichromatic coordinates by analyzing the spectra using appropriate mathematical techniques
(McClements et al., 1998). The details of these techniques have been described elsewhere and
are beyond the scope of this book (Francis and Clydesdale, 1975).
5.2.1.5. Tristimulus and Special Colorimeters
Since visual color judgments can be affected by several factors (lighting conditions, angle of
observation, individual differences in perception) instruments for color measurement
provide a subjective alternative. Due to the difficulty in objectively describing the colors of
materials using everyday language, a number of standardized methods have been developed
to measure and specify color in a consistent way (Francis and Clydesdale, 1975; Hutchings,
1994). These methods are based on the principle that all colors can be simulated by combining
three selected colored lights (red, green, and blue) in appropriate ratios and intensities.
This trichromatic principle means that it is (almost) possible to describe any color in
terms of just three mathematical variables (i.e., hue, value, and chroma) (Francis and
Clydesdale, 1975).
The color of a food suspension is determined by the absorption and scattering of light
waves from both the particles and continuous phase (Dickinson, 1994). The absorption of
light depends on the type and concentration of chromophores present, while the scattering
of light depends on the size, concentration, and relative refractive index of any particulate
matter. Whether a suspension appears ‘‘red,’’ ‘‘orange,’’ ‘‘yellow,’’ ‘‘blue,’’ etc. depends
principally on its absorption spectra. Under normal viewing conditions, a suspension is
exposed to white light from all directions. When this light is reflected, transmitted, or
scattered by the suspension, some of the wavelengths are absorbed by the chromophores
present. The color of the light that reaches the eye, is a result of the nonabsorbed wavelengths
(e.g., a juice appears red if it absorbs all the other colors) (Francis and Clydesdale, 1975). The
color of a suspension is modified by the presence of the particles or any other particulate
matter. As the concentration or scattering cross-section of the particles increases, a suspension becomes lighter in appearance because the scattered light does not travel very far through
the emulsion and is therefore absorbed less by the chromophores. It is therefore possible to
modify the color of a suspension by altering the characteristics of the emulsion particles or
other particulate matter.
Tristimulus colorimeters employ three glass filters (red, green, and blue) corresponding
to the response of the cones in the human eye, a light source, and a detector system.
Recombination of these three primary colors (RGB) can match almost any unknown color
(Fig. 5.3). As not all colors can be obtained by the addition of three primaries, the problem
was overcome by adding one of the primaries to the unknown color and matching the
combined color by the addition of the other two primaries. In this way an imaginary
mathematically negative color was generated and RGB source was changed to XYZ coordinates. Moreover, a trained panel can match the spectrum color using the RGB sources, and
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Color, Turbidity, and Other Sensorial and Structural Properties
105
B
G
R
i
Figure 5.3. Color match with addition of primary lights, red (R), green (G), and blue (B).
data can be recalculated in terms of XYZ and a curve of the standard observer can be
obtained (Fig. 5.4).
The response of the human eye was standardized, giving origin to the CIE system.
The Commission Internationale de L’Eclerage (CIE) also specifies the following for color
measurement (King, 1980): (i) The use of standard light sources, (ii) the conditions for
the observation and measurement of color, and (iii) the use of ‘‘standard observer’’ curves.
The amounts of the three theoretical primaries or tristimulus values required to match a
given color can then be determined. The most common approach to determining the tristimulus values of a material is the weighted ordinate method (Nassau, 1996). CIE XYZ
coordinates are given by the following equations:
750
ð
X¼
RE
x dl
(5:2)
380
Relative
response
z
y
x
x
400
550
Wavelength (nm)
Figure 5.4. Standard observer curves.
700
106
Fruit Manufacturing
750
ð
Y¼
REy dl
(5:3)
REz dl
(5:4)
380
750
ð
Z¼
380
where R is the reflectance (or transmittance) of the sample, E is the energy distribution of the
standard light source, and l is the wavelength. Tristimulus colorimeters replace the integration with the mentioned filters.
Spectrophotometers introduced microelectronics to perform the tristimulus values’ calculation also. It is recommended that reflecting materials be illuminated at an angle of 458
and viewed at an angle of 908. When defining colors there are a number of attributes that need
to be accounted for:
(1) Hue, which is the color of the material black; white and grays are colors devoid of hue.
(2) Lightness or luminosity is simply the brightness of a color, the paler colors have
greater lightness than the dark colors.
(3) Saturation is used to indicate the strength of the chromatic response: pale or pastel
colors have low saturation, while deep and vivid colors have high saturation.
These three independent variables allow colors to be arranged logically in a threedimensional space. Although the X, Y, Z tristimulus values can be used to define a color,
new parameters are required because interpreting the appearance of that color from the
former is very difficult. The parameters usually chosen are the chromaticity coordinates:
(Nassau, 1996):
x¼
X
Y
Z
; y¼
; z¼
X þY þZ
X þY þX
X þY þX
(5:5)
As x þ y þ z ¼ 1 only two of the coordinates need to be specified, generally x and y. Thus
the three parameters required to define a color are x, y and Y; x, and y to define the hue and
saturation and Y the brightness. If the x, y chromaticity are plotted, the CIE horseshoeshaped spectrum locus is obtained (Fig. 5.5), including all real colors. The lightness of the
color will be represented by an axis perpendicular to the x, y plane. Colors can be located in a
three-dimensional space (color solid).
5.2.1.6. CIELAB Method
In 1976 a useful method for quantifying the appearance of a surface color was introduced.
Three new parameters L , a , and b were defined as:
L ¼ 116(Y =Yn )1=3 when (Y=Yn > 0:008856)
L ¼ 903:3(Y =Yn )when (Y=Yn < 0:008856)
a ¼ 500[ f (X =Xn ) f (Y =Yn )]
b ¼ 200[ f (Y =Yn ) f (Z=Zn )]
where f(Y =Xn ), f(Y =Yn ) and f(Z=Zn ) are defined elsewhere (King, 1980).
(5:6)
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Color, Turbidity, and Other Sensorial and Structural Properties
107
0.9
515
520
0.8
530
505
0.7
545
500
555
0.6
495
565
0.5
575
Y
C Illuminant
490
590
0.4
605
485
0.3
780
480
0.2
470
0.1
380
0
0
0.2
0.4
0.6
0.8
X
Figure 5.5. CIE chromaticity diagram.
X, Y, and Z are the tristimulus values of the material, while Xn , Yn , and Zn are the
tristimulus values of a white object, and these correspond to the normalized tristimulus values
of the illuminant. L , a , and b are the axes of a three-dimensional color space. It has the
advantage of having the same configuration as those derived logically by arranging colors
visually. Hunter parameters (Fig. 5.6) are based in similar consideration (Hunter, 1975). The
system measures the degree of lightness (L), the degree of redness or greenness (a), and the
degree of yellowness or blueness ( b). Additionally Psychometric chroma (C ) and hue (H)
were defined as:
L = 100
White
+b
Yellow
–a
Red
+a
Green
Blue
−b
L=0
Black
Figure 5.6. Hunter color space.
108
Fruit Manufacturing
C ¼ [(a )2 þ (b )2 ]1=2
(5:7)
H ¼ tan1 (t =a )
(5:8)
The total difference AE between two colors each given in terms of L , a , and b can be
calculated from:
AE [(D L )2 þ (Da )2 þ (Db )2 ]1=2
(5:9)
Other possible values are lightness difference DL , chroma difference DC , and hue difference
DH :
DL ¼ L sample L standard
(5:10)
DC ¼ C sample C standard
(5:11)
DH ¼ [(DE)2 (DL )2 (DC )2 ]1=2
(5:12)
5.2.1.8. Measurement of Tristimulus Values
Reflectance and transmittance measurements over the visible spectrum can be readily made
using a spectrophotometer. The surface of the sample will not be subjected to elevated
temperatures, which could cause a change in coloration. When the sample is fluorescent, as
most spectrophotometers irradiate the sample with monochromatic radiation, the higher
energy-excitation wavelengths will not be incident on the sample when wavelengths in the
emission region are being measured.
The problem will not occur in instruments that irradiate the sample with polychromatic
light. Commercial systems for tristimulus measurements were compared by Francis and
Clydesdale (1975) and Little (1976) among others. In all cases, tristimulus values, Hunter
values, etc. have to be computed from reflectance or transmittance data.
A number of tristimulus colorimeters are available; these are instruments that, after
irradiation of the sample with polychromatic light, examine the reflected light with three or
four filters and a photocell. Visual colorimeters are also used, e.g., the Lovibond Tintometer,
in which red, yellow, and blue filters are used to obtain a visual match with the sample. The
color is then defined in Lovibond units of red, yellow, and blue; conversion of these units into
X, Y, and Z values is possible by graphical methods.
5.2.1.9. Application of Colorimetry
Colorimetry provides an objective method for specifying the color of food. Most fruits have
been subjected to an investigation of their color properties. Francis and Clydesdale (1975)
have reviewed the application of colorimetry to most foods, including tomatoes and tomato
products, green vegetables, citrus products, potato products, cereal products, meat color,
sugars, beer and wine, and tea and coffee. Kramer (1976) has discussed the application of
colorimetry to quality control.
Many studies relating pigment concentration to color have been made. The color of a
material is dependent on a number of factors, e.g., the pigment concentration, the nature of
the surface and particle size, and as a result of these factors light scattering may also be
significant (McClements et al., 1998). Scattering may alter the observed color of the food
considerably.
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Color, Turbidity, and Other Sensorial and Structural Properties
109
5.3. FOOD DISPERSIONS
5.3.1. Definitions
The classical definition of the colloidal state is in terms of size alone; the lower limit is
generally taken to be in the neighborhood of 10–50 Å, and the upper size limit, as 1---5 mm.
(Fig. 5.7)
Table 5.3 shows how any two of three states of matter (solid, liquid, and gas) can be
mixed to form a colloid. Substances in the same state (except gases) can also be mixed to form
colloids. Particles in a colloid may adhere together and form aggregates of increasing size
( flocculation), which may settle down due to gravity. If flocks change to a much denser form
it is said to undergo coagulation, which is an irreversible process.
A typical example of food dispersion is a cloudy or opalescent fruit juice. A cloudy juice is
a colloidal system where the dispersing medium is water, and the dispersed (colloidal) matter
is formed by the rest of cellular tissue released after fruit processing (milling and pressing).
During cloudy juice processing a small percentage of insoluble particles is retained in
suspension, giving it a light opaque color. Nagel (1992) described cloudy apple juice as a light,
whitish-yellow juice, clearly showing cloudiness, which presents no sedimentation, is full
bodied and juicy, but has no astringent or bitter taste. Figure 5.8 compares cloudy and
clarified apple juice. Typical cloud fruit particles are also included in Fig. 5.8.
Particularly, cloudy fruit juices have solids of various dimensions distributed in a serum
(mainly sugars and organic acids) or clarified juice (Moyls, 1966; Beveridge and Tait, 1993).
One of the main problems with cloudy juice production is the assurance of cloud stability
(Chobot and Horulaba, 1983; Beveridge and Harrison, 1986; Gierschner, and Baumann,
1988; Genovese et al., 1997). Even after prolonged storage none or only a very small part
of the cloud particles should precipitate.
Colloidal stability. Derjaguin, Verway, Landau, and Overbeek (McClements, 1999)
developed a theory, which explains the colloidal stability as the result of the sum of the
electrical double layer repulsive and van der Waals’ attractive forces that the particles
experience as they approach one another (Fig. 5.9).
µ
Å
0.0001
0.001
0.01
0.1
1
1
10
100
1000
10,000
Molecular
Dispersion
10
100
1000
Suspensions
Colloidal Dispersion
Bacteria
Yeast
Turbidity
Figure 5.7. Particle classification by size.
110
Fruit Manufacturing
Table 5.3. Different types of colloidal systems.
Dispersed phase
Dispersing phase
Type of system
Familiar examples
Gas
Gas
Liquid
Liquid
Liquid
Solid
Solid
Solid
Aerosol
Solid aerosol
Foam
Emulsion
Sol, Suspension
Solid foam
Gel
Solid sol
Fog
Smoke
Whipped cream
Mayonnaise, milk
Paint, ink Cloudy juice
Foam rubber, Marshmallow
Gelatin, cheese
Colored gemstones, Some alloys
Liquid
Solid
Gas
Liquid
Solid
Gas
Liquid
Solid
b
Apple
Juice
r
x
0.5 µm
a
Clarified
Cloudy
Figure 5.8. (a) Visual comparison between cloudy and clarified apple juice, and (b) cloudy apple juice particles.
r is the particle radius, and x the particle separation. (Genovese and Lozano, 2005)
- - + -+
-
- + -+
Hydrophobic and
+
Hydration interactions
--+ + -- + + --- Steric
- Repulsion
London-van der Waals
- +- - + --- - +--- ++
--+ - - ++
--- + +- - - Electrostatic forces
--
Figure 5.9. Possible forces acting on particles in colloidal dispersion.
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
111
The DVLO theory proposes that an energy barrier resulting from the repulsive force
prevents two particles approaching one another and adhering together. However, if two
particles collide with sufficient energy to overcome that barrier, the attractive force will pull
them into contact and they will adhere strongly and irreversibly together. For colloidal
stability, the repulsive forces must be dominant. There are different mechanisms that affect
dispersion stability:
Steric repulsion: This involves macromolecules absorbed onto the particle surface and
causing repulsion.
Electrostatic (charge) repulsion: This is the effect on particle interaction due to the
distribution of charged species in the system.
Figure 5.10 shows a submicron positive fruit particle completely surrounded by negatively charged pectin (based on Yamasaki et al., 1964, proposal). Development of a net charge
at the particle surface affects the distribution of ions in the surrounding interfacial region,
resulting in an increased concentration of counterions (ions of opposite charge to that of the
particle) close to the surface. Thus an electrical double layer exists around each particle. The
liquid layer surrounding the particle exists as two parts: an inner region (Stern layer), where
the ions are strongly bound; and an outer (diffuse layer) region, where they are less firmly
associated. Within this diffuse layer is a hypothetical boundary within which the particle acts
as a single entity. The potential at this boundary is the Z-potential (z). In Fig. 5.10 k1 is the
Debye length, a measure of the ‘‘thickness’’ of the electrical double layer.
Non-DLVO interaction forces have been found in aqueous suspensions of very hydrophobic or very hydrophilic particles (Molina-Bolı́var and Ortega-Vinuesa, 1999). Recently,
Genovese and Lozano (2005) have claimed that repulsive hydration forces might play a
significant role in the stability of CAJ.
5.3.2. Food Dispersion Characterization
The proper characterization of food suspensions depends on the purposes for which the
information is sought, because the total description is an enormous task (Trottier, 1997).
Diffuse layer
+
+
Pectin
+
+
+
κ
−1
+
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+ + +
+
+
+
+ + +
+
+
+
+
+
+
+
+ + + +
+
Nerst layer
+
+
+
+
+
Shear plane
Figure 5.10. Charges at a particle surface.
Particle
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Fruit Manufacturing
Anyway, one or more of the following properties should be considered: size and size distribution, shape, number and size distribution of pores, morphology of the primary particles,
surface area, state of agglomeration, and or chemical and phase composition.
5.3.3. Particle Size, Shape, and Size Distribution
Particle size and particle size distribution (PSD) are fundamental properties, which have a
strong influence on many other properties and can be used to predict them. While the size of
spheres or cubes can be completely specified by a unique dimension (diameter or length), food
engineers will rarely be so fortunate as to be dealing with regularly shaped particles. Although
an irregularly shaped particle does not possess a unique linear dimension, its size is usually
expressed as the diameter equivalent sphere. At least three possibilities for spheres that are
equivalent to a given particle may be considered:
A sphere:
(1)
(2)
(3)
(4)
With
With
With
With
the same volume (dv ),
the same surface area (ds ),
the same projected area as that of the particle (da ), and
the sieve diameter (dA ).
The sieve diameter, which is the width of the smallest aperture through which a particle
can pass, has little application in food suspensions at the colloidal level. This diameters and a
number of additional equivalent diameter are listed in Table 5.4.
PSD is usually inferred, via Stokes’ law, from the sedimentation time of dispersed
particles. The most direct methods for determining the particle size, shape, and distribution
of food dispersions are scanning and transmission electron microscopy. Indirect methods for
determining size and particle size distribution include sedimentation and centrifugation, conductimetric techniques, light scattering, X-ray diffraction, gas and solute adsorption, ultrafiltration, and diffusiometric methods (McClements, 1999). The analytical choice depends on the
physical properties of the food dispersion. Modal, median, and mean diameters should also be
calculated. Modal diameter is the diameter that occurs most frequently in the sample, median
diameter is the diameter such that 50% of the sample diameters are smaller than it, and mean
diameter is obviously the sum of all the sample values divided by the size of the sample.
5.3.3.1. Electron Microscopy
Microscopy is the only method in which particles are observed and classified individually. The
lower limit of particle size that can be resolved by the microscope using visible light is about
0:2 mm. Colloids and many suspensions require transmission or scanning electron microscopy, which would permit the measurement of smaller particles. Electron microscopy is
applied when particle identification and shape, as in the case of most food particles, are
important in addition to size.
Sample preparation is in general more complex, which involves drying and gold covering
in a metal evaporator. Recent developement of enviromental scanning electron microscopes
(ESEM) has simplified sample preparation. The increasing power of data processing software,
coupled with the falling cost of TV cameras and scanners, had led to a generalization of image
processing systems. Three basic types of diameters are used in particle diameter analysis,
Martin, Feret, and equivalent surface area, and are described in Table 5.4. (Trottier, 1997)
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
113
Table 5.4. Diameters frequently used in particle size analysis.
Diameter
Name
Definition
dv
Equivalent volume
Diameter of a sphere having
the same volume as the particle
ds
Equivalent surface
da
Projected area
Diameter of a sphere having the
same surface as the particle
Diameter of a circle having the
same surface as the particle
dst
Stokes
Diameter of a sphere with the same
Stokes’ free falling velocity, as the
particle in the laminar regime
dF
Feret
Distance between two parallel tangents
on opposite sides of particle profile
dM
Martin or horizontal
Distance between opposite sides of the
projected particle
dc
Perimeter
Diameter of a circle with the same
perimeter as the projected particle
Representation
Source: Trottier, 1997; Genovese and Lozano, 2000.
The processing of hundreds, even thousands, of particles may be required to establish
statistical significance. Digital images of food particles can be statistically analyzed with
powerful software, such as the AnalySIS 2.1 (Soft Imaging Software GmbH) version.
5.3.3.2. Sedimentation
Measurement of the settling rate for particles under gravitational or centrifugal acceleration
in a quiescent liquid is the basis of techniques for determining particle size and size distribution. The particle size determination by sedimentation is based on an equivalent spherical
114
Fruit Manufacturing
diameter. The upper size limit for sedimentation methods is established by the value of the
particle Reynolds’ number:
Re ¼ dvr=m
(5:13)
where d is the particle diameter, r is the particle density, m is the viscosity, and v the terminal
velocity of the particle, which can be determined from Stokes’ law, given by equation:
1=2
18h
h
(5:14)
d¼
(r rl )g
t
where rl is the dispersant density. Instruments of different configurations are used to determine particle diameter (McClements, 1999). Centrifugal particle size analyzers utilize the
sedimentation method and detects particle concentration photometrically (Fig. 5.11). A builtin microcomputer converts the absorbance changes into particle size distribution, based on
Stokes’ law. It must be noted that variation in distance from the center of rotation and the
location of particle may not be correctly introduced by the manufacturers of the analyzer, and
deviations up to 12.5% can be expected. When using the same cuvette and sample volume the
mentioned error is systematic and constant, and it can be easily overcome by calibrating the
equipment with standards of known particle size.
Centrifugal sedimentation under constant speed rotation is described by the following
equation:
!
rp rl
du
18:m
¼
u
(5:15)
:R:v2 dt
rp :d 2
rp
where v is the angular velocity, d the particle diameter, R the distance from sedimentation
uppermost surface to detection beam, rp the particle density, rl the dispersant density, m the
dispersant viscosity, t the time, and u the sedimentation velocity of particles. As Eq (1) shows,
particle density must be known. Genovese et al. (1997) used starch granules of known size and
density for calibration of the method.
Detector
Sample
Light source
Figure 5.11. Scheme of a simplified photometric/sedimentation method for particle size determination.
5
Color, Turbidity, and Other Sensorial and Structural Properties
.
115
5.3.3.3. Photon Correlation Technique
Photon correlation is a technique for (among other things) measuring the size of colloidal
particles, from a few nanometers to approximately 1 mm. It should not be confused with laser
diffraction, which measures from 1 mm upward to several hundred micrometers.
Photon correlation operates by measuring the temporal fluctuations in the light scattered
by the particles, while diffraction measures the angular distribution of scattered light. A basic
PCS setup is shown in Fig. 5.12. A laser illuminates the sample, which is a dilute suspension of
the particles to be measured. The scattered light is viewed by a photomultiplier, usually at a
908 angle. The light intensity is not constant, but varies randomly, as the particles diffuse
around in the beam, and the wavefronts of light scattered from them overlap and interfere.
The photomultiplier sees a time-varying signal, not a constant one.
If the particles are small, they move around by diffusion and the scattered light shows rapid
fluctuations. Contrarily large particles diffuse slowly, and light scattered from such particles
varies on a slower time scale. Variations in the light level give information about the particle size.
Problems arise when the particles are all not of the same size, in other words there is a
particle size distribution. PCS is less reliable in suspensions with broad size distribution,
because of difficulties associated with interpreting the more complex autocorrelation decay
curves (Horne, 1995). PCS is capable of measuring the size of extremely small particles, up to
individual large macromolecules.
In brief:
.
.
.
Photon correlation analysis is fast and reliable. Range: 0:01---5:00 mm.
Electron microscopy needs more sample treatment. New software and computational
peripherics made the particle analysis easier. Range: 0:002---15 mm.
Sedimentation methods need a standard and knowledge of the density of the particle.
Range: 0:02---500 mm.
5.3.4. Cloudy Fruit Juice Viscosity
It is useful to study the flow behavior of cloudy juices under well-defined conditions
and to link the rheological behavior with the microstructure. Particle size, shape, and
Laser
Sample
Correlator
Photomultiplier
Figure 5.12. Simplified scheme of the photon correlation technique.
116
Fruit Manufacturing
volume fraction, as well as electroviscous effects, modify juice viscosity, compromising
the colloidal system stability. Rao (1987) reviewed the flow properties of plant food suspensions. While the rheology of fruit pulps and cloudy citrus juices has received continuous
attention (Vitali and Rao, 1984; Nogueira et al., 1985; Rao et al., 1985; Ibarz and Lozano,
1992), there are also some published works on the viscoelastic properties of cloudy
juices (Saravacos, 1970; Ibarz and Graell, 1986; Genovese et al., 1997; Genovese and Lozano,
2000).
Characterizing the cloudy juice microstructure is difficult, but most of the variables
involved are reflected in one parameter: the volume fraction of particles (f). Moreover,
diluted and moderately concentrated regions can be modeled in terms of the intrinsic viscosity
[m] (Sherman, 1970), which depends on particle shape and size distribution, and also on the
applied shear stress (t). Other factors influencing the viscosity of cloudy juices are serum
viscosity (mo ), pH, electrolyte concentration, and electroviscous effects.
Because of the absence of distortion of the cloudy particles as a result of strong electrostatic
forces, two electroviscous effects (Krieger, 1972) may be present: (a) When diluted dispersions
are sheared, the electrical double layer (shear layer) is distorted. This distortion leads to an
increased viscosity, or first electroviscous effect. (b) In more concentrated dispersions, viscosity
increases due to particle repulsion effect, which was claimed to be proportional to f2 and
inversely proportional to pH (Sherman, 1970). This effect is known as second electroviscous
effect.
The liquid microstructure under given conditions of stress, particle concentration, shape,
size distribution, and interparticle affinity will be the primary determinant of the rheology.
The viscosity of a diluted dispersion (m) containing spherical nondeformable particles is given
by the well-known Einstein relationship (Metzner, 1985):
mr ¼ (1 þ af)
(5:16)
where mr is the relative viscosity, f is the particle volume fraction, and a is a constant.
Provided that f is low enough to prevent particle interaction, the distance between particles is
much greater than their diameter, there is no slippage at the particle–fluid interface, and m
arises only from viscous drag, then a ¼ 2:5, independent of particle size. Sherman (1970)
found that for f$0:05 then a ¼ [m], where [m] is the intrinsic viscosity:
hr 1
[m] ¼ Lim
(5:17)
f!0
f
On the other hand, the first electroviscous effect may significantly increase the a value in the
case of water dispersion of charged particles. It was then proposed (Russel, 1980) to modify
Eq. (5.3) as follows:
mr ¼ 1 þ 2:5bo f
(5:18)
where bo is a coefficient that takes into account the mentioned electroviscous effect. However,
in the case under study bo will also be a function of the sphericity, size, and distribution of
particles in the juice. Genovese and Lozano (2000) found that cloudy apple juice particles
could be considered ellipsoidal rather than spherical. In this case, the axial ratio of the particle
(pa ) should be considered:
pa ¼ La =Ba
(5:19)
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
117
where La and Ba are the major and minor axis, respectively. Mooney (1951) worked on a
different extension of Eq. (5.16):
hr ¼ 1 þ af þ 0:4075(pa 1)1:508 f
(5:20)
when 1 < pa < 15; and:
p2a
1
1
þ
hr ¼ 1 þ 1:6f þ
f
5 3( ln 2pa 1:5) ln 2pa 0:5
(5:21)
when pa > 15.
Depending on the axial ratio of particles Eq. (5.20) or (5.21) gives information about
the influence of shape (sphericity) on the viscosity of cloudy juice. Figure. 5.13 shows a
typical frequency histogram obtained through the statistical analysis of apple juice cloud
particles (Genovese et al., 1997). Cloudy apple juice resulted in a suspension of irregular
shape particles ranging from 0.25 to 5 mm in size, with a mean diameter of f ¼ 0:84 mm.
Calculated maximum and minimum mean diameters resulted in La ¼ 1:01 m and
Ba ¼ 0:74 mm, respectively
It was found that relative cloud material in apple juice was < 0.5% (Ruck and Kitson,
1965; Stähle-Hamatschek, 1989). A 108Brix juice had a particle volume fraction of
fo ¼ 3:93 103 (Genovese and Lozano, 2000).
50
%N
40
30
20
10
D, µm
0
0
1
2
3
4
5
Figure 5.13. Particle size distribution histogram: particle relative number (%N) versus particle diameter (D)
(Genovese et al., 1997 with permission).
118
Fruit Manufacturing
Figure 5.14 shows a typical log shear stress (t) versus log shear rate (g_ ) curve for different
apple juice soluble solids’ concentration. It can be observed that t increased linearly with g_
and all curves go to the origin. Cloudy apple juice was shown to be Newtonian up to 508 Brix.
Genovese and Lozano (2000) also compared viscosity with soluble solids for both clarified
and cloudy apple juice (Fig. 5.15). Fitting relative viscosity values versus f data to the modified
Einstein’s equation (5.18) resulted in an equation with slope a ¼ 96:23. A particle suspension
with f < 0:05 (cloudy juice) conformed with the Sherman (1970) assumption [m] ¼ a ¼ 96:23.
Calculated bo resulted in bo ¼ hr =2:5 ¼ 37:43. This elevated value indicates that the first
electroviscous effect cannot be neglected in this type of products. Finally, with the estimated
La and Ba particle axial values, pa parameter resulted equal to 1.365 and Eq. (3.20) should be
considered. It can be easily calculated that the term accompanying a coefficient in Eq. (5.20) is
practically irrelevant and the effect produced in the viscosity due to nonsphericity of particles
can be neglected. When the size of particles is considered, below about 0:5 mm diameter, a
higher relative viscosity is always to be expected.
5.4. FRUIT AROMA
5.4.1. Activity Coefficients of Fruit Juice Aroma
It is well known that citrus fruits contain peel oil, the essence from which oil is obtained
during the juice processing. This oil is rich in terpene hydrocarbons (limonene), which
100.00
t, Pa
25ⴗC
10.00
ⴗBrix
10
20
30
40
1.00
50
0.10
g, s−1
0.01
10
100
1000
Figure 5.14. Log–log plot of shear rate (g_ ) and shear stress (t) at 258C for cloudy apple juice at various soluble solids
(Genovese and Lozano, 2000 with permission).
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
119
40
h, cp
25ⴗC
30
Cloudy juice
20
Clarified juice
10
X, ⴗBrix
0
10
20
30
40
50
Figure 5.15. Viscosity of cloudy and clarified apple juice as a function of soluble solids at 258C
(Genovese and Lozano, 2000 with permission).
contribute little to the aroma and are susceptible to oxidation. However, many aldehydes,
esters, and alcohols contribute to the aroma of citrus oils. Other fruits, like apples and pears,
contain much less volatile compounds and cannot form essential oils in distillates but essence
can be used as flavorants after separation and rectification. Processing of fruit juices often
involves aroma recovery, an operation by which volatile aromatic compounds contained in
natural juice are stripped, together with a certain amount of water vapor, by thermal
evaporation. Fruit aroma are very dilute solutions of esters, aldehydes, and alcohols, never
exceeding levels of few parts per million in the juice (Carelli and Lozano, 1989).
This water solution of aromatics, which can be considered at infinite dilution from a
practical standpoint, is further rectified in a packed column up to a concentration of 150 –200
fold, condensed and cooled to avoid evaporation of more volatile compounds. Design and
optimization of aroma recovery operations requires the knowledge of thermodynamic properties. To design or optimize the performance of evaporation units and rectification columns in
aroma concentration information on relative volatilities of the aroma compounds is needed.
The rate of the partial pressure of a given volatile in the vapor phase to its mole fraction
in the liquid phases is called volatility of the volatile. The ratio of the volatilities of two
compounds (a and b) is called the relative volatility:
0
aa=b
0
P g
¼ b0 b
Pa g a
(5:22)
where Pi are the vapor pressure of pure compounds and gi are the activity coefficients.
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Fruit Manufacturing
Activity coefficients were introduced to extend Raults’ law application to real solutions.
Relative volatility may be calculated if the activity coefficients of the substances are known.
Volatile aroma compounds are present in fruit products at very low concentrations and
solute–solute interactions may be neglected. Therefore, at infinite dilution the activity coefficient is a constant value g1 . From reliable values of these infinite dilution coefficients it is
possible to predict the vapor–liquid equilibrium over the entire range of compositions (Loncin
and Merson, 1979). While data on g1 of many compounds in organic solvents are easily
available (Tiegs et al., 1987), information on fruit aroma compounds’ in aqueous solution is
scarce. Aroma compounds values of g 1 may be obtained experimentally or estimated
through thermodynamic models.
5.4.2. Experimental Method
Carelli et al. (1991) measured the infinite dilution coefficients (g1 ) of aroma compounds in
model solutions simulating apple juice, by following the dilution exponential method (Leroi
et al., 1977). This method, also called the dynamic method, is based on the stripping of a
solute from a solvent by a constant flow of inert gas. The variation of solute concentration in
the carrier gas is then measured by gas–liquid chromatography. Figure 5.16 shows a sketch of
a typical equilibrium cell used in this method.
A working equation for g1 calculation valid for volatile solvent can be derived from the
equilibrium conditions and mass balance in the cell (Duhem and Vidal, 1978; Carelli et al., 1991):
g1 ¼ [(X (t)=Y (t)) þ 1] (Pav =Ps )
(5:23)
where:
X (t) ¼ ln (Si =Sit¼0 )
Y (t) ¼ ln [1 (Pt DPav t=( pt Pav )NRT)]
Carrier gas
outlet
Septum
Aroma solution
Carrier gas inlet
Fritted disk
Water bath
Figure 5.16. Sketch of a typical equilibrium cell used for aroma stripping. Temperature and pressure must be
carefully controlled.
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
121
and N is the total moles of solvent in the dilution cell, D is the flow of carrier gas through the
cell (cm3 =min), R is the gas constant, T is the cell temperature (K), Si is the area of solute
peak, Pt is the total pressure in the cell (kPa), Pav and Ps are the vapor pressure of solvent and
solute at T (kPa), respectively, and t is time (min).
Vapor pressure of fruit aroma compounds may be calculated with the Antoine equation
(Reid et al., 1977) or calculated from nonlinear regression of Ps versus T data. Table 5.5 lists
Antoine constants of some volatile compounds, valid at the usual processing temperatures.
Vapor pressure of solvent may be estimated from nonlinear regression of Psv versus T data of
glucose solutions (Taylor and Rowlinson, 1955), a reasonable assumption taking into account
the concentration and composition of fruit juices (Crapiste and Lozano, 1988). The g 1 values
were obtained from linear regression of X(t) versus Y(t) data.
A more simplified expression of Eq. (5.23) has been previously used for g1 calculation of
volatile in food model systems (Lebert and Richon, 1984; Sorrentino et al., 1986), but it failed
to represent solvent volatility.
5.4.3. Thermodynamic Models
Several models have been proposed for estimating activity coefficients. Carelli et al. (1991)
compared experimental g 1 values of apple aroma with those predicted from one-parameter
Wilson, NRTL, and UNIQUAC equations. In food engineering practice, UNIFAC has
achieved general acceptance (Saravacos et al., 1990; Sancho et al., 1997).
5.4.3.1. Wilson Equation
The Wilson expression for a binary mixture with the solute at infinite dilution is (Kruming
et al., 1980)
ln g 1 ¼ ln (1 A12 ) þ A21
(5:24)
where Aji ¼ 1 (Vj =Vi ) exp [ (gji gii )=RT]
(5:25)
and
Table 5.5. Antoine constants of aroma compounds.
Compound
Benzaldehyde
Butanol
Butyl acetate
Butyl isobutyrate
Ethanol
Ethyl acetate
Ethyl butyrate
Ethyl valerie
Hexanal
Hexanol
2-Methyl- 1- butanol
Pentyl acetate
Propanol
Trans-2-hexenal
.
.
A
B
C
14.3351
15.2010
14.1686
14.8788
16.8969
14.1366
13.9837
15.3818
15.4971
16.0848
14.2558
15.3830
15.5285
15.3857
3748.62
3137.02
3151.09
3515.11
3803.98
2790.50
3127.60
4074.13
3952.08
4055.45
2752.19
4103.45
3166.38
4007.22
66:12
94:43
69:15
40:76
41:68
57:15
60:15
40:59
38:12
76:49
116:30
40:94
80:15
47:56
Adapted from Carelli et al. (1991).
Antoine equation: In P ¼ A B=(C þ T); P is vapor pressure in kPa; T is temperature in K.
122
Fruit Manufacturing
g21 ¼ g12 ¼ (g11 g22 )1=2 (1 c12 )
(5:26)
In these equations Vij is the liquid molar volume of components i and j (m3 =mol), gii is a
constant proportional to the energy of interaction between molecules of species, and C12 is a
parameter to be determined experimentally. Hiranuma and Honma (1975) proposed that for
systems where the infinite dilution activity coefficient is of the order of 10 or greater values
gii ¼ DUi=3
(5:27)
Vj =Vi ¼ 1
(5:28)
and
can be used, where AU is the energy of vaporization of the ith component (J/mol), and can be
calculated from:
DUi ¼ [RT 2 d( ln Poi )=dT] RT
where
Poi
(5:29)
is the vapor pressure of the ith component.
5.4.3.2. NRTL Equation
The NRTL equation is a one-parameter expression valid at infinite dilution that can be
expressed, after the nonrandomness parameter a12 is set equal to 0.4 and on the basis of
V2 > V1 , as
ln g 1
2 ¼ (V2 =V1 0:4RT)(g12 g22 ) þ exp [ (g21 g11 )=RT](g12 g11 )
(5:30)
where the parameter gij was assumed to be given by Eq. (5.26) and the parameter gii by the
expression:
gii ¼ 0:08 DUi =qij
(5:31)
q12 ¼ 1; q21 ¼ (V2 =V1 )1=2
(5:32)
where
5.4.3.3. UNIQUAC Model
The UNIQUAC model was derived from statistical mechanical arguments (Abrams and
Prausnitz, 1975). It was expressed as the combination of a combinatorial term, which takes
into account liquid-phase nonidealities due to differences in molecular size and shape; and a
residual term, which takes into account nonidealities due to intermolecular interactions.
When activity coefficients are calculated at infinite dilutions, UNIQUAC equations can be
simplified considerably to give the expressions:
ln g1 ¼ ln (r2 =r1 ) þ 5q2 ln (q2 r1 =q1 r2 ) þ 5(r2 q2 ) (r2 1) r2 =r1
[5(r1 q1 r1 1)]
(5:33)
ln g1 ¼ q2 (1 þ g12 =RT g22 =RT) exp [ (g12 g11 )=RT ]
(5:34)
ln g 1
2
¼
ln g 1
c2
þ
ln g1
r2
Where
gii ¼ 0:5 DUi =qi
(5:35)
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
123
On the other hand, gij is given by Eq. (5.26); r and q are pure components structural
parameters, obtained from Gmehling et al. (1982).
5.4.3.4. UNIFAC Model
This model calculates the activity coefficients as the sum of two contributions: molecular size
and molecular interactions. A general equation for predicting infinite dilution coefficients
based in the UNIFAC model was used by Reid et al. (1987):
ln g1
1 ¼ A1,2 þ B2 n1 þ C 1 =n1 þ F 2 =n2
(5:36)
where n1 and n2 are the total number of carbon atoms in molecules 1 and 2. The other
constants are temperature dependent and specific for each binary system. Values of g 1 of
selected aroma compounds obtained experimentally and predicted with the UNIFAC model
are presented in Fig. 5.17 (Sancho et al., 1997).
5.4.4. Fruit Aroma Properties
Carelli et al. (1991) found that the activity coefficients at infinite dilution for alcohols, esters,
and aldehydes increase with the length of the carbon chain, particularly in the case of esters
and alcohols.
The behavior is in agreement with the trends obtained in previous studies (Lebert and
Richon, 1984; Sorrentino et al., 1986). Activity coefficients of lower alcohols and esters also
increased with temperature. Contrarily, heavier aromas reduced their values when the temperature was increased. On the other hand, g 1 values for ethyl butyrate, butyl acetate, ethyl
isobutyrate, and butanol remained practically constant with temperature in the range of
practical interest.
6000
Experiemental
5000
UNIFAC
4000
3000
2000
1000
Hexenal
Trans- 2hexenal
Pentyl
acetate
Butyl acetate
Ethyl
butyrate
Ethyl acetate
0
Figure 5.17. g1 values of some aroma compounds predicted with the UNIFAC model
(adapted from Sancho et al., 1997).
124
Fruit Manufacturing
The observed behavior for alcohols is in agreement with g 1 in pure water calculated
from Pierotti et al. (1959), which predicted increasing values of g1 with temperature for
ethanol, propanol, and butanol and a decrease in g 1 for hexanol. Kieckbusch and Judson
King (1979) studied the effect of temperature on partition coefficients of n-acetates in water
and polysaccharide solutions. The authors reported that partition coefficients for the series
methyl acetate–pentyl acetate increase with temperature in the range 25–408C. According to
this, it appears that a change of behavior could be expected at higher temperatures. Since the
slope of ln g1 ’versus 1/T is associated with the excess molar enthalpy, this would imply a
change from exothermic to endothermic mixtures.
Experimental values of activity coefficients at 20–258C of some aroma compounds
diluted in pure water and sugars-organic acid solution simulating apple juice are presented
in Figs. 5.18 and 5.19. It can be seen that g1 values in model solutions of most of the volatiles
are higher than those reported in pure water. It appears that the presence of apple juice
solutes leads to an increase in activity coefficients of aroma components.
EXAMPLE
Aroma stripping by flash condensation
Due to the heat sensitivity of fruit juices, multiple-effect evaporators with aroma
recovery are commonly used (see Chapter 2). Single-strength fruit juices are evaporated,
and volatile is captured by flash condensation. This process, based more on industrial
practice than theory, is schematized in Fig. 5.20 (Carelli et al., 1996), and is presented as
a computer program for the simulation of fruit aroma recovery by flash evaporation.
The PC program can estimate the volatile composition of the flash outlet streams for
different juice composition and flash temperatures. The program is valid for adiabatic
(using a preheater) and isothermal flash processes. Figure 5.21 shows a printed sheet of
results for the flash recovery of a specified fruit juice.
9000
8000
Water
7000
Model solution
ga
6000
5000
4000
3000
2000
1000
Hexanal
Trans- 2-hexenal
Pentyl acetate
Ethyl valerate
Butyl acetate
Ethyl butyrate
Ethyl acetate
0
Figure 5.18. Activity coefficients at infinite dilution of different fruit volatile in water and solution simulating fruit
juice. Adapted from Pierotti et al. (1959) and Carelli et al. (1991).
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
125
1200
Water
Model solution
1000
800
g a 600
400
200
0
Ethanol
Propanol
Butanol
Hexanol
Figure 5.19. g 1 values of some alcohols present in fruit juice aroma in pure water and model solution. Adapted from
Sorrentino et al. (1986), Chandrasekaran and Judson King (1972), Lebert and Richon (1984), and Carelli et al. (1991).
Clarified fruit juice
Evaporator
Flash
separator
Vapor and volatiles
Stripped fruit
juice
Rectification
column
Concentrated
essence
Figure 5.20. Volatile captured by flash condensation of a single-strength fruit juice.
5.4.5. Fruit Shrinkage During Dehydration
Shrinkage of fruits during dehydration is an observable physical phenomenon that occurs
simultaneously with moisture diffusion, and has a significant effect not only in drying process
but also in product quality. Shrinkage directly determines the structural properties of the
product as well as its rehydration characteristics, while it indirectly influences flavor and taste
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Fruit Manufacturing
AROMA RECOVERY BY FLASH
***************************
FLASH STREAMS
****************
FEED
LIQUID
Q(kg/h)
VAPOR
10000.00
9000.00
Q(kmol/h)
502.83
447.37
1000.00
55.47
P(mm. Hg)
4578.44
758.54
758.54
T(K)
433.10
373.80
373.80
BRIX
11.00
12.2
----
HEAT REQUIREMENT ISOTHERMAL FLASH : 29921.67 KCAL/H
AROMA COMPOSITION (ppm)
************************************
VOLATILE
FEED
LIQUID
VAPOR
RECOVERED
(%)
ETHANOL
64.93
4.64
407.45
66.01
PROPANOL
2.00
0.56
14.23
74.8
BUTANOL
6.00
1.15
47.2
82.78
2METHYL-BUTANOL
1.86
0.24
15.60
88.24
HEXANOL
2.03
0.21
17.51
90.75
ETHYL ACETATE
1.39
0.05
12.81
96.90
ETHYL
ISOBUTYRATE
12.69
0.14
119.43
99.00
ETHYL BUTYRATE
8.38
0.1
78.69
98.77
BUTYL ACETATE
4.58
0.08
42.86
98.44
PENTYL ACETATE
5.28
0.08
49.49
98.59
ETHYL VALERATE
10.26
0.13
96.40
98.83
HEXANAL
5.69
0.1
53.09
98.14
TRANS-2-HEXENAL
5.55
0.35
49.80
94.38
BENZALDEHYDE
0.47
0.06
3.9
87.83
AROMA CONCENTRATION: 7.69 FOLD
Figure 5.21. Volatile recovery by flash condensation, estimated with Carelli et al. (1996) computer program.
(This software is downloadable free of charge at the site http://www.upv.es/dtalim/)
too. The details of fruit structure at a cellular level determine the pathway of water occurring
in fruit processing. In drying fruits the development of the physical structure is characterized
by indices such as bulk, particle density, porosity, and shrinkage. Shrinkage affects the
product quality in terms of loss of rehydration capacity and decrease in rehydration rate
(Mavroudis et al., 1998). Some generalized correlations that predict bulk shrinkage coefficient
by taking into account only the initial moisture content of the food were proposed (Lozano
et al., 1983; Ratti, 1994; Mavroudis et al., 1998).
Drying is a key food preservation process of many food products. During drying, stresses
are formed due to nonuniform shrinkage resulting from nonuniform moisture and/or temperature distributions. This may lead to stress crack formation, when stresses exceed a critical
level, resulting in products of undesired quality.
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
127
5.4.5.1. Shrinkage coefficient, sb
The shrinkage coefficient may be defined as the ratio between the bulk volume of the sample
after processing and that of the fresh sample:
sb ¼
Vb
r
¼ bo (Xs þ Xw Xwo )
Vbo
rb
(5:37)
where Vb , rb , and X are bulk volume, bulk density, and mass fraction, respectively. Subscripts
o, s, and w are for initial (fresh product), solids, and water, respectively.
Experimental data for shrinkage of fruits during processing were previously reported
(Lozano et al., 1980, 1983). They showed shrinkage is dependent on processing conditions. A
few models for shrinkage were also published (Suzuki et al., 1976; Lozano et al., 1980, 1983;
Ratti, 1991). Different equations for calculating shrinkage during dehydration of selected
fruits are listed in Table 5.6.
Fictitious length model z (Roman et al., 1982) transforms every change of real length Dx
into change of fictitious length. The above model requires data on porosity and bulk density
as a function of moisture content.
Kilpatrick et al. (1975) studied volume shrinkage of potatoes and other vegetables as
drying proceeds. Charm (1978) reported on volumetric contraction of meat and potatoes.
Suzuki et al. (1976) developed three equations that are applicable to three different drying
models: uniform drying, core drying, and semicore drying. The first model results in two
alternate equations: one needs data for equilibrium moisture contents and bulk density, while
the other requires the initial moisture content and bulk density of the material. The second
and third models need the initial and equilibrium values for moisture and bulk density.
Shrinkage of fruits at different moisture contents were reported by Lozano et al. (1980,
1983), who also provided equations to predict Sb in the entire range 0 < Xw < Xwo for a
variety of foods, requiring only knowledge of the fresh food moisture content.
The linear relation between Sb and water content is well established for a wide variety of
fruits in air drying. Figure 5.22 shows the change in bulk shrinkage coefficients of pear
Table 5.6. Literature equations for the calculation of shrinkage during fruit dehydration.
Model
Basic equation
Description
Reference
Fictitious length
Dz ¼ rb Dz=[r(1 þ x)]
X ¼ real length
Román
et al. (1982)
Early stage
of drying
Sb ¼ (Xx þ 0:8)=(Xwo þ 0:8)
r ¼ rb (1 «X¼0 )
X ¼ water mass fraction
Uniform
drying
Sb ¼ (Xx þ a)=(Xwo þ a)
Core drying
Sb ¼ kXw =Xwo þ 1
Semicore
drying
Volume
shrinkage
modeling
Sb ¼ rXw =Xwo þ n0
Sb ¼ 0:161 þ 0:816=Xw =Xwo )
þ0:022 e0:018=(Xwþ0:025)
þf (1 (Xw =Xwo )
Kilpatric
et al. (1955)
Xo ¼ Initial water mass fraction
a ¼ Xe (1=rb,e 1) þ 1=rb,e
* Subscript ‘‘e’’ is
for equilibrium condition.
(Xw,e þ 1)rb,o
Xo
k¼1
(Xo þ 1)re Xo Xo,e
Suzuki
et al. (1976)
R and n0 are complex
functions of (Xw , Xwo , Xe , rb,o rb,e )
f ¼ 0:209 Sb,f
Sb,f ¼ 0:966=(Xo þ 0:796)
Lozano
et al. (1983)
Suzuki
et al. (1976)
128
Fruit Manufacturing
0.9
Sb
0.8
Pear
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
X/X o
Figure 5.22. Bulk shrinkage coefficient of pear as a function of moisture content (adapted from Lozano et al., 1983).
(Lozano et al., 1983), as a result of drying. For some products the slope of Sb becomes
noticeably less steep for X =X0 < 0:15. This is important because it indicates that all linear
predictive equations for Sb will fail to cover the entire range 0 < X =X0 < 1. No less important
is the fact that the range 0 < X =X0 < 0:15 is very significant in modeling drying operations.
In other words, this range of X at which there is a change of slope in Sb is the one where most
of the modeling and drying simulation is done.
While Suzuki’s and Kilpatrick’s equations fit the Sb data reasonably well at high X =X0
values, they fit the data less well than Lozano et al.’s (1983) model when X =X0 < 0:15.
Moreover, they fail to indicate the curvature in Sb versus X =X0 , which is encountered when
X =X0 < 0:15.
A valid question is how sensitive the data are to different drying conditions and sample
shape. Data by Kilpatrick et al. (1975), Suzuki et al. (1976), and Mazza and Lemaguer (1980)
are quite close to those reported here. All authors used conventional air drying. Kilpatrick
et al. (1975) did not report sample shape or drying conditions, although they referred to
tunnel drying. Suzuki et al. (1976) used 408C dry bulb temperature, 30% relative humidity and
air at 0.6 – 0.7 m/sec. Mazza and Lemaguer (1980) used 40.5 – 608C, an unreported relative
humidity (36%) and air at 0.30 – 0.55 m/s. Thus, as long as it is conventional air drying and
changes in drying conditions are not too drastic, the results are valid. As far as the authors
know, there are no similar data available for other drying procedures. As to the influence of
sample shape, this study reports data corresponding to cylinders of 1 cm in diameter, 4 cm in
length and, in the case of garlic, there are additional data corresponding to slicing the original
cylinder. Suzuki et al. (1976) used 1 in. cubes, while Mazza and Lemaguer (1980) dried onion
slices. The implication is that the correlations suggested are not sensitive to shape.
5
.
Color, Turbidity, and Other Sensorial and Structural Properties
129
5.4.6. Structural Damage During Freezing
Freezing is not a common operation in the fruit and fruit juice industry. During freezing the
ice crystals grow to a size that depends on the rate of heat removal. When heat is rapidly
removed, ice crystals tend to be small. On the contrary, during slow cooling the ice crystals
grows slowly outside the cell. Under such conditions cell shrinks, a phenomenon associated to
the osmotic transfer of water from inside the cell to the forming ice. In addition to this
shrinkage, there are other mechanisms of freezing damage (Reid, 1996).
(1) Cells may be destroyed during freezing due to the increasing concentration of the
unfrozen matrix, especially at high salt concentration.
(2) During fast freezing, the ice crystal formation within the cell may destroy the
membrane structure and organelles of the cell. This may result in the liberation of
enzymes responsible for undesirable reactions.
(3) While nondesirable enzymatic reactions may be controlled by blanching, this heat
process cannot prevent loss of cell turgor, associated to changes in the semipermeable
properties of the cell membrane. Loss of turgor due to freezing is more evident in
fruits that are eaten raw.
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CHAPTER 6
CHEMICAL COMPOSITION OF
FRUITS AND ITS TECHNOLOGICAL
IMPORTANCE
In spite of their popularity, fruits are relatively unimportant as major nutritional items. Fruits
are selected largely for their agreeable taste. Most fruits are juicy, with high water and sugar
content, and they become important mainly for the vitamins, minerals, and fibers they
contain. Fruits add variety and flavor to the diet.
Whole fruit may be fresh, frozen, canned, dried, made into preserves or a variety of
desserts. Concentrated fruit flavors are also used in food and drinks. Figure 6.1 shows a
simplified scheme of apple components. Fruits are living complex systems, and it is obvious
that after the liberation of these chemical reactive components during size reduction, mashing, trimming, and any other destructive process, different deteriorative reactions will take
place. Therefore, studies on the composition and changes occurring during processing and
storage might be equally helpful to the nutritionist and the processor, the latter to optimize
the processing parameters to avoid browning and other undesirable reactions affecting
organoleptic properties.
6.1. PROXIMATE COMPOSITION OF FRUIT AND FRUIT PRODUCTS
In the early 1990s the long-standing, traditional basic four food groups, consisting of meat,
dairy products, grains, and fruits and vegetables, were reworked into a balanced and healthy
food guide pyramid. This pyramid has as its base the grain group; on the second level are the
fruit and vegetable groups; on the third level are the meat and dairy groups; and at the top is
the fats, oils, and sweets’ group (Anonymous, 1992). The sources of most vitamins and
minerals belong to fruits and vegetables. This pyramid suggests three to five servings (One
serving ¼ half cup) of vegetables and two to four servings of fruit should be eaten every day.
They also provide fiber, which contains no nutrients but aids in moving food through the
digestive system (Fig. 6.2).
More recently, a new version of food pyramid, was published by the USDA (2005). This
new pyramid (Fig. 6.3) symbolizes a personalized approach to healthy eating and physical
activity, and was developed to ensure consumers make healthy food choices and active every
day.
The widths of food group bands suggest how much food should be chosen from each
group. This food pyramid recommends two cups of fruits every day. Personalized information on the amounts of food to eat each day may be accessed at the website, mypyramid.gov.
A vast amount of data have been accumulated on the compositional characteristic in
different fruits (Nagy et al., 1990, 1992; Somogy et al., 1996). Sugar and organic acids are the
133
Water
(84%)
Solids
(soluble and insoluble :
16%)
Aldehydes
Ethers
Volatiles
Esters
Malic
Alcohols
Organic
acids
Citric
Waxes and
essential
oils
Ethylene
Vitamins
Proteases
Enzymes
Nitrogen
Catalases
Oxidases
Amino
Tannins
Amino
acids
Diastases
Pectinases
Basic
Amides
Lysine,
arginine,
hystidine,
aspartic
acid, etc.
Asparagine
Pigments
Minerals
Anthocyanins
Ca, K, Na,
Mn, Mg, S,
P, etc.
Chlorophylls
Flavonoids
Carbohydrates
Dextrins
Pectin
Sugars
Starch
Cellulose
Figure 6.1. Simplified schematic representation of the most remarkable components of a fruit.
Fat, oil
sweets
Milk &
cheese
group
Meat,
eggs,
Fruits
Vegetables
Bread, cereals, pasta
Figure 6.2. Food guide pyramid (USDA,1992).
6
.
Chemical Composition of Fruits and its Technical Importance
135
Figure 6.3. USDA new food pyramid (USDA, 2005).
major constituents of soluble substances. The nutrients known to be essential for humans are
proteins, carbohydrates, fats and oils, minerals, vitamins, and water.
Compositions of fruit not only vary according to botanical variety, cultivation
practices, and weather, but also change with the degree of maturity prior to harvest, the
condition of ripeness, and storage conditions. Most fresh fruits are high in water content, and
low in protein and fat. In these cases water contents will be greater than 70% and frequently
greater than 85%. Fruits are also important sources of both digestible and indigestible
carbohydrates. The digestible carbohydrates are present largely in the form of sugars and
starches, while indigestible cellulose provides fibers that are important to normal digestion
(Table 6.1).
Fruits are also important sources of minerals and certain vitamins, especially vitamins
A and C. It is well known that citrus fruits are excellent sources of vitamin C. Beta-carotene
and certain carotenoids, and vitamin A precursors are present in the yellow-orange fruits.
Table 6.1. Typical percentage composition of edible portion selected fruits.
Fruit
Bananas
Oranges
Apples
Strawberries
Carbohydrate
Protein
Fat
Ash
Water
24.0
11.3
15.0
8.3
1.3
0.9
0.3
0.8
0.4
0.2
0.4
0.5
0.8
0.5
0.3
0.5
73.5
87.1
84.0
89.9
Source: Chen (1992) and Konja and Lovric (1993).
136
Fruit Manufacturing
6.1.1. Proteins and Amino acids
Nitrogen-containing substances are found in fruits in different combinations: proteins, amino
acids, amides, amines, nitrates, etc. In fruits, nitrogen-containing substances are less than
1% in most cases. Among nitrogen-containing substances proteins are most important
(Dauthy, 1995). Proteins are colloidal in structure, and heating makes them insoluble above
approximately 508C. This behavior needs to be considered in heat processing of fruits.
Proteins are source of amino acids, necessary for growth and tissue repair. However, fruit
proteins are less valuable than animal proteins due to the lack of essential amino acids.
Good plant sources of proteins are beans, peas, nuts, bread, and cereals. Amino acids are
defined as any group of organic molecules that consist of a basic amino group (NH2 ), an
acidic carboxyl group (COOH), and a specific organic side chain that is unique to each
amino acid. Arginine, glycine, cystine, histidine, and tryptophan are a few examples of amino
acids. The human body is unable to synthesize the so-called nine essential amino acids. In the
case of fruits they provide less than 3 g/100 g of proteins (Fig. 6.4).
6.1.2. Organic Acids
Fruit contains organic acids, such as citric acid in oranges and lemons, malic acid in
apples, and tartaric acid in grapes (Dauthy, 1995). These acids give fruits, tartness and slow
down bacterial spoilage. Acidity and sugars are the main elements determining the taste of
fruits, and sugar/acid ratio is very often used in order to give technological characterization
of fruit products.
Protein (g)
Apple
Tomato 3.5
Strawberry
3
Quince
Pumpkin
2.5
Pomegranate
Plum
Pineapple
Persimmon
Apricot
Avocado
Banana
Carambola
Cherry
2
Fig
1.5
1
Grape
0.5
Grapefruit
0
Pear
Guava
Peach
Jackfruit
Passion fruit
Orange
Olive green
Nectarine
Mulberry
Kiwifruit
Lemon
Lime
Lychee
Mandarin
Mango
Melon honeydew
Figure 6.4. Protein content of fresh fruits (g/100g) (Wills, 1987; Nagy, 1990; Somogyi et al., 1996).
6
.
Chemical Composition of Fruits and its Technical Importance
137
Malic acid is found in juices and fruits, such as apples, gooseberries, rhubarbs, and
grapes. Tartaric acid is a widely distributed plant acid with many food and industrial uses,
and is obtained from by-products of wine fermentation. Its forms include several salts
(Fig. 6.5), cream of tartar (potassium hydrogen tartrate), and Rochelle salt (potassium sodium
tartrate). It is used in effervescent tablets, gelatin desserts, fruit jellies, and as an acidifying
agent in carbonated drinks; and belongs to dicarboxylic group. Carambola fruit is rich in
oxalic acid (Swi-Bea Wu et al., 1992). Among other organic acids present in minor amounts
lactic, succinic, pyruvic, glyceric, shikimic, maleic, and isocitric acids must be included
(Fig. 6.6). One of the consequences of the organic acid content in fruits is the relatively
wide range of pH encountered in fruit products (Table 6.2).
6.1.3. Carbohydrates
Carbohydrates are the main component of fruits, representing more than 90% of their dry
matter. They are produced by the process of photosynthesis and function as structural
Figure 6.5. SEM micrograph of grape juice tartrates (Buglione, 2005).
800
700
600
500
400 mg/100 g
300
200
100
0
Citric
rry
e
ap
ac
h
a
Pe
r
Ki
Tartaric
ple
Ap
Gr
Pe
Malic
e
Ch
Quim
wi
ry
ge
er
wb
it
fru
ra
St
an
Or
Figure 6.6. Organic acid content of selected fruits (Wills, 1987; Nagy, 1990; Somogyi et al., 1996).
138
Fruit Manufacturing
Table 6.2. pH values of selected fruit products.
Food product
pH
Apple butter
Apple sauce
Apples
Apricots
Cherries
Cucumbers
Grapefruits
Lemons
Olives
Oranges
3.1–3.5
3.6 –3. 9
3.0 –3.3
3.7–3.8
3.4 –4.0
3.0 –3.5
3.2–3.5
2.3 –2.6
2.9–3.2
3.2–38
Food product
pH
Orange juice
Peaches
Peas
Pineapple juice
Plums, currants
Prune juice
Pumpkins
Raisins
Strawberries
Tomato juice
3.7– 4.1
3.4 –3.6
6.1–6.4
3.3 –3.6
2.9 –3.2
3.7– 4.1
4.1– 4.4
3.6– 4.2
3.3 –3.4
4.0 – 4.5
Source: Dennis, 1983; Friend, 1982; Goodenough and Atkin, 1981; Jackson and
Shinn, 1979; Salunkhe, 1991; Wills et al., 1989; Hui, 1991.
components as in the case of cellulose. On the other hand, as starch, carbohydrates account
for the energy reserves; they function as essential components of nucleic acids as in the case of
ribose, and also as components of vitamins such as ribose and riboflavin (Dauthy, 1995).
Carbohydrates account for more than half of the calories’ intake for most people, daily
adult intake should contain about 500 g carbohydrates. Starches and sugars are the main
source of the body’s energy.
However, sugars are not essential foods; they provide energy but not nutrients. Figure 6.7
shows that fruit sugars mainly consist of glucose, fructose, and sucrose. Maltose and minor
12
10
8
% 6
4
Apple
Cherry
Grape
2
Peach
Pear
0
Kiwifruit
Fructose
Glucose
Strawberry
Sucrose
Sorbitol
Figure 6.7. Sugar content of selected fruits.
6
.
Chemical Composition of Fruits and its Technical Importance
139
Figure 6.8. Scanning electron photomicrograph of an isolated apple starch granule (5 kV 4,400).
percentage of other mono- and oligosaccharides are also present in fruits (McLellan and
Acree, 1992). In mango purée heptulose and xylose have been detected. Carambola juice is
rich in arabinose (Swi-Bea Wu et al., 1992).
6.1.3.1. Starch
Starches provide a reserve energy source in plants and supply energy in nutrition; they are
found in seeds and tubers as characteristic starch granules. Apple juice is one of the juices that
can contain considerable amounts of starch, particularly at the beginning of the season.
Unripe apples contain as much as 15% starch (Reed, 1975). Apple starch granules could be
considered practically spherical (Fig. 6.8). In this case, major (La ¼ 9:21mm) and minor axes
(Ba ¼ 7:86mm) are very similar (Carrı́n et al., 2004).
The starch content varies from fruit to fruit (Table 6.3), variety to variety, and season to
season within a given fruit. As the fruit matures on the tree, starch hydrolyzes into sugars.
Table 6.3. Starch content of selected fruits.
Fruit
Guava
Apple
Jackfruit
Carambola
Mango
Pumpkin
Banana
Starch (g/kg)
1
2
4
5
5
17
31
Source: Somogyi et al., 1996; Sanchez-Castillo et al., 2000;
Carrin et al., 2004.
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Fruit Manufacturing
Decrease in starch usually begins a few weeks before harvest. Apple starch resulted in
particles of regular shape with a mean diameter D ¼ 9:21mm (s ¼ 2:74) (Carrin et al., 2004)
6.1.3.2. Pectin
Pectin is a ‘‘gum’’ found naturally in fruits that causes jelly to change to gel. Tart apples, crab
apples, sour plums, Concord grapes, quinces, gooseberries, red currants, and cranberries are
especially high in pectin. Apricots, blueberries, cherries, peaches, pineapples, rhubarbs, and
strawberries are low in pectin. Underripe fruit has more pectin than fully ripe fruit.
Pectin consists of a backbone, in which ‘‘smooth’’ a-d-(1–4)-galacturonan regions are
interrupted by ramified rhamnogalacturonan regions, highly substituted by neutral sugar side
chains (Oakenfull, 1991) (Fig. 6.9). An important feature of galacturonans is the esterification
of the galacturonic acid residues with methanol. The degree of methoxylation (DM) is defined
as the number of moles of methanol per 100 moles of galacturonic acid.
6.1.4. Lipids
Fats and oils are a concentrated source of energy. Fats make certain vitamins available for use
in the body, they cushion vital organs, help to maintain body temperature, and make up
part of all body cells. Most fruits have fat content <0.5 g/100 g edible portion (Watt and
Merrill, 1963). However, significant quantities are found in nuts (55%), apricot kernel (40%),
grape seeds (16%), apple seeds (20%), and tomato seeds (18%). Table 6.4 lists fruits with
relatively high fat content.
6.1.5. Minerals
Most of foods contribute to a varied intake of essential minerals. Calcium builds bones and
teeth, and it is necessary for blood clotting. The best sources are milk and hard cheese. Others
are leafy greens, nuts, and small fishes (sardines) with bones that can be eaten. Calcium content
in fruits generally rarely exceeds 40 mg/100 g edible portion (Fig. 6.10). (Dauthy, 1995)
6
4
COOCH3
O
OH
OH
1
OH
α
OH
O
O
COOH
O
COOCH3
O
OH
OH
O
OH
O
OH
O
COOCH3
Figure 6.9. Polygalacturonic acid molecule.
Table 6.4. High fat content fruits.
Fruit
Avocado
Lychee
Olive green
Coconut
Source: Watt and Merrill, 1963.
Fat (g/100g)
16
1
13
35
6
.
Chemical Composition of Fruits and its Technical Importance
141
Calcium
Apple
Tangerine
Sapote
Sapodilla
Raspberry
40
Apricot
Avocado
Banana
Blackberry
30
Quince
Plum
Breadfruit
Carissa
20
Pineapple
Cherimoya
10
Pear
Cherry,Sour Red
0
Peach
Cherry, Sweet
Passion -Fruit
Crabapple
Papaya
Fig
Orange
Gooseberry
Mulberry
Grapefruit
Meloncantaloupe
Mango
Loquat
Guava
Jackfruit
Lime
Lemon
Jujub
Kiwifruit
Figure 6.10. Calcium content in 100 g edible portion of fruits (Watt and Merrill, 1963; Wills, 1987).
Mineral substances are present as salts of organic or inorganic acids, or as complex
organic combinations (chlorophyll, lecithin, etc.); they are in many cases dissolved in
cellular juice. Mineral-rich fruit includes strawberries, cherries, peaches, and raspberries.
Important quantities of potassium (K) and absence of sodium chloride (NaCl) give a high
dietectic value to fruits and to their processed products. Phosphorus is mainly supplied by
vegetables.
Potassium is a major mineral present in fruits and ranges from 30 (cherimoya) to 600 mg/
100 g (avocado) edible portion (Fig. 6.11). Phosphorus works with calcium to make strong
bones and teeth. A diet that has enough protein and calcium also provides enough phosphorus (Fig. 6.12). Fruits with relatively high levels of sodium and magnesium are shown in
Figs. 6.13 and 6.14, respectively. Copper is of particular interest since it is a cofactor for PPO
and can also serve as a catalyst for numerous oxidative reactions.
Iron helps to build red blood cells and aids the blood in carrying oxygen to the cells. Iron
content of selected fruits is given in Fig. 6.15.
6.1.6. Vitamins
Many reactions in the body require several vitamins, and the lack or excess of any one can
interfere with the function of another. As the body cannot manufacture all vitamins, it must
absorb them from food. Vitamins are also added to fruit products mainly for nutritional
purposes.
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Fruit Manufacturing
Potassium
LONGANS
Watermelon
Tangerine
400
Loquat
Lychee
Mango
300
Strawberry
Soursop
Melon honeydew
200
Mulberry
Sapote
100
Sapodilla
Nectarin
0
Orange
Rose apple
Papaya
Rhubarb
Passion fruit
Raspberry
Peach
Quince
Pear
Persimmon
Pomegranate
Plum
Pineapple
Figure 6.11. Potassium content of selected fruits (100g edible portion) (Watt and Merrill, 1963; Wills, 1987).
High P fruits (mg/kg)
Apricot
Strawberry
Soursop
700
600
Avocado
Banana
500
Sapote
Blackberry
400
300
200
Passion fruit
Breadfruit
100
0
Mulberry
Cherimoya
Lychee
Cherry, sweet
Loquat
Gooseberry
Guava
Longans
Kiwifruit
Jackfruit
Jujub
Figure 6.12. Selected fruits with high phosphorous content (Watt and Merrill, 1963; Wills, 1987).
6
.
Chemical Composition of Fruits and its Technical Importance
143
Sodium
Avocado
30
Strawberry
Mammy apple
20
Soursop
Melon cantaloupe
10
Sapote
Melon honeydew
0
Sapodilla
Mulberry
Passion fruit
Rhubarb
Quince
Persimmon
Figure 6.13. Selected fruits with high sodium content (100g edible portion) (Watt and Merrill, 1963; Wills, 1987).
Magnesium
Longans
Watermelon
Tangerine
Strawberry
30
Loquat
Lychee
25
Mango
20
Soursop
Sapote
Melon honeydew
15
Mulberry
10
5
Rose apple
Nectarine
0
Rhubarb
Orange
Raspberry
Papaya
Passion fr, purple
Quince
Pomegranate
Peach
Plum
Pineapple
Pear
Persimmon japan
Figure 6.14. Selected fruits with high magnesium content (100g edible portion) (Watt and Merrill, 1963; Wills, 1987).
144
Fruit Manufacturing
2
Metal concentration (mg/kg)
1.8
Copper
1.6
Zinc
1.4
Iron
1.2
1
0.8
0.6
0.4
0.2
Mulberry
Avocado
Sapodilla
Lemon
Soursop
Breadfruit
Raspberry
Kiwifruit
Cherimoya
Fig
Stberry
Gooseberry
Cherry, Sour
Pomegranate
Rhubarb
Carambola
Cranberry
Watermelon
Mango
Persimmo
Grapefruit
Papaya
Tangerine
Rose apple
0
Figure 6.15. Metal composition of selected fruits (Wills, 1987; Nagy, 1990; Somogyi et al., 1994).
Vitamin B group and vitamin C (or ascorbic acid) are water-soluble vitamins that are not
stored in the body for long, hence should be consumed every day. Table 6.5 lists major
vegetable source of vitamins, and Fig. 6.16 shows a selection of fruits with relatively high
ascorbic acid content.
Some vitamins also serve multiple functional purposes: vitamins C and E act as antioxidants, prevent undesirable color changes, and retard the development of rancidity. Provitamin A (or b-carotene) and riboflavine (vitamin B2 ) are used as natural colorants. Four
vitamins (A, D, E, and K) are known as the fat-soluble vitamins. They are digested and
absorbed with the help of fats that are in the diet.
6.1.7. Water
Water plays an active part in many chemical reactions and is needed to carry other nutrients,
regulate body temperature, and help eliminate wastes (Dauthy, 1995). Water makes up about
60% of an adult’s body weight. Requirements for water are met in many ways. Most fruits are
more than 80% water (Fig. 6.17).
6.1.8. Aroma
Aroma components exist in very small quantities in fruits and are composed of various
chemical species: alcohols, aldehydes, esters, terpenes, etc. Ripe fruits, especially bananas,
Table 6.5. Major vegetable sources of vitamins.
Vitamin
Major vegetable sources
Vitamin B1 (thiamine), mg
Vitamin B6 , mg
Folic acid, mg
Vitamin C, mg
Vitamin E, IU
Vitamin K
Cereal grains, nuts
Cereal grains
Green leafy vegetables, wheat bran and germ
Citrus fruits, green peppers, broccoli, cantaloupe
Vegetable oils, margarine, cereal grains
Green leafy vegetables, vegetable oils
IU is international unit, mg stands for milligram, indicates no data available.
Source for RDA data: US Department of Health, Education, and Welfare.
.
Chemical Composition of Fruits and its Technical Importance
145
300
250
200
150
100
Tomato
Passion Fruit
Cherry
Pineapple
Mango
Carambola
Grapefruit
Quince
Lime
Lemon
Lychee
Orange
Kiwifruit
Guava
0
Mandarin
50
Strawberry
Ascorbic acid (g/kg)
6
Figure 6.16. Ascorbic acid content of selected fruits (Watt and Merrill, 1963; Wills, 1987).
Water
Apple
Tomato
Strawberry
Quince
Pumpkin
Pomegranate
Plum
95
90
Apricot
Avocado
Banana
85
Carambola
80
Cherry
75
Fig
70
Pineapple
Grape
65
Persimmon
Grapefruit
60
Pear
Guava
Peach
Jackfruit
Kiwifruit
Passion Fruit
Lemon
Orange
Lime
Olive, green
Nectarine
Melon honeydew
Lychee
Mango
Mandarin
Figure 6.17. Water content of fruits (100g edible portion) (Wills, 1987; Nagy, 1990; Somogyi et al., 1994).
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Fruit Manufacturing
Table 6.6. Esters usually found in fruits.
Name
Formula
Identified in
Butyl-acetate
Octyl-acetate
Ethyl-butyrate
CH3 COOC4 H9
CH3 COOC8 H17
CH3 COOC2 H5
Bananas
Oranges
Pineapples
oranges, and pineapples, owe their odors to the presence of esters. Some common esters
formed with acetic acid (CH3 COOH) are found in Table 6.6. Fruit processing techniques
must be designed and operated to reduce loss and modification of aroma components.
The volatile components of fruit aroma are usually recovered by removing them by a
partial evaporation of the fresh juice, prior to the clarification and/or concentration operations. These dilute aqueous aromas are usually concentrated by distillation and then
returned to the juice. The process of aroma recovery and concentration can be optimally
designed and efficiently operated if the composition of the aroma is known.
Figure 6.18 compares the aroma quality of volatile extracts obtained from whole and
peeled apples, based only on the most desirable compounds (Carelli and Lozano, 1989). Results
indicated that, in the case of the GS aroma, only those volatiles with relatively high retention
times increased their relative composition when the whole fruit was processed. On the contrary,
‘‘whole RD apple’’ aroma was rich in those desirable compounds with low retention times.
However, organoleptic assessment indicated ‘‘whole apple’’ aroma to be more ‘‘fruity’’
and ‘‘characteristic’’ than that of peeled apples independent of the apple variety. Figure 6.19
compares the volatiles present in the aroma extracted from a commercial apple essence.
120
1/2 h
1h
Recovery (%)
100
80
60
40
20
Benzanal
Hexanol
Ethyl acetate
2-Methyl-i-butanol
Butanol
Hexanol
Ethanol
Ethylbutyrate
0
Figure 6.18. Contribution of peel to aroma: comparison between desirable volatiles of ‘‘whole apple’’ and
‘‘peeled apple’’ aroma. Expressed as relative area (A%) of total desirable (Adapted from Carelli and Lozano, 1989.)
A very significant increase in ethanol content was also determined and attributed to extensive fermentation during
the grinding and pressing operations.
6
Chemical Composition of Fruits and its Technical Importance
.
147
1000
Volatile (ppm)
100
10
1
0.1
0.01
Ethanol
Butanol
Ethyl acetate
Propanol+ethly butyrate
Hexanal
Ethyl valerate +pentyl acetate
2-Methyl-1-butanol
Butyl acetate
Hexanol
Trans-2-hexenal
Ethylisobutyrate
Trans-2-hexnol
Hexyl acetate
Benzanal
Acetophenone
4-Methoxyallyl-benzene
0.001
Figure 6.19. Volatile composition of apple aroma (Source: Carelli and Lozano, 1989).
The values indicated that substantial losses of very valuable components (e.g., ethylisobutyrate, pentyl acetate, and trans-2-hexenal) occurred during the industrial process.
6.1.9. Color compounds
Different pigments complete the proximate composition of fruits. Color is an important
aspect of both natural and processed fruits. Natural colorants are in general unstable, and
color of fruits and fruit products may change during processing and storage.
Natural pigments may be defined as the pigments occurring in unprocessed fruits, as well
as those formed upon processing and storage. Major fruits and fruit products pigments can be
grouped into chlorophylls, carotenoids, flavonoids (anthocyanins and anthoanthins), Melanoidins, and caramels (Dauthy, 1995).
.
.
Chlorophyll. This is the most abundant of all these pigments. In living plant tissues,
chlorophyll is present in chloroplast cells as colloidal suspension and is associated to
carbohydrates and protein. There are two types of chlorophyll:
(a) Blue-green. Its chemical formula is C55 H72 O5 N4 Mg.
(b) Yellow-green. Found in most green tissues; its formula is C55 H70 O6 N4 Mg.
In many fruits chlorophyll is present in the unripe state and gradually disappears
during ripening. Chlorophyll is water insoluble. Sodium copper chlorophyllin salt,
obtained after the hydrolysis of chlorophyll with sodium hydroxide and the replacement of magnesium with copper, is a heat-stable coloring food.
Carotenoids. Pigments belonging to this group are fat soluble and range in color from
yellow through orange to red. Important fruit carotenoids include the orange carotenes
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Fruit Manufacturing
.
of apricot, peach, and citrus fruits; the red lycopene of watermelon and apricot; and the
yellow-orange xanthophyll of peach. These and other carotenoids seldom occur singly
within plant cells. The content of these pigments rarely exceeds 0.1%. In fruits,
b-carotene is an indication of provitamin A content. The carotenoids g, b-carotene,
and phytofluene were reported as the three main carotenoids in passion fruit (purple)
(Chan, 1994).
Flavonoids. Flavonoids are polyphenolic compounds possessing 15 carbon atoms;
two benzene rings joined by a linear three-carbon chain. This class of pigments are
water soluble and commonly present in the juices of fruits. Flavonoids include the
purple, blue, and red anthocyanins of grapes, berries, plump, eggplants, and cherries;
the yellow anthoxanthins of apples, and the colorless catechins and leucoanthocyanins,
which are tannins present in apples and grapes. These colorless tannin compounds are
easily converted to brown pigments upon reaction with metal ions. Anthocyanin
pigments (red and purple) occur in the sap of cells. Anthocyanins give the familiar
color to fruits such as red apples, blueberries, cherries, cranberries, strawberries, and
plums. Anthocyanins are responsible for color in most berries. Anthocyanin concentration is usually expressed as cyanidin-3-glucoside/100 g pulp sample. The color of
concord grapes is due to anthocyanin pigments, the major contributor being delphidin
monoglucoside (McLelland and Acree, 1992).
Phenolic acids are not practically found in free forms in plants because the carboxyl
groups are very active and easily transform into esters or amides when combined with
aliphatic alcohols and phenols or amino compounds. Phenolic acids are divided into two
subgroups, which are the derivatives of hydroxybenzoic acid and hydroxycinnamic acid.
The most important derivatives of hydroxybenzoic acid are ferulic acid, caffeic acid,
and coumaric acid, which occur in trace amounts naturally in plants. The caffeic acid ester of
d-quinic acid is one of the most important polyphenols naturally occurring in apples.
Flavonoids are divided into five subgroups according to their chemical structure (Fig. 6.20).
All flavonoids are derived from flavan (2-phenol-benzo-dihydropyran). The general structure
(C6 C3 C6 ) is given in Fig. 6.21.
Anthocyanidins
Anthocyanidins are found in nature as glycosides and are called anthocyanins. Anthocyanins are natural color pigments, which have a color range varying from rich red to blue, the
characteristic color of most fruits.
Flavones and Flavonols
Flavones and flavonols, which have slight yellow color, are practically found in all
plants. Flavones differ from flavonols in the absence of OH-group at C3 -atom of the
center ring.
Flavonones
Flavonones do not have a double bond at the center ring. Their glycosides are mainly
found in citrus fruits, like naringin (Fig. 6.22).
6
.
Chemical Composition of Fruits and its Technical Importance
Anthocyanidins
Flavones and
flavanols
Flavanones
Flavonoids
Found in nature as
glycosides called
anthocyanins
Familiar Color of red
apples, blueberries,
cherries, cranberries,
strawberries, and plums
Flavones and flavonols
(slight yellow in color—
practically found in
any plants)
Flavones differ from
flavonols in the absence
of OH-group at C3-atom
of the center ring
Flavonones do not
have a double bond at
the center ring
Their glycosides are
mainly found in citrus
fruits
149
The most frequent
catechins are diastereoisomer pair (+)–
catechin and
(–)–epicatechin, as well
as (+)-gallocatechin
and (–)–pigallocatechin
Catechins and
leucoanthocyanidins
Catechins are very
reactive if exposed to
atmospheric oxygen
Proanthocyanidins
They can condense
chemically and
enzymatically into
oligomers and polymers,
forming
proanthocyanidins
Figure 6.20. General classification of flavonoids (Source: Herrmann, 1976; Cook and Samman, 1996).
8
2
1
o 2 1
7
6
4
6
3
5
3
5
4
o
Figure 6.21. General structural formula of flavonoids.
O
Aa-L/ Rh(1-6)-b-Gl-O
OH
OH
O
Figure 6.22. Chemical structure of naringine.
Catechins and Leucoanthocyanidins
Catechins are flavan-3-ol monomers comprising one OH-group at C3 -atom. The leucoanthocyanidins flavan-3,4-diols, have one OH-group at C3 - and C4 -atoms. The most
frequent catechins are diastereoisomer pair (þ)-catechin and ()-epicatechin, as well as (þ)gallocatechin and ()-epigallocatechin. Catechins are very reactive if exposed to atmospheric
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Fruit Manufacturing
oxygen. They can condense chemically and enzymatically into oligomers and polymers
forming proanthocyanidins.
Caramel. It is an amorphous dark brown-coloring material formed by heating saccharides, alone or in the presence of amino products or selected accelerators. Maillard-type
reactions are involved and the products are extremely complex in composition. Detailed
information on nonenzymatic browning is given in Chapter 7.
Finally, proanthocyanidins are colorless if their chain is short. Yellowish to brown color
is formed with increasing polymerization. When they are heated in acidic media, they
transform into corresponding anthocyanidins, getting the typical reddish-violet color.
6.2. INFLUENCE OF PROCESSING AND STORAGE ON THE
COMPOSITION OF FRUITS
As soon as fruits are harvested, deterioration of quality attributes or nutrients begins and
increases with time. Nutrients are lost by:
(a)
(b)
(c)
(d)
Food processing operations
Sensitivity of nutrients to pH, oxygen, light, and heat
Enzyme action
Measures to control enzyme activity, e.g., blanching.
6.2.1. Vitamin Destruction During Processing and Storage
Vitamin losses may occur in canned fruits when stored at high temperatures (>378C).
Vitamin C (ascorbic acid) is probably the most unstable vitamin, and it is readily oxidized
by many nonenzymatic processes. Although frozen storage temperatures between 18 and
288C result in satisfactory vitamin C retention levels in fruits, at temperatures above 108C,
it is easily oxidized and will be drastically reduced in a short period of time. The use of
package materials impermeable to oxygen and light is recommended. The enzyme ascorbate
oxidase, which is not present to any great extent in vitamin C sources, causes oxidation. It was
also found that the mixing of orange juice with mashed bananas naturally containing
phenolase, act similar to ascorbate oxidase. All vitamins are subject to enzyme hydrolysis
and the above illustrates the point. Particular food combinations can lead to nutrient loss.
Benterud (1977) studied thermal stability of vitamins in a hot melt of carbohydrates free
of oxygen. The author found some vitamins were extremely heat resistant (e.g., vitamin E,
riboflavine), whereas thiamine was the most labile of the vitamins. Some vitamins (A, D, B12 ,
and C) show a gradual degradation as temperature is raised from 100 to 1308C. However, in
fruit processing vitamin stability condition is more complicated because pH, oxygen, ions,
reducing agents, etc. influence the rate of decomposition. Figure 6.23 shows the loss in canned
fruit products. Canned or bottled fruit juice stored at ambient temperature for extended
period is likely to lose all its vitamin C content (Cameron, 1978).
The principal causes for vitamin C destruction in bottled juice are oxidation by residual
air in the head space, anaerobic decomposition, and the effect of light. It was observed that
after some fruit processing operation significant loss of ascorbic acid occurs (Nagy, 1980). Up
to 47% loss in vitamin C occurred in canned fruits after two years’ storage at 278C.
Adisa (1986) studied the influence of storage and molds on the ascorbic acid content of
orange and pineapple fruits. He found that about 40% loss of ascorbic acid was recorded in
both fruits stored at 308C for 8 weeks. The rate of loss of vitamin C was observed to be faster
in fruits infected with mold than in healthy fruits.
6
.
Chemical Composition of Fruits and its Technical Importance
151
120
Vitamin,%retained
Vit.C
100
B6
Panth.acid
80
60
40
20
0
Furit juices
Grapefurit juice
Orange juice
Peaches
Apricots
Figure 6.23. Loss of vitamins C and B6 , and panthotenic acid in canned fruit and fruit juices
(adapted from Benterud, 1977).
Maeda and Mussa (1986) indicated that ascorbic acid depletion with time in bottled and
canned orange juice was almost linear. The ascorbic acid levels in canned orange juice stored
for 8 weeks at room temperature were significantly higher than in bottled (glass) juice.
Ascorbic acid is also present in relatively high concentration in raspberry. Ochoa et al.
(1999) measured the percentage of residual ascorbic acid during long-term storage of raspberry pulp at 4, 20, and 378C (Fig. 6.24). These values are in general agreement with those
Ascorbic acid (%)
100
10
37ⴗC
20 ⴗ C
4ⴗC
1
56
52
48
44
40
36
32
28
24
20
16
12
8
4
0
0.1
Storage time(days)
Figure 6.24. Semilogarithmic plot of the ascorbic acid reduction during storage of raspberry pulp at 48, 208 and 378C.
Reprinted from Lebensm. Wilss. u Technol. 32(3): 149–153. Ochoa, M.R. Kesseler, A.G., Vullioud, M.B. and
Lozano, J.E. Physical and chemical characteristics of raspberry pulp: storage effect on composition and color.
(copyright) 1999, with permission from Elsevier.
152
Fruit Manufacturing
reported for other fruits and vegetables in the same range of temperatures (Villota and
Hawkes, 1992).
Ochoa et al. (1999) found that ascorbic acid decrease in raspberry pulp obeys the
following linear regression equation:
AA(%) ¼ 100ekt
(6:1)
where AA is the ascorbic acid content (%), k is the fitted reaction rate constant, and t is
the storage time (days). Parameters and correlation coefficients of Eq. (6.1) are k37 C
¼ 0:0980 (r2 ¼ 0:978), k20 C ¼ 0:0424 (r2 ¼ 0:979), and k4 C ¼ 0:0275 (r2 ¼ 0:995).
Calculated values of the rate constant, k, were temperature dependent and the effect of
this variable was calculated according to the Arrhenius model. The calculated activation
energy for ascorbic acid degradation in raspberry pulp was Ea ¼ 6:45 kcal=mol (r2 ¼ 0:970).
6.2.2. Effect of Storage on Metal Content
Acid liquid foods, as many fruit juices are, interact with the components of the container. In
the case of canned juices, corrosion of tinplate increases the heavy metal content, especially
tin, lead, and iron. Table 6.7 lists the increase in heavy metal content in canned orange
juice.
Packing of fruit juice in tin cans causes a higher contamination with heavy metals than
that in paperboard boxes or laminated pouches.
6.2.2.1. Influence of Storage on Fruit Juice Aroma
The effect of storage condition on fruit juice aroma has been extensively studied. Velez et al.
(1993) studied changes of orange juice aroma due to storage time and temperature, by gas
chromatographic analysis of nearly 40 volatile constituents. The authors found that at 08C
changes in orange volatile were very slow and difficult to detect. Contrarily, at 358C the
quality of the orange aroma deteriorates very fast. Figure 6.25 shows orange aroma deterioration of the principal volatile, at 208C after 3 months’ storage.
During aroma stripping of the single-strength juice when the amount of evaporated
water is in the 10 –12% range, up to 90% of the aroma compound evaporates. Moreover,
only a slightly greater evaporation of volatile may be achieved by doubling the amount of
water evaporated. Vélez et al. (1993) also found that only 7 out of 38 volatile were significantly affected by storage time and temperature.
Table 6.7. Heavy metal content of orange juice when
affected by type containers.
Metal
Tin
Lead
Iron
Zinc
Copper
Fresh fruit
Tin can
Other containers
–
–
0.1
0.07
0.045
55
0.16
2.75
1.05
0.09
0.4
0.1
0.16
0.12
0.03
Adapted from Mesallam, 1987.
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.
Chemical Composition of Fruits and its Technical Importance
153
Acetoin
Acetoin
Terpinolene
Volatile (mg/L)
Linalool
Alfa-pinene
Ethyl butyrate
Octanal
Terpinene-4-ol
Initial
1 month
Dodecanal
2 months
Citronellon
3 months
0
1
2
3
Figure 6.25. Changes in the main orange volatile after 3 months’ storage at 208C (adapted from Kirchner and
Miller, 1957; Petersen et al., 1998).
6.2.3. Fruit Juice Change in Amino Acid Content During Storage
During storage fruit juices are exposed to temperatures that have an adverse influence on
quality. In these fruit juices the major constituents believed to be involved in browning are the
reducing sugars, amino acids, polyphenols, and organic acids (Joslyn, 1956; Cornwell and
Wrolstad, 1981). Wolfrom, Kashimura, and Norton (1974) studied browning mixtures constituted by different amino acids and glucose in 1:1 molar ratio simulating orange juice stored
at 658C and reported that g-aminobutiric acid and l-arginine were the main contributors to
browning. del Castillo et al. (1998) found a loss of 65.1% of original amino acids’ content,
when storing dehydrated orange juice (aw ¼ 0:44) at 508C for 14 days. About 78% of the loss
was attributed to the amino acids in major proportions, namely proline, arginine, asparagine,
and g-aminobutiric acid.
Babsky et al. (1986) evaluated changes in the composition of clarified apple juice
concentrate during prolonged storage at 378C. Results showed that storage caused an 87%
loss in the total free amino acids, which was mostly due to decreases in glutamic acid,
asparagine, and aspartic acid (Fig. 6.26).
The major constituents were asparagine (Asn), aspartic acid (Asp), and glutamic acid
(Glu). The other individual amino acids amounted to less than 10%. These values are similar
to those found by Burroughs (1957) and Czapski (1975). Bielig and Hofsommer (1982),
working with about 90 samples of apples, apple juices, and concentrates, found that every
apple juice has a characteristic amino acid spectrum and no mean value can be specified.
The concentration changes of total amino acids during storage of apple juice were very
large (Fig. 6.27). Asp and Glu decreased more markedly. Several studies (Warmbier et al.,
154
Fruit Manufacturing
500
450
Total
% Retention
400
Amino acids (mg/L)
350
300
250
200
150
100
50
0
0
50
100
150
Time (days)
Figure 6.26. Free amino acid composition of apple juice concentrate (758Brix). Variation during storage at 378C
(adapted from Babsky et al., 1986).
350
Asn
300
Glu
AA (mg/L)
250
Asp
200
150
100
50
0
0
20
40
60
Time (days)
80
100
120
Figure 6.27. Decreases in glutamic acid, asparagine, and aspartic acid (adapted from Babsky et al., 1986).
1976; Spark, 1969; Eichner and Karel, 1972; Reyes et al., 1982) reported Maillard browning
of reducing sugars with only one or two amino acids other than Asp, Asn, and Glu.
Wolfrom et al. (1974) and Ashoor and Zent (1984) studied the influence of different
amino acids in model systems. None of these studies have shown Glu and Asn to be very high
browning producing compounds.
6
.
Chemical Composition of Fruits and its Technical Importance
155
Buedo et al. (2001) studied the change of free amino acid (AA) composition of peach
juice concentrate (PJC), as a result of the Maillard reactions. The authors observed that
total AA content decreased 8, 35, and 60%, after 112 days of storage at 15, 30, and 378C,
respectively (Fig. 6.28). The main constituent, asparagine, contributed 71% of the total
loss while aspartic acid increased its concentration, probably as a result of the asparagine
degradation.
Buedo et al. (2001) also found that during storage of peach juice at 378C glutamine
concentration drops 60 fold, while alanine reduces only to half at the same time and conditions.
However, it must be taken into account that the contribution of each AA to the juice browning
is not directly related to the consumption rate, as each AA can produce different chromophores, having different light absorbance characteristics (Labuza and Baisier, 1992).
6.2.4. Effect of Storage on Fruit Sugars
Sucrose in an acid media, as many fruit products are, can hydrolyze under a rate corresponding to a first-order process (Babsky et al., 1986). The reducing sugars increased at a rate
determined by the inversion of sucrose. It is well known that the rate of hydrolysis is a
function of the concentration of reactants, temperature, and acid–catalyst concentration
(Glasstone, 1946). If excess water is present the rate of disappearance of sucrose can be
represented by a pseudo first-order reaction rate equation:
S ¼ S0 exp (Kt)
(6:2)
where S0 is the initial sucrose concentration, moles/100 g concentrate, S is the sucrose
concentration at time t; K is the rate constant (0:00822 day1 under studied conditions), t is
the time, min. Hydrolysis, also called inversion because it is accompanied by an inversion of
the angle of polarization, yields two simple sugars, d-glucose and d-fructose. The rate of
appearance representing total reducing sugars is described by Eq. (5.3):
R ¼ 2So (1 eKt ) þ Ro
(6:3)
where: R ¼ reducing sugars (glucose þ fructose) concentration at time t moles/100 g concentrate, Ro is the reducing sugar concentration at t ¼ t0 ; and t is time, mm.
AA (mg/L) after 11 days, storage
6000
Peach juice: 12ⴗBrix
5000
4000
3000
2000
1000
0
Initial
37ⴗC
30ⴗC
15ⴗC
Figure 6.28. Decrease in PJC’s total AA content during storage at 37, 30, and 158C (adapted from Buedo et al., 2001).
156
Fruit Manufacturing
Schoebel et al. (1969) obtained experimental data on the dependence of the first-order
reaction rate on pH. Figure 6.29 shows the development of sucrose and total reducing sugars
during storage of concentrated apple juice, which increased in concentration in accordance
with the predicted kinetics (Eq. 6.3). Hence, hydrolysis appeared to be the major cause of
sucrose reduction (and reducing sugars increase) in apple juice at a rate determined by pH and
temperature.
Akhavan and Wrolstad (1980) verified that slight losses (6%) in total sugars occur after
112 days of storage at 378C of pear concentrate. Stadtman (1948) considered the possibility
that relatively small chemical changes are required to produce brown pigment of intense
color. If this is the case, the changes in reducing sugars necessary to produce large changes in
color might be hard to be detectable.
Beveridge and Harrison (1984) detected no loss of reducing sugar after heating 72.50
Brix-pear juice at temperatures up to 808C for 2 h. Reyes et al. (1982) found that glucose
undergoes more browning than fructose with glycine at 608C and pH 3.5. Any detectable
variation in the fructose/glucose ratio may indicate unbalanced consumption of these reducing sugars due to nonenzymatic browning reaction.
0,4
0,35
0,3
Sugar (mol/100 g)
0,25
Reducing
sugars
0,2
Sucrose
0,15
0,1
0,05
0
0
20
40
60
Time (days)
80
100
120
Figure 6.29. Sucrose hydrolysis and increase in reducing sugars in apple juice as a function of time of storage, at 378C
(from Babsky et al., 1986 with permission).
6
.
Chemical Composition of Fruits and its Technical Importance
157
6.2.5. Effect of Processing and Storage on Fruit Pigments
Changes in fruits and fruit products color are extensively considered in Chapter 7. However,
some aspects on fruit pigment degradation are revised here. Pigments may oxidize resulting in
color fading of highly colored canned fruits. As carotenoids are highly sensitive to oxygen and
light, particularly in the presence of metals such as iron, copper, and manganese, processing
and storage can produce carotenoid degradation. Anthocyanins show low stability in products manufactured from fruits.
Moreover, the stability of these natural pigments is poor in dehydrated fruits, unless
packaged in inert gas. Temperature, light, and initial composition of fruits are considered as
responsible factors for the instability of fruit anthocyanins.
For many apple varieties, red skin color is important for marketability. Apple harvest is
largely based on the amount of red color consistent with the natural tendency of the variety.
Unfortunately, this may not be the best time to harvest for optimum quality after storage.
Whereas color development of fruit maturing on the tree generally increases with time, the
fruit also begins to ripen and will not store as well. Different apple varieties showed different
storage temperature optima for red color development. Precooling the tissue for 48 h at 28C
(to simulate cold nights) increased the amount of red pigment that accumulated.
6.2.6. Changes in Organic Acid Content
The role of organic acids appears to be essentially catalytic (Reynolds, 1965). Reduction of
organic acids in apple juice was only 9% (Babsky et al., 1986). The slight decrease in acidity
might be partly due to copolymerization of organic acids with products of the browning
reactions. Lewis et al. (1949) also suggested that organic acids can react with reducing sugars
to produce brown pigments. Sample pH did not change during storage, keeping its initial
value of 3.72 + 0.02 almost constant.
Urbicain et al. observed that the titratable acidity in peach juice rose with time and
temperature. Major organic acids present in stone-free peaches are malic, citric, and quinic
(Wang et al., 1993). The basic amino groups disappear during Maillard reaction, hence pH
lowers as the reaction proceeds in systems with no buffers (Spark, 1969). In the case of peach
juice, the organic acids present in major proportion act as a strong buffer, hence no variations
of pH may be expected. Moreover, the consumption of AA would increase titrable acidity.
Spark (1969) has reported that buffers increase browning rate, which boosts the color damage
in concentrated peach juice.
6.2.7. Changes in Phenolic Compounds
Phenolic compounds present in fruit products may react to form brown polymeric compounds (Abers and Wrolstad, 1979). If this reaction plays any role in the color development
of apple juice, total phenolic content will not increase during storage as Babsky et al. (1986)
found, using the Folin–Ciocalteau reagent (Singleton and Rossi, 1965). Cornwell and
Wrolstad (1981) proposed that reductone compounds present in the juices interfere with the
Folin–Ciocalteau reagent increasing the apparent phenolic contents.
Market demands for ‘‘natural’’ juices and pulps devoid of food additives have prompted
food scientists to study the quality deterioration of fruits during processing and storage. The
raspberry (Rubus ideaus) is a bush fruit of the rosaceous family, whose economic importance
is increasing because of the use of raspberry products in the food industry.
158
Fruit Manufacturing
Major problems confronted in the production of raspberry pulp include operations that
may affect their properties. The color of raspberry fruits is not significantly affected by
freezing and cold storage. However, when crushed, most fruit berries yield a highly pectinous
pulp, releasing little free run juice with poor color stability on storage. Anthocyanins, which
are responsible for color in most berries, easily degrade following various reaction mechanisms affected by oxygen, ascorbic acid, pH, and temperature among other variables (Abers
and Wrolstad, 1979; Skrede, 1985).
Ochoa et al. (1999) found that total anthocyanin (TA) pigment in raspberries decreased
significantly through storage, at a rate strongly dependent on temperature. After 40 days,
pulp stored at 378C had lost the majority of the anthocyanins. Semilogarithmic plots of
percentage of residual anthocyanin during long-term storage of raspberry pulps were linear
(Fig. 6.30), showing that decrease followed first-order reaction kinetics, in accordance with
previous findings in other berries (Skrede, 1985).
It was suggested that anthocyanins may be destroyed either through direct oxidation by
quinones formed from catechin by PPO action or through copolymerization of anthocyanins
into brown pigments (tannins) formed from catechin–quinone polymerization (Jackman
et al., 1987).
Pigment instability is an undesirable consequence of processing of canned syrup strawberries and products that contain them (Garcı́a-Viguera et al., 1999). Processing was found to
cause a 50% decrease in the flavanol concentration and the formation of a polar compound.
The conversion of leucoanthocyanidin to anthocyanin when heated at acidic condition (Lee
and Wicker, 1991) was responsible for the pink discoloration in canned fruits like lychee.
A number of researchers have shown that the rate of ascorbic acid oxidation influences
total anthocyanin loss in strawberry products. Loss of natural color was reported to be
affected by AA content by Skalsky and Sistrunk (1973). However, studies on the effect of
ascorbic acid on the destruction of anthocyanin pigment were in general carried out at
elevated temperatures and under relatively low storage temperatures. No significant correlations were found between ascorbic acid content and any of the other quality factors.
% Antocyanin
100
10
37 ⬚C
20⬚C
4⬚C
1
0
10
20
30
40
Storage time (days)
50
60
Figure 6.30. Percentage of retention of anthocyanin in heritage raspberry pulp at 4, 20, and 378C during storage
(from Ochoa et al., 1999). Reprinted from Lebensm. Wilss. u Technol. 32(3): 149–153. Ochoa, M.R. Kesseler, A.G.,
Vullioud, M.B. and Lozano, J.E. Physical and chemical characteristics of raspberry pulp: storage effect on
composition and color. (copyright) 1999, with permission from Elsevier.
6
.
Chemical Composition of Fruits and its Technical Importance
159
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Schoebel, T., Tannenbaum, S.R. and Labuza, T.P. (1969). Reaction at limited water concentration. 1. Sucrose
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Singleton, V.I. and Rossi, J.A. (1965). Colorimetry of total phenolics with phosphomolybdic–phosphotungstic acid
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Skrede, G. (1985). Color quality of blackcurrant syrups during storage evaluated by Hunter L0 , a0 , b0 values. J. Food
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Chem. 22: 796 –800.
CHAPTER 7
FRUIT PRODUCTS,
DETERIORATION BY BROWNING
7.1. INTRODUCTION
Food processing, defined by the predictability of the product–process interactions, is
turning into a science. Processing and storage of fruit products affect composition in
many ways. How to develop the best process knowledge of the reaction and a description
of the engineering processes involved, the latter already studied in previous chapters, is
required.
Knowledge of deterioration factors, including the rates of deterioration, means that it is
possible to find ways of lowering or stopping those deteriorative actions, thereby gaining fruit
preservation. In order to maintain their nutritional value and organoleptic properties and
because of technical–economical considerations, not all the identified methods against deterioration actually have practical applications.
7.1.1. Different Mechanisms of Deterioration
Appearance, which is significantly impacted by color, is one of the first attributes used by
consumers in evaluating food quality. Color may be influenced by naturally occurring
pigments such as chlorophylls, carotenoids, and anthocyanins in fruits; or by pigments
resulting from browning reactions. Browning of fruits and fruit products is one of the
major problems in the fruit industry and is believed to be probably the first cause of quality
loss during postharvest handling, processing, and storage. Browning can also adversely affect
flavor and nutritional value.
Extraction of fruit juices, or elaborating fruit pulps and purées, requires the grinding of
fruit, which results in the rupturing of the fruit cells and the mixing of the fruit components
with the atmospheric oxygen. The same is valid for the cutting of fruits before dehydration.
This incorporation of oxygen into the fruit pulp, or on the surface of the cut fruit, causes the
oxidation of the phenolic compounds to quinones, which are naturally present in the fruit
tissue. The endogenous enzyme polyphenoloxidase (PPO) catalyzes this oxidation, known as
enzymatic browning (EB).
EB is one of the most devastating reactions for many exotic fruits, in particular tropical
and subtropical varieties. It is estimated that over 50% loss in fruits occurs as a result of EB
(Whitaker and Lee, 1995). Projected increases in fruit markets will not occur if EB is not
understood and controlled.
On the other hand, browning reactions of nitrogen compounds, mainly free amino acids
and proteins, with carbohydrates cause deterioration by color and off-flavor development
during processing and storage of fruits. These deteriorative reactions are generally known as
nonenzymatic browning (NEB) reactions. However, browning reactions in fruits are more
163
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Fruit Manufacturing
complex than suggested by the simple classification as enzymatic or nonenzymatic, because of
the large number of secondary reactions that may occur. This is reflected in the range of color
produced even in the same product (e.g., raspberries may develop red or brown discolorations; Ochoa et al., 1999). Browning may occur in some fruits in which endogenous ascorbic
acid (AA) is oxidized to dehydroascorbic acid (DHAA), which then reacts with free amino
acids to yield deep brown colors by the Maillard reaction (Kacem et al., 1987). These
deteriorative reactions need to be described in some more detail.
Much research has been concentrated in recent years to find effective and economical
ways to prevent browning in various fruit products. Concerted efforts have been made to
understand the basic biochemistry involved in enzymatic browning reactions and to find
practical techniques to prevent the browning reactions in fresh and processed products. On
the other hand, the complexity of EB and NEB reactions, and the various compounds
involved (Hodge, 1953; Spark, 1969) makes it difficult to study the reactions by means of a
simple analytical chemical method. However, kinetic approach can be used in solving the
problem.
7.2. ENZYMATIC BROWNING
EB occurs in fruits after bruising, cutting, or during storage, and its control during the
processing of fruits is of great importance to fruit manufacturing. EB is a significant problem
in apples, pears, bananas, peaches, and grapes, particularly. Acceptability of browning also
depends on the product:
.
.
In clarified fruit juice, like apple juice, a little browning is accepted and the typical
amber-like hue is commercially desirable.
However, both apple purée and cloudy juice must retain the yellowish or greenish
color, which characterizes the fresh product.
It must be remembered that enzymatic problem is not always a problem to be avoided:
the color of products such as raisins and prunes is obtained thanks to a controlled PPO
reaction (Vámos-Vigyázó, 1981).
7.2.1. Phenolic Compounds and Oxidases
POP is an example of an enzyme that can lower the quality of a food product by catalyzing
the oxidation of phenolic compounds. The susceptibility to browning may depend on PPO
activity and/or phenolic content (Coseteng and Lee, 1987). Polyphenol oxidase catalyzes the
initial step in the polymerization of phenolics to produce quinones, which undergo further
polymerization to insoluble dark brown polymers known as melanins. These melanins form
barriers and have antimicrobial properties, which prevent the spread of infection or bruising
in plant tissues. The formation of yellow and brown pigments in fruit products during EB
reactions is controlled by the levels of phenols, the amount of PPO activity, and the presence
of oxygen (Spanos and Wrolstad, 1992).
The phenolic composition of apple, pear, and white grape juices was reviewed by Spanos
and Wrolstad (1992) (see also Chapter 6). These authors classified the phenolic constituents of
importance in fruit juices into two groups: (a) phenolic acids and related compounds, and
(b) flavonoids.
7
.
Fruit Products, Deterioration by Browning
165
During the browning of fruit tissue, the enzyme PPO, also called orthodiphenol oxidase or
catecholase, catalyzes the oxidation of phenolic compounds related to catechol and containing two o-dihydroxy groups to the corresponding o-quinone (Joslyn and Ponting, 1951;
Vamos-Vigyazo, 1981).
Relatively few of the phenolic compounds in fruits serve as substrates for polyphenol
oxidase (Table 7.1). Compounds with minor differences may or may not be substrates for
polyphenol oxidase. For example, Shannon and Pratt (1967) found that when comparing
quercetin and dihydroquercetin, differing only in the bonding between carbons at the 2 and 3
positions, only the latter was a substrate for apple polyphenol oxidase. It was assumed that
quercetin is more stable than dihydroquercetin, due to the presence of a double bond
conjugated to an aromatic ring, affecting compound solubility.
Most raw fruits contain polyphenols and PPOs, located in different compartments in the
cell structure. When through damaging or processing (e.g., milling) enzyme, substrates and
oxygen come into contact with each other, and a lot of reactions start that finally lead to the
formation of insoluble brown pigments (melanins).
The EB of fruit and vegetables is always considered as a quality loss of both fresh and
processed food products. Simple representation of EB reactions is given in Fig. 7.1.
7.2.2. Kinetics of Enzymatic Browning
PPO activity, as for most of enzymes, may be minimized by reducing agents, heat inactivation, lowering the pH of the fruit product, and the presence of enzyme inhibitors, among
other techniques, which are reviewed in Chapter 8. To effectively inhibit or control the EB in
fruit products, an accurate determination of the kinetics of these catalyzed-oxidative reactions
is required. The kinetics of deterioration can be followed through color measurements, which
is a simple and effective way for studying the phenomenon.
The substrate specificity of polyphenol oxidase varies in accordance with the source of the
enzyme. Phenolic compounds and polyphenol oxidase are in general directly responsible for EB
reactions in damaged fruits, during postharvest handling and processing. The relationship of
Table 7.1. Phenolic substrates of PPO in fruits.
Fruit
Phenolic substrates
Apple
Chlorogenic acid (flesh), catechol, catechin (peel), caffeic acid, 3,4-dihydroxyphenylalanine (DOPA),
3,4-dihydroxy benzoic acid, p-cresol, 4-methyl catechol, leucocyanidin, p-coumaric acid, flavonol
glycosides
Isochlorogenic acid, caffeic acid, 4-methyl catechol, chlorogenic acid, catechin, epicatechin, pyrogallol,
catechol, flavonols, p-coumaric acid derivatives
4-methyl catechol, dopamine, pyrogallol, catechol, chlorogenic acid, caffeic acid, DOPA
3,4-dihydroxyphenylethylamine (dopamine), leucodelphinidin, leucocyanidin
Chlorogenic acid, caffeic acid, coumaric acid, cinnamic acid derivatives
Catechin, chlorogenic acid, catechol, caffeic acid, DOPA, tannins, flavonols, protocatechuic acid,
resorcinol, hydroquinone, phenol
Dopamine-HCl, 4-methyl catechol, caffeic acid, catechol, catechin, chlorogenic acid, tyrosine,
DOPA, p-cresol
Chlorogenic acid, pyrogallol, 4-methyl catechol, catechol, caffeic acid, gallic acid, catechin, dopamine
Chlorogenic acid, catechol, catechin, caffeic acid, DOPA, 3,4-dihydroxy benzoic acid, p-cresol
Chlorogenic acid, catechin, caffeic acid, catechol, DOPA
Apricot
Avocado
Banana
Eggplant
Grape
Mango
Peach
Pear
Plum
Adapted from Marshall, Kim and Wei, 2000.
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Fruit Manufacturing
OH
O
HO
OH
PPO
HO
OH
OH
OH
O
O
PPO. O2
O
O
HO
OH
OH
OH
OH
OH
OH
O
HO
OH
OH
o-dihydroxyphenol
OH
o-quinone
Melanins
Figure 7.1. Simplified mechanism for the transformation of a diphenol to dark colored melanins by PPO.
the rate of browning to phenolic content and polyphenol oxidase activity, has been reported for
various fruits such as apples (Coseteng and Lee, 1987), grapes (Lee and Jaworski, 1988), and
peaches (Lee et al., 1990). In addition to serving as polyphenol oxidase substrates, phenolic
compounds act as inhibitors of polyphenol oxidases (Walker, 1995). Their inhibitory action
decreased in the following order: cinnamic acid > p-coumaric acid > ferulic acid > benzoic acid.
Although relatively few of the phenolic compounds in fruits serve as substrates for
polyphenol oxidase, as catechins, cinnamic acid esters, 3,4-dihydroxy phenylalanine
(DOPA), and tyrosine (Table 7.1), the stecheometry of complex reactions like EB in fruits
as substrate is practically unknown. Therefore, instead of determination of consumption of
reactives (phenols), or formation of products (melanins), the kinetics of color development is
commonly used for studying the browning reactions.
As found by Sapers and Douglas (1987), tristimulus reflectance values were strongly
nonlinear, and changes in the rate of browning may be better understood when plotting
colorimetric parameters against log time.
EB in Golden Delicious apple juice was monitored by measuring CIE L value (Lozano
et al., 1994). The authors observed a significant influence of degree of ripeness on the rate of
EB. Pulp made with unripe (GA) apples browned at a faster rate. This behavior was
attributable to differences in AA content and PPO activity in young fruits. Figure 7.2
70
T = 5˚C
OA
MA
GA
65
CIE L*
60
55
50
45
40
35
30
1
10
Time, min
100
Figure 7.2. Relationship of CIE ¼ l value in Golden Delicious apple pulp to time and degree of ripeness at 58C.
GA stands for green apple, MA for mature apple, and OV for overmature apple (Lozano et al., 1994
with permission).
7
Fruit Products, Deterioration by Browning
.
167
shows the variation of CIE L parameter as a function of log time with degree of ripeness of
processed apple as a parameter, at constant temperature.
The rate of luminosity decrease can be divided into three periods: (i) the first period,
characterized as an induction or flat period, attributable to the inhibition action of the
naturally occurring ascorbic acid (Ponting and Joslyn, 1948), (ii) the second period, which
looks linear when represented in this semilog plot, attributed to the consumption of the
enzymes’ substrates (Sapers and Douglas, 1987), and (iii) the third period that approaches a
plateau at a time depending on the degree of ripeness of apples.
It was also observed (Fig. 7.2) that the lower induction time corresponded to unripe
fruits. Koch and Bretthauer (1956) also found considerable seasonal variations in the amount
of AA and DHAA in apples. Another relevant information given by Koch and Bretthauer
(1956) is that PPO activity is greatest in young fruits than in fully ripe apples.
Lozano et al. (1994) found that a reduction in b values and change in sign (from to þ) in
a parameter clearly indicated that browning development occurred in apple pulp. Negative a
values were given by the green pigmentation of apples and it was pronounced in green samples.
7.2.2.1. Effect of the Temperature in the Color Change
Color development during pulping of fruits certainly includes Michaelis–Menten type reaction kinetics followed by several reactions, both reversible and irreversible, up to the formation of dark brown pigments. The combined effect of these browning reactions may result in a
nonlinear behavior strongly dependent on temperature. Experimental data obtained with
apples on the dependence of the rate of CIELAB L with temperature were fitted by Lozano
et al. (1994) to the equation:
L ¼ a k log t
ð7:1Þ
where a and k are fitting parameters and t is the time in min. The selected range is in
agreement with the second linear period in the L versus log t plot.
However, it must be noted that browning mechanism can be strongly nonlinear and
simplified kinetics equations may not be applicable. Kinetic measurements on complex
systems, such as fruit pulp, usually give reaction constant values, which may or may not be
the dissociation constant, but it is frequently the combination of the rate constants for several
steps.
In general, for production of light-colored apple pulp, the time between milling of the
fruit and the heat treatment must be as short as possible.
7.3. NONENZYMATIC BROWNING
NEB via Maillard-type reactions is the most important route of color deterioration in fruit
juices. The reaction is followed by undesirable color, odor, and flavor changes (Pribella and
Betusova, 1978; Toribio and Lozano, 1984; Cornwell and Wrolstad, 1981). Three basic NEB
reactions have been identified (Fig. 7.3):
.
.
Pyrolysis: which results in a burnt and inedible flavor;
Caramelization: when the simpler sugars lose water molecules from their structure,
through a 1:2 and 2:3-enolization. This process is affected by pH. Through many
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Fruit Manufacturing
Pyrolysis
Involves the total loss of water from the
sugar molecule and the breaking of
carbon–carbon linkages
NEB
Caramelization
Heat-induced transformation of
reducing sugars alone in a
concentrated solution
Maillard
Browning reactions involve simple
sugars, and amino acids and
simple peptides
Figure 7.3. Basic NEB reactions.
.
intermediates, and in the pH 2–7 range, d-fructose for example can give rise to furans,
isomaltol, and maltol, well-known bread crust flavor/aromas;
Maillard-type reaction: of amino acids and proteins with carbohydrates, which is
discussed extensively in the following section.
7.3.1. Maillard Reactions
The reaction begin to occur at lower temperatures and at higher dilutions than caramelization,
as in clarified fruit juices (Toribio and Lozano, 1984). The rate can increase by 2–3 times for
each 108C rise in temperature. Maillard reactions have three basic phases (Fennema, 1986):
.
.
.
.
The initial reaction is the condensation of an amino acid with a simple sugar, which
loses a molecule of water to form N-substituted aldosylamine. This is unstable and
undergoes the Amadori rearrangement to form 1-amino- 1-deoxy-2-ketoses (or
ketosamines), which can cause complex subsequent dehydration, fission, and polymerization reactions. One of the Maillard paths is a simple caramel reaction, catalyzed
by amino acids.
The ketosamine products of the Amadori rearrangement can then react three ways in
the second phase. One is simply further dehydration into reductones and dehydroreductones, which are essentially caramel products. Second is the production of shortchain hydrolytic fission products such as diacetyl, acetol, pyruvaldehyde, etc. These
then undergo Strecker degradation with amino acids to aldehydes and by condensation
to aldols. Negative aromas like 2 and 3-methyl-butanal are also formed.
Third path is the Schiff’s base/furfural path. This involves the loss of 3 water molecules, then a reaction with amino acids and water. These also undergo aldol condensation and polymerize further into true melanoidins.
These products react further with amino acids in the third phase to form the brown
pigments and flavor active compounds collectively called melanoidins. These can be
off-flavors. The outcome will depend not only on which amino acids and sugars are
available, but also on pH, temperature, and concentration.
7
.
Fruit Products, Deterioration by Browning
169
In general, high levels of amino acids favor both caramel and Maillard reactions, but
dilution eliminates caramel reactions. At temperatures >1008C pyrazines are produced. High
levels of polyphenols favor Strecker degradation. Table 7.2 lists principal reactions and
characteristics of identified NEB reactions.
Fruit juice concentrates containing more than 65% total solids are normally stable
from the standpoint of fermentation at any temperature, but when stored at relatively
high temperatures, NEB reactions occur. NEB via Maillard-type reactions is the most
important route of color deterioration in apple juice (Czapski, 1975; Toribio and Lozano,
1984).
The reaction takes place between amino acids and reducing sugars present in the juice,
decreasing the alpha-amino nitrogen content followed by undesirable color, odor, and flavor
changes (Pribella and Betusova, 1978; Toribio and Lozano, 1984). The same behavior was
found in pear juice concentrate (Cornwell and Wrolstad, 1981), citrus juices (Kanner et al.,
1982; Cornwell and Wrolstad, 1981), and intermediate moisture foods (Resnik and Chirife,
1979; Waletzko and Labuza, 1976).
Color deterioration was reported for many fruit products, such as citrus juices (Reynolds, 1965; Kanner et al., 1982; Cornwell and Wrolstad, 1981), intermediate moisture foods
(Waletzko and Labuza, 1976; Johnson et al., 1969; Czapski, 1975), and apple juice (Toribio
and Lozano, 1984, 1986).
Babsky et al. (1986) studied the effect of storage on the composition of clarified
apple juice concentrate and concluded that natural juices were very complex mixtures in
which the role of the many components in browning reactions was difficult to elucidate.
Kinetics of NEB in fruits and fruit products can be simplified as seen in the following
scheme (Fig. 7.4).
Table 7.2. Basic Maillard-type reactions (adapted from Fennema, 1985).
Stage
Principal reactions
Characteristics
Initial (Colorless)
Condensation, enolization,
Amadori rearrangement. With
proteins, glucose and free amino
groups combine in 1:1 ratio
Intermediate (Strong absorption in
near-ultraviolet range)
Sugar dehydration to
3-deoxyglucosone and its -3,
4-ene, HMF, and
2-(hydroxyacetyl)furan; sugar
fragmentation; formation of
alpha-dicarbonyl compounds,
reductones, and pigments
Aldol condensations,
polymerization, Strecker
degradation of alpha amino acids
to aldehydes and N-heterocyclics
at elevated temperatures. Carbon
dioxide evolves
Reducing power in alkaline solution
increases. Storage of colorless 1:1
glucose–protein product
produces browning and
insolubility
Addition of sulfite decolorizes,
reducing power in acidic solution
develops, pH decreases, sugars
disappear faster than amino
acids. Positive test for
amino sugars (Amadori
compounds)
Acidity, caramel-like and roasted
aromas develop, colloidal and
insoluble melanoidins form,
fluorescence, reductone reducing
power in acid solution, addition
of sulfite does not decolorize
Final (Red-brown and
dark-brown color)
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Fruit Manufacturing
Reactives
Intermediates
Hexoses
Amino-acids
Organic acids
Glucids
Vitamins
Anthocyanins,
etc.
Standard
methods of
determination
are available
Final
products
CO2
Amadori
compounds
Aldehydes
Most of them very
reactives and unstable
(Exceptions: CO2 and
5-HMF)
Melanoidines
Large Absorbance on
visible range
Figure 7.4. Simplified scheme for NEB reactions in most fruit juices.
7.3.1.1. Tristimulus Parameters and Absorbance as a Measurement of Browning in Fruit Juices
The absorption at one fixed wavelength, although reliable for kinetics studies, is not adequate
for comparing the visual color changes in both browned apple juice and model systems. For
this reason, the Hunter a, b, L color parameters and C.I.E. x, y, z parameters are also
measured (see also Chapter 5).
In general during NEB of fruit juices, except for the differences given by the initial color,
the tristimulus values are grouped within a narrow band drawn from the standard light source
‘‘C’’ so as to approach asymptotically the spectrum red locus (Fig. 7.5).
This behavior could denote that the same NEB reactions occurred in juices and model
systems, resulting in polymers (melanoidines) with the same color attributes.
7.3.1.2. Kinetics of Nonenzymatic Browning (NEB)
The kinetics of NEB is generally dependent upon product characteristics and storage conditions, including:
.
.
.
.
Influence of temperature, soluble solids’ concentration, pH, acidity, and water activity.
Amino acids and reducing sugars’ content.
5-HMF formation during NEB
Effect of polyphenols, galacturonic acid, and other minor compounds.
As pointed out by Labuza and Riboh (1982) most of the quality-related reaction rates are
either zero- or first-order reactions, and the statistical difference between both types may be
small. Besides real fruit products, the technique of using simplified model food systems to
simulate the effect of storage and processing on quality has been widely used.
7.3.1.3. Effect of Soluble Solids
It is well known that by increasing the concentration of food solids (or reducing water
content) browning reactions are significantly enhanced (Eichner and Karel, 1972). The
7
.
Fruit Products, Deterioration by Browning
171
0.9
515
520
0.8
505
0.7
530
Green
545
500
555
0.6
495
565
0.5
Y
Yellow
575
490
0.4
590
Daylight
485
0.3
605
Pink
Red
780
480
0.2
Blue
470
0.1
380
0
0
0.2
0.4
0.6
0.8
X
Figure 7.5. Effect of browning during storage of apple juice (adapted from Toribio and Lozano, 1987).
occurrence of a maximum reaction rate at a certain water activity was also described (Labuza
et al., 1970). A further increase in solids’ content resulted in rate decrease. It was suggested
that at these high concentrations, the rate of reaction was controlled by the mobility of the
reactants. Toribio et al. (1994) found that clarified apple juice has a slower nonenzymatic
browning reaction rate (NEBr) at low water activities increasing up to the maximum point
between aw 0:53---0:55 (about 828Brix) as shown in Fig. 7.6.
A further increase in aw significantly reduces the color formation. It is assumed, in this
case, that the increase in aw tends to dilute the concentration of reactants, decreasing chemical
reaction rate.
This NEBr maximum is typical in nonenzymatic Maillard-type reactions. The aw values
at maximum NEBr for model mixtures was found to be aw ¼ 0:87 (Labuza et al., 1970).
7.3.1.4. Effect of Reducing to Total Sugars’ Ratio (R/T)
The rate of sucrose hydrolysis is a function of the concentration of reactants, temperature,
and acid–catalyst concentration. However, if excess water is present as in fruit juices, the rate
of disappearance of sucrose follows a pseudo-first-order reaction rate equation. As expected,
an increase in the reducing sugars (fructose þ glucose) content, when keeping the total sugars’
content constant, resulted in a faster browning of model mixtures (Lozano, 1991).
7.3.1.5. Effect of the Fructose to Glucose Ratio (F/G)
Lozano (1991) found that glucose was more reactive than fructose, at least during the zeroorder reaction rate period in model solutions simulating apple juice. Wolfrom et al. (1974)
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Fruit Manufacturing
0.025
37ⴗC
NEBr (Abs 420nm/day)
0.02
0.015
0.01
Storage time
0 days
0.005
40 days
80 days
0
0
0.1
0.2
0.3
0.4
aw
0.5
0.6
0.7
0.8
Figure 7.6. NEBr as a function of water activity (aw ) and time of storage (adapted from Toribio et al., 1984).
working with a simulated orange juice, found that fructose had higher initial rate of browning
than glucose during the initial stage of reaction but was dependent on the kind of amino acid.
7.3.1.6. Effect of Amino Acids (AA)
Lozano (1991) also found that an increase of total amino acids’ content (AA) from 4.34 to
6.51 g/L, resulted in a noticeable increase in the browning rate. It can be calculated that for 1.5
times greater AA content, there was an approximately 1.5 increase of the reaction constant K.
Similar results were also found during storage (Babsky et al., 1986) and processing (Toribio and
Lozano, 1987) of clarified apple juice. Asparagine was found to represent nearly 70% of the
total amino compounds in clarified apple juice, and 90% of it disappeared after 100 days of
storage at 378C (Babsky et al., 1986). Calculated increase in reaction constant K was 5% when
the Asn content was increased from 60 to 70%. Del Castillo et al., (1998) found a loss of 65.1%
of original AA content, when storing dehydrated orange juice (aw ¼ 0:44) at 508C for 14 days.
About 78% of that loss was attributed to the AA in major proportions, namely proline,
arginine, asparagine, and g-aminobutiric acid.
Buedo et al. (2001) studied the change of free amino acids’ (AA) composition of peach
juice concentrate (PJC), as a result of the Maillard reactions, in particular the effect of the
storage at 15, 30, and 378C for 6 weeks, since those are conditions likely to be found in
commercial practice. A decrease in total AA content was observed to be 8, 35, and 60%, after
112 days of storage at those temperatures, respectively. The main constituent, asparagine,
contributed to 71% of the total loss, while aspartic acid increased its concentration, probably
as a result of the asparagine degradation.
Decrease in total AA content was exponential, pH remained constant during the storage,
while titratable acidity increased with both time and temperature, assumed by disappearance
of amino groups.
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Fruit Products, Deterioration by Browning
173
7.3.1.7. Effect of the Content of Organic Acids
The malic acid participation in the NEB, via Maillard reaction, was judged to be essentially
catalytic (Reynolds, 1965). However, Lozano (1991) found that: (i) an increase in malic acid
actually accelerated the rate of browning during storage, and (ii) the pH was reduced only by
0.1 unit when malic acid content was increased from 2 to 6 g/L. It must be noted that NEB
reaction is not very sensitive to low pH changes in the pH ¼ 2– 4 range (O’Beirne, 1986).
Titratable acidity in peach juice was observed to rise with time and temperature. Also, at 30 and
378C the increase rate is maximum on the initial storage days. Major organic acids present in
stone-free peaches are malic, citric, and quinic (Wang et al., 1993).
7.3.1.8. Effect of Other Minor Components
NEBr increased with ascorbic acid, because of the participation of vitamin C in the Maillard
reactions. During the enzymatic clarification process of fruit juices, the natural pectic substances (mainly polymers of the galacturonic acid) are broken by specific enzymes (pectinases), which are able to hydrolyze pectin to their basic units. This treatment is also applied
during the pressing stage, to improve the juice extraction, and depending on fruit variety and
maturity, considerable amount of free galacturonic acid could be present in clarified fruit
juices after enzyme treatments.
Lozano (1991) found that the adding of 60 mg/L of galacturonic acid to a model sugar–
malic acid–amino acid solution accelerated the color formation. It can be concluded that
galacturonic acid, produced during the enzymatic treatment of pulps and juices, may accelerate browning, thus reducing the storage capacity of apple juice concentrate.
7.3.1.9. Effect of Temperature
Figure 7.7 shows the change in absorbance at 420 nm for different apple juice concentrates
and a model solution over 120 days, at 378C. The rate of NEB of this model solution can be
divided into the following two stages:
Red Del.
Granny Smith
Model system
Absorbance (420 nm)
2
1.5
1
0.5
37ⴗC
0
0
25
50
75
100
Storage time (days)
125
Figure 7.7. Color development as a function of time of apple juice (708Brix) and a model solution (fructose/glucose:
3.13, reducing/total sugars: 0.90, total amino acids: 3.5 g/L, malic acid: 6.4 g/L at 128Brix) (adapted from Toribio and
Lozano, 1984; Lozano, 1991).
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Fruit Manufacturing
(1) An induction period, already observed in other model food systems (Warmbier et al.,
1976), is attributed to the formation of colorless intermediates. This period was
exponential rather than linear and the color development could be expressed as a
typical first-order reaction equation.
(2) A linear period of reaction where the color formation follows a zero-order kinetics. It
was assumed that the behavior with temperature would follow the same trend at any
given time when the storage temperature was reached.
Moreover, when concentrate was stored for 30 days at 378C and the next 30 days
at 208C, color formation was 40% higher than that during the inverse condition (first 30
days at 208C and the next 30 days at 378C). Clearly this shows how important it is to
cool the product as soon as it is produced. The information is relevant because a sizeable
amount of concentrate is made during the summer season and the product is stored in the
open air.
When the color formation during storage of model solutions at any temperature is
compared with the color development of a natural apple juice, some differences are readily
observed (see Fig. 7.7):
(1) Color increase was very much higher in AJC than in model solutions,
(2) Induction time was not detected during the AJC storage,
(3) Obtained maximum NEBr had different values.
The first two differences could be attributable both to the influence of minor components
like galacturonic acid and to the heating during processing (clarification, aroma recovery, and
concentration stages), which could reduce or eliminate the induction time.
Eichner (1975) and Eichner and Karel (1972) found in very viscous food systems that the
viscosity remarkably affected the NEBr. As the viscosity of both the model systems and apple
juice had practically the same values at the same soluble solids, it is difficult to exclusively
attribute the limitation of NEB reactions and the occurrence of a maximum to the reactant
mobility.
It must be noticed that the use of liquid model systems for kinetic studies, besides the
previously considered limitations, resulted in a very good tool to individually quantify the
participation of juice components in deteriorative reactions.
To put it succinctly, browning rates in fruit juice are mostly dependent on reducing
sugars and amino acids’ content. However, independent of juice composition, the lower the
storage temperature, the less is the darkening. Thus, from a practical standpoint, thermal
history is crucial in obtaining lighter color products.
7.3.2. 5-HMF Formation During Storage and Processing of Fruit Products
The Maillard reaction among hexoses and amino components leads to the formation of
5-hydroxymethylfurfural (5-HMF) (Shallenberger and Mattick, 1983) as intermediate. The
HMF increase during processing and storage of fruit products was positively identified and
quantified (Resnik and Chirife, 1987; Toribio and Lozano, 1979). Figure 7.8 shows the HMF
increase of apple and grapefruit juice during prolonged storage.
As Fig. 7.8 shows, the rate of accumulation can be divided into three periods. The first
period is characterized as an induction time of approximately 2 weeks. During the second
7
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Fruit Products, Deterioration by Browning
175
Kcal/mol.
HMF (mg/100 g)
50
40
30
20
Apple, 37ⴗC
10
Grapefruit, 40ⴗC
0
0
25
50
75
100
Time (days)
125
Figure 7.8. Rate of accumulation of 5-HMF with time of storage in apple and grape juices (Saguy et al., 1978;
Babsky et al., 1986).
period the rate showed a rapid increase of HMF with a maximum at about 7–8 weeks. After
that maximum the rate of formation diminished rapidly, and the HMF production
approached a plateau. A similar behavior attributable to a second-order autocatalytic reaction (Frost and Pearson, 1961), was recognized by Schallenberger and Mattick (1983) during
the acidic degradation of hexoses.
It would appear that after 50 days of storage under the present conditions, HMF started
to form brown pigments (melanoidins) in apple juice at such a rate that after some period the
consumption equaled the formation via Amadori rearrangement of hexose degradation.
Calculated activation energy/valid for the HMF formation in the range of temperatures
considered, resulted Ea ¼ 35 Kcal=mol.
Similar results were obtained by O’Beirne (1986) for apple juice concentrate. Petriella
et al. (1985) found that the NEBr in different food systems reduced with decreasing pH.
However, the range of work was pH ¼ 5 –7 and the authors showed an apparent change in the
browning mechanism at pH ¼ 5 and lower. Although Wolfrom et al. (1974) also found that
the browning rate decreased with pH, they worked with apple juice at pH ¼ 6 –7. It was
speculated that an increase in the malic acid content was more effective in accelerating NEB
than the consequent pH reduction.
Concentration by evaporation ideally reduces costs and increases shelf life by removing
water without changing the solid composition. However, in practice, clarified fruit juices are
susceptible to color and flavor changes during evaporation. In the previous section we
considered the reaction of the hexoses and amino components present in apple juice, leading
to the formation of 5-hydroxymethylfurfural. HMF can also be produced by acid-catalyzed
splitting of sugars (Shallenberger and Mattick, 1983).
Extrapolation of the straight-line portion to the time axis, gives a value for the induction
period. A similar induction period in the formation of HMF was observed by Shallenberger
and Mattick (1983) and was attributed to some autocatalytic mechanism. Only an initial flat
period and rapidly increasing rates at the outset of reaction were observed in apple juice
(Toribio and Lozano, 1987).
The formation of 5-HMF was also proposed to be used to complement color data
in estimating the severity of heating during processing and storage of fruit juices (Askar,
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Fruit Manufacturing
EXAMPLE 7.1
NEB estimation during storage
Urbicain and Lozano devised a nomogram for calculating the relative color increase
from the initial one, with concentration (in Brix), time (in days), and temperature (in8C)
given. This is shown in Fig. 7.9. In this example, the increase in color after 60 days’
of storage of a 708Brix apple juice concentrate at 308C needs to be estimated. A line is
drawn passing through 708Brix and 60 days’ points, and extrapolated to the reference
line. A second line is plotted joining the focus point (Fo) with the intersection between the
first plotted line and the reference line. Finally, a line normal to temperature line,
beginning at the intersection between (Fo-Ref) line and 308C line, indicates DC value
at the corresponding scale. The result indicates an absorbance increase DC ¼ 0:68 can be
estimated.
1984; Toribio and Lozano, 1987). The formation of HMF depends on the duration and
temperature of processing and storage. In fresh, untreated juices the HMF content is practically 0 (Babsky et al., 1986; Askar, 1984). The HMF level is important because it indicates
the severity of heating that has been applied during processing, as reported for milk (Burton,
1984), honey (Jeuring and Kuppers, 1980), dehydrated apples (Resnik and Chirife, 1979), and
tomato paste (Allen et al., 1980). Both HMF and furfural (2-furaldehyde) are useful as
indicators of temperature abuse in orange juice (Meydav and Berk, 1978).
8Brix t days
65
DC Ref
T 8C
Fo
5
10
20
25
Result:
DC = 067
100
30
1
37
200
15
Figure 7.9. Nomogram to estimate the relative color increase (DC) in Abs. 420 nm of concentrate as a function of
time of storage or transportation, concentration (8Brix), and temperature.
7
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Fruit Products, Deterioration by Browning
177
EXAMPLE 7.2
NEB during refrigeration of a juice barrel
50-gallon plastic barrels, although practically displaced by high-volume containers, are
still used in small processing plants. The problem is to estimate the necessary time to
reduce the center temperature to values low enough to retard the NEB. Urbicain and
Lozano (1992) found that by maintaining clarified concentrated apple juice (CAJ) for
60 min at 508C (outlet evaporator temperature), 10% increase in color and 5-HMF
formation occurs. Barrels are filled at 508C to facilitate pumping and improve sanitary
conditions. A typical barrel is 1 m high and has 0.55 m diameter. As viscosity increases
during cooling from 0.06 Pas to 2.0 Pas, convective movement is rapidly restricted, and
barrels are normally piled in threes. The well-known solutions for conductive cooling of
an infinite cylinder can be applied (Carslaw and Jaeger, 1965; Welty et al., 1976).
Assuming the following average properties for CAJ:
r ¼ 1,360kg=m3
k ¼ 0:86 kcal=sm C
cp ¼ 0:64 kcal=kg C
And considering a convective coefficient Hc as (Charm, 1963):
Hc ¼ 0:095(Ti Ta)r1 (BTU=h ft F)
where r is the barrel radius.
By solving analytically or graphically, temperature reduction at different barrel
diameters was calculated and plotted in Fig. 7.10. Under normal storage condition, a
CAJ barrel may need up to 2 days to reduce its temperature to sufficiently low NEB
rate. It must be indicated that any efforts made to optimize the process will become
useless if packaging and refrigeration is not properly done.
The presence of excessive amounts of HMF is considered evidence of overheating. Askar
(1984) indicated that HMF is also responsible for the cooked taste of apple juice. Multipleeffect evaporators were designed to concentrate apple juice at reduced temperatures, but in
practice temperatures become very high in the initial effects (Lozano et al., 1984).
Toribio and Lozano (1987) heated apple juice in a set of thin, rectangular cells, made of
stainless steel, designed to have a relatively high sample capacity and a short come-up time
(less than 40 s under the more adverse conditions) to evaluate the buildup of HMF in apple
juices at the soluble solid concentrations and temperatures that usually prevail during
concentration by evaporation. The various apple juices were heated in the range 100 –1088C
for selected times (from 4 to 80 min).
HMF, which has been shown to be essentially absent in fresh manufactured singlestrength apple juice (Askar, 1984), increased to significant amounts during high-temperature
heating. The relationship between buildup of HMF and time and temperature, and soluble
solids is shown in Fig. 7.11. These results indicate that HMF increases linearly with time after
an initial induction period, which depends on soluble solids and temperature.
More research is needed to establish relationships between material properties and rates
of oxidation, NEB, and enzymatic changes.
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Fruit Manufacturing
55
CAJ Barrel
distance from center
Temperature (ⴗC)
45
r = 0.01 m
r = 0.11 cm
r = 0.25 cm
35
25
15
5
−5
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Time (h)
Figure 7.10. Cooling rate of a 708Brix CAJ in barrel, stored in a –58C cooling chamber.
The induction periods are comparable with the residence times in individual evaporator
effects. Kurudis and Mauch (21) reported 32.5 min as the mean residence time in an industrial
sugar evaporator similar to those commonly found in the fruit juice industry.
160
140
70ⴗBrix 108ⴗC
70ⴗBrix 100ⴗC
30ⴗBrix 108ⴗC
30ⴗBrix 100ⴗC
HMF (mg/L)
120
100
80
60
40
20
0
0
10
20
30
40
50
Time (min)
60
70
80
Figure 7.11. HMF formation as a function of time and temperature, and soluble solids, for clarified apple juice
(adapted from Toribio and Lozano, 1987).
7
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Fruit Products, Deterioration by Browning
179
The effect of variety on HMF formation was noticeable. The results show that the rate of
buildup of HMF was especially dependent on juice composition. It was three to four times
more rapid in Granny Smith than in Red Delicious apple juice. As noted previously, HMF
can be formed either by heating of reducing sugars in acid solution, or by the reaction
between hexoses and amino acids. Both pH and total amino acid content favor the formation
of HMF in fruit juice.
Calculated activation energies for HMF formation in apple juice and dehydrated apples
ranged from 33.8 to 46.8 kcal/mol. (Resnik and Chirife, 1979; Toribio and Lozano, 1987).
Toribio and Lozano (1987) considered 30 mg/l of HMF as a reasonable limit after heat
treatment of apple juice. Time of heating required at various temperatures to attain this
level is plotted in Fig. 7.12.
The data in Fig. 7.12 indicate that variety has a more drastic effect on HMF buildup than
on the development of NEB. Therefore, the measurement of HMF in fruit juice may provide
a useful complement to color data. Depending on the composition of the juice, the HMF level
can reach very high values. As HMF is an important intermediate in NEB via Maillard
(Babsky et al., 1986), its production during processing may accelerate browning during
storage. HMF content can then be used to complement color data in estimating the severity
of heating during processing and the storage capacity of fruit juice concentrates.
Nonenzymatic browning kinetics as affected by glass transition
Although glass transition may control physical changes in foods, its effect on reaction
kinetics is not well established. Significant issues are whether reactions become diffusion
controlled and importance of glass transition on reaction rates.
Lievonen et al. (1998) studied the effects of physical state, water plasticization, and glass
transition on kinetics of NEB in a water solution, and concentrated polyvinylpyrrolidone
(PVP) and maltodextrin (MD) model systems with 0.23, 0.33, and 0.44 aw at 248C with the
same concentration of reactants, xylose, and lysine 1:1, (10%, w/w) in the water phase. Water
contents of the MD and PVP systems increased from 6.3 to 9.7 and from 8.2 to 17.3 g H2 O/
100
Time to attain 30 mg/L HMF (min)
Granny Smith
Red Delicious
10
98
102
106
Temperature (ⴗC)
110
Figure 7.12. Effect of temperature on rate of accumulation of 5-HMF in apple juice for Granny Smith (698Brix) and
Red Delicious (70.68Brix) varieties (adapted from Toribio and Lozano, 1987).
180
Fruit Manufacturing
100 g dry matter, respectively, as aw increased from 0.23 to 0.44. The rate of NEB was the
highest at all temperatures (10 –1008C) in water solution. The rate in PVP systems (Tg ranging
from 30 to 608C) was higher than in MD systems (Tg ranging from 30 to 808C) both as a
function of temperature at constant water content and as a function of water content at a
constant temperature. Above Tg , reaction rates increased more rapidly than below Tg . These
results may be useful in controlling NEB in processing and storage of concentrated food
materials.
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CHAPTER 8
INHIBITION AND CONTROL
OF BROWNING
8.1. INTRODUCTION
As previously discussed in this book (Chapter 6) fruits are complex systems, which, after size
reduction (pulping, milling, or cutting) are transformed into a mixture of chemical and
biochemical active components reacting in aqueous media. Moreover, process conditions
cover a wide range of temperatures (Fig. 8.1), which make the modeling and prediction of
deteriorative reactions even more difficult. Voluminous literature on the interplay of these
parameters during processing of foodstuffs is available.
Browning of fruits is a major problem in the fruit industry and is believed to be one of the
main causes of quality loss during postharvest handling and processing. The mechanism of
browning in fruits and fruit products is well characterized and can be enzymatic or nonenzymatic in origin (Chapter 7).
8.2. INHIBITION AND CONTROL OF ENZYMATIC BROWNING
Enzymatic browning (EB) is the result of fast reactions. Even an optimized processing
technology cannot completely avoid the EB during pulping and pressing of fruit juice, unless
special care is taken to avoid oxygen. Enzymes and reactions responsible of discoloration in
fruits are described in Chapter 7. In brief, during EB reactions, polyphenol oxidase catalyzes
the oxidation of phenols to o-quinones, which are highly reactive compounds. o-Quinones
thus formed undergo spontaneous polymerization to produce high molecular weight compounds or brown pigments (melanins). These melanins may in turn react with amino acids
and proteins leading to enhancement of the brown color produced.
Many studies have focused on either inhibiting or preventing polyphenol oxidase activity
in foods. Vámos-Vigyázó (1995) classified the principles of EB prevention into:
.
.
Inhibition or inactivation of the enzyme, and
Elimination or transformation of the substrate.
The author indicated that it is not easy to classify an inhibitor as belonging only to one of
these categories. Moreover, many inhibitors act on both enzyme and substrate. It must be
emphasized that EB in fruits and fruit products can be controlled or reduced also by:
.
.
.
Selecting cultivates of slight browning tendency.
Improving the agricultural techniques.
Identifying PPO activity, phenolic composition, and browning kinetics.
183
184
Fruit Manufacturing
Removal of heat
Ambient
temperature
Application
of
heat
Freeze drying & freeze
Concentration
Freezing
Chilling
Storage
−358C
Raw material preparation
Size reduction
Mechanical separation
Enzyme treatment
Use of membranes
Dehydration
Blanching
Pasteurization
Evaporation
Ultra high temperature
Processes
150 8C
Figure 8.1. Range of temperatures during typical fruit processing and storage.
Various techniques and mechanisms have been developed for controlling EB. These
techniques attempt to eliminate one or more of the essential components (oxygen, enzyme,
copper, or substrate) from the reaction.
As EB is an oxidative reaction, it can be retarded by the elimination of oxygen from the
cut surface of the fruit. However, browning restarts rapidly when oxygen is reintroduced.
Oxygen exclusion is possible by immersion in syrup, deoxygenated water, or the coating of
fruit with films not permeable to that gas (McEvily et al., 1992).
As copper prosthetic group of polyphenol oxidases must be present for the EB reaction
to occur, these chelating agents capable of removing Cu may be effective to control ED
deterioration. Inactivation of the polyphenol oxidase by heat treatments, such as steam
blanching, is effectively applied for the control of browning in fruits that are to be canned
or frozen. Chemical modification of phenolic substrates, such as chlorogenic acid, caffeic
acid, and tyrosine, can however prevent oxidation. Certain chemical compounds react with
the products of polyphenol oxidase activity and inhibit the formation of the colored compounds produced in advanced, nonenzymatic reaction steps, which finally lead to the formation of brown compounds.
Other techniques, such as the use of naturally occurring enzyme inhibitors and ionizing
radiation, have been used as alternatives to heat treatment and the health risks associated
with certain chemical treatments. It must be realized that inactivation of enzymes responsible
for browning in fruits can be irreversible (e.g., heat treatment) or reversible (e.g., use of
ascorbic acid). A general classification of the methods used to inhibit EB is sketched in
Fig. 8.2.
8.2.1. Thermal Treatments
8.2.1.1. Elevated Temperatures
Although steam blanching is one of the most effective methods for controlling EB in canned
or frozen fruits (Vámos-Vigyázó, 1995), it is not a practical alternative for treatment of fresh
8
.
Inhibition and Control of Browning
185
Thermal inactivation
EB
Inhibition
and control
Chemical
treatment
Nontraditonal
methods
Figure 8.2. General classification of methods for inhibition of enzymatic browning.
foods. In such cases, the exclusion of oxygen and/or the application of inhibitors should be
considered. Moreover, blanching should not be used as it affects the texture and flavor of fruit
products.
Adams (1991) reviewed enzyme inactivation during heat processing of foodstuffs.
He concluded that enzymes have complex covalent and noncovalent structures, which are
susceptible to heat-induced chemical degradation and disruption. In general enzyme inactivation as a function of temperature can be described by the Arrhenius or activated-complex
model.
It is also known that refrigeration (0– 48C) retards browning; however during fruit juice
processing the cellular tissue is practically destroyed, and low temperatures are not enough to
control oxidation. It is also true that about 10 s at 908C inactivates PPO (Dimick et al., 1951),
which are conditions easily provided during heating of pulps. However, in practice a long
delay occurs between crushing (or pulping) and thermal processing.
Yemeniciogı̈lu et al. (1997) studied the heat-inactivation kinetics of crude polyphenol
oxidase (PPO) from six apple cultivars (Golden Delicious, Starking Delicious, Granny Smith,
Gloster, Starckinson, and Amasya) at three temperatures (688, 738, and 788C). PPO activity
initially increased and then decreased with heat, following a first-order kinetic model
(Fig. 8.3). The authors attributed the increase in activity to the presence of latent PPO.
Calculation of activation energies (54.7–77.2 kcal/mol) indicated that PPO in apples was
generally more heat stable than PPO in other fruits, like banana (Galeazzi et al., 1981),
grape (Lee et al., 1983), and pear (Halim and Montgomery, 1978).
Thermal enzymatic inactivation is described in accordance with kinetic parameters such
as decimal reduction times (D), inactivation rate constant (k), z values (z), and activation
energies (Ea ). The D value, or decimal reduction value, is defined as the time required to
inactivate 90% of the original enzyme activity at a given temperature. An inactivation
reaction, which follows first-order kinetics, has a D value equivalent to 2.303/k. Temperature
dependence of the D value is given by the z value, which represents the temperature increase
required in order to obtain a 10-fold (1-log cycle) decrease in D value. For a first-order decay
process, the D value is equivalent to ln (10) k.
Similar to the z value is the activation energy (Ea ), which expresses the temperature
dependence of the k value as indicated in the Arrhenius relationship:
ln k ¼ Ea =RT þ ln A
k ¼ A(eEa =RT )
186
Fruit Manufacturing
1000
Activity, as % of original activity
68ⴗC
100
73ⴗC
78ⴗC
10
0
10
20
30
Time (min)
40
50
Figure 8.3. Effect of heating time and temperature on PPO in apple (Gloster cultivar) (Yemeniciogı̈lu et al., 1997)
with permission.
The Q10 value is the change in the rate of a reaction that occurs with a 108C change in
temperature and can be related to the Arrhenius equation as, Q10 ¼ e10Ea =RT (Tþ10) (Labuza
and Riboh, 1982).
While chemical reactions of single polyphenols have been described step by step, in
complex food systems only secondary effects, such as color development, can be recorded.
Color parameter was useful for studying the kinetics of EB reactions. Sapers and Douglas
(1987) reported that decreases in the CIE L value correlated well with increases in fruit
browning. Labuza et al. (1990) proposed the normalized DL=L0 (%) values as a measure of
browning, when initial Hunter L0 values varied slightly between samples. Genovese et al. (1997)
used deviation from initial Hunter parameters to study the EB in cloudy apple juice. These
authors prepared different types of samples: natural juice (without steam treatment during
crushing), not centrifuged (NJnC); natural juice, centrifuged (NJnC); cloudy juice (steam
treated), not centrifuged (CJnC); and cloudy juice, centrifuged (CJnC). Figure 8.4 shows the
variation of DL ¼ L L0 with time, for the different cloudy apple juice assayed.
Luminosity decreases monotonically in the case of centrifuged natural juice (NJC) and
remains practically constant for cloudy juices, either centrifuged (CJC) or not (CJnC). The
small increase in DL for CJnC samples was attributed to partial precipitation of insoluble
particles. Not centrifuged natural juices (NJnC) showed a completely different behavior and
the rate of luminosity variation could be divided into two periods. The first period was
characterized as a rapid increase in DL, attributed to the fast precipitation of unstable
particles. During the second period, after a maximum at 10 min the luminosity decreased
exponentially due to the oxidative darkening. The combined effect of particle precipitation
and EB resulted in a strong nonlinear behavior.
Hunter hue angle and saturation index (Chapter 5) was practically constant in all
cases other than natural juices without treatments, in this case it is attributable to particle
8
.
Inhibition and Control of Browning
187
8
NJnC
NJnC
NJC
CJnC
6
Precipitation
CJC
4
Enzymatic
browning
DL
2
CJnC
0
CJC
NJC
2
0
10
20
30
40
50
Time (min)
Figure 8.4. Variation of DL with time, for the different samples assayed (Genovese et al., 1997) with permission.
precipitation. While hue angle decreases, the saturation index increases with time for NJnC
samples. The color difference (DE) development in apple juice samples is shown in Fig. 8.5.
Analysis of the data concerning the color deterioration of apple juice suggests that cloud
characteristics and EB effect on hue should not be independently considered. Moreover,
steam treatment of juice was very effective not only in inactivating oxidative enzymes, but
also in stabilizing cloudiness. Similar results were obtained by McKenzie and Beveridge
(1988), during the blanching of Spartan apple juice. The authors attributed apple particulate
stabilization to the formation of a protective colloid that prevented aggregation.
It was also observed that centrifugation (4,200g per 5 min) had a positive effect in
controlling, or at least in retarding, color changes, when applied to natural juices without
heat treatment (NJC). Table 8.1 lists some thermal fruit treatments for the inhibition of EB.
60
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Fruit Manufacturing
12
NJnC
NJC
CJnC
10
CJC
8
DE
6
4
2
0
0
10
20
30
Time (min)
40
50
60
Figure 8.5. Total color difference (DE) as a function of time, for natural and cloudy apple juices, at 208C
(Genovese et al., 1997) with permission.
8.2.1.2. Refrigeration Temperatures
In general, for every 108C reduction in temperature a similar decrease in the rate of enzymecatalyzed reactions occurs, which is referred to as the temperature coefficient (Q10 ). This
effect was attributable to a decrease in both mobility and ‘‘effective collisions’’ necessary for
the formation of enzyme–substrate complexes and their products.
Freezing temperatures of 188C or below are often used for the long-term preservation
of food. Some fruits (berries) may be precooled or stored at chilling temperatures. However,
others like bananas, mangoes, and avocados are susceptible to chill injury and should
Table 8.1. Inhibition of EB by thermal treatment.
Fruit/product
Inhibition method
Reference
Apricot substrate is catechin
and chlorogenic acid
Plum juice
Cloudy apple juice
808C at 10 min
Dijkstra and Walker (1991)
658C at 20 min
Steam heating of the mash
in the range 65 –70C for 15–20 s
Siddiq et al. (1994)
Genovese et al. (1997)
8
.
Inhibition and Control of Browning
189
therefore not be stored below their respective critical temperatures (Fennema, 1975). Cold
preservation and storage during distribution and retailing are necessary for the prevention of
browning in fruit, since refrigerated temperatures are effective in lowering polyphenol oxidase
activity.
8.2.2. Chemical Inhibition
Chemical antibrowning agents have been commonly used to prevent browning of fruits and
fruit products. Antibrowning agents are compounds that either act primarily on the enzyme
or react with the substrates and/or products of enzymatic catalysis in a manner that inhibits
colored product formation. The enzyme PPO can be inhibited by acids, halides, phenolic
acids, chelating agents, sulfites, reducing agents such as ascorbic acid, quinone couplers such
as cysteine, and some other substrate-binding compounds. Figure 8.6 shows the evolution of
the chemical methods for the inhibition of EB applied to fruit products.
The most widespread methodology used in the fruit industry for control of EB is the
addition of sulfiting agents. The major effect of sulfites on EB is described in Fig. 8.7. As a
reducing agent, sulfites reduce the o-quinone produced by PPO catalysis to the less reactive
diphenol, preventing the development of later condensation of complex brown melanins.
Inactivation of PPO by application of sulfur dioxide (SO2 ) has been successful in preventing
EB, but its use was restricted by regulations. Sulfites have been linked to allergic reactions, the
Food and Drug Administration (FDA) prohibited the use of sulfite preservatives in fresh
vegetables and fruits (Langdon, 1987).
The effect of reducing agents is temporary because these compounds oxidize irreversibly
by reaction with pigments, enzymes, and metals. Their role is based in their ability to
reduce o-quinones (Fig. 8.7). Sulfydryl compounds transform o-quinones in stable, colorless
products.
More
traditional
More
innovative
Cyclodextrin
EDTA
Organic
acids
Sulfiting
Cysteine
Glutathione
Ascorbic
acid and
analogs
Chitosan
Chelating
agents
Reducing
agents
Chemical
inhibition
of EB
Aromatic
enzyme
inhibitor
Proteolitic
enzymes
Acidulant
Citric acid
Phosphoric
acid
Peptides
Hexylresorcinol
Aliphatic alcohols
Anions
Ficin
Papain
Bromelain
Carbohydrate
derivatives
Honey
Figure 8.6. Description of chemical methods for the inhibition of EB.
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Fruit Manufacturing
Reducing agent
OH
PPO + O2
R
R
Monophenol
OH
PPO + O2
O
OH
O
R
Melanins
o-Quinone
Diphenol
Figure 8.7. Effect of reducing agents on the first stages of EB.
Alternative inhibitors of PPO were investigated extensively (Shannon and Pratt, 1967;
Park and Luh, 1985; Sapers and Ziolkowski, 1987; Oszmianski and Lee, 1990; Siddiq
et al., 1994; Lozano-de-Gonzalez et al., 1993). Tables 8.2–8.4 list some alternatives to sulfite
antibrowning agents, according to their primary mode of action (McEvily et al., 1992):
reducing agents, chelating agents, enzyme inhibitors, complexing agents, and miscellaneous
methods.
Different EB inhibitors were assayed in raw apple juice and on cut surfaces of apple
plugs, using quantitative measurements of color changes to evaluate treatment effectiveness during storage by Sapers et al. (1989). While ascorbic acid-6-fatty acid esters showed
Table 8.2. Nonsulfite antibrowning agents applied to fruits and fruit products. Reducing agents.
Name
Mechanism of inhibition
Comments
Reference
Ascorbic acid
Free radical scavenger.
Reduces o-quinone
to diphenols.
Effect on PPO activity is controversial.
It is easily decomposed to form
dehydroascorbic acid. Insufficient
penetration into the cellular
matrix of fruits
Gola-Goldhirsh
et al. (1984)
Erythorbic acid
Idem
Ascorbyl phosphate
esters (APE)
and ascorbyl fatty
acid esters (AFAE)
Releases ascorbic acid
when hydrolyzed by
acid phosphatases
Sulfydryl compounds
These agents react
with o-quinones
to produce stable,
colorless products
Erythorbic and ascorbic acid
application depends on the fruit.
One compound cannot be
substituted for the other
without prior evaluation
In APE inhibition power depends
on the acidity of the fruit and
the activity of endogenous acid
phosphatase. AFAE needs
emulsifying agents, which have
detrimental effect on
antibrowning ability
This category is reduced to
sulfur-containing amino acids
(e.g., cysteine and methionine).
High concentrations affect
taste of treated fruit products
Janovitz-Klapp
et al. (1990)
Borestein (1965)
Seib and Liao
(1987)
Sapers et al. (1989)
Sapers et al. (1991)
Pierpoint (1966)
8
.
Inhibition and Control of Browning
191
Table 8.3. Nonsulfite antibrowning agents applied to fruits and fruit products: Chelating agents
and enzyme inhibitors.
Name
Mechanism of inhibition
Comments
Reference
Ethylenediamine
tetraacetic acid
Chelating agents bind
to the active site of
PPO, or reduce Cu
availability for the
enzyme
Idem
EDTA or its sodium salt is used
in the food industry as
a metal chelating agent
McEvily et al. (1992)
Acidic polyphosphate mixture
has been evaluated as EB
inhibitor in combination
with ascorbic acid
4-hexylresorcinol inhibits
browning, is water soluble,
stable, and nontoxic (GRASS)
Ashoor and Zent
(1984)
Phosphate-based agents
Substituted resorcinols
Frankos et al. (1991)
Table 8.4. Nonsulfite antibrowning agents applied to fruits and fruit products. Miscellaneous agents.
Name
Mechanism of inhibition
Comments
Reference
Enzyme
treatments
o-Methyl transferase converts
PPO substrates to ferulic acid
(inhibitor of PPO)
Sodium, calcium, and zinc
chloride are pH-dependent
inhibitors of PPO, explained
by the interaction between
charges of halides and
active site of PPO
Method too expensive
Finkle and Nelson (1963)
The order of decreasing
inhibition power
of halides is F > Cl > Br > I
Martinez et al. (1986)
Anions
Janovitz-Klapp et al. (1990)
antibrowning activity in juice, ascorbic acid-2-phosphate (AAP) and -triphosphate was
effective for cut fruit surfaces.
Combinations of ascorbic acid (AA) with an acidic polyphosphate were highly effective
with both juice and cut surfaces. Cinnamate and benzoate inhibited browning in juice, but
induced browning when applied to cut surfaces. On the contrary, combinations of betacyclodextrin with AA were effective in juice, but not on cut surfaces.
Sapers (1991) infiltrated ascorbic acid-2-phosphate (AAP) and ascorbic acid into apple
tissue to control browning. AAP hydrolysis by endogenous acid phosphatase (APase) yielded
AA, which became oxidized to dehydroascorbic acid. APase activity varied greatly with
commodity, method of sample preparation, and sample pH. Variation in the ability of
AAP to inhibit browning in different products could be explained by these factors.
Montgomery (1983) treated pear juice concentrates with 0.2 mM cysteine and observed
color changes of concentrates during storage for 6 months at different temperatures. Initial
browning of concentrates was eliminated by cysteine treatment of pear juice. Cysteine
appeared to retard Maillard reaction in pear juice concentrates and no deleterious changes
in flavor intensity were noted.
During milling and finishing operations EB is difficult to control even with high levels of
SO2 or vitamin C because of the incorporation of air. Regarding opalescent and cloudy juices,
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Fruit Manufacturing
several production procedures effective for reducing EB reactions were published (Chobot
and Horulaba, 1983), including the use of ascorbic acid and nitrogen (Fukutani et al., 1986),
blanching of the pulp (McKenzie and Beveridge, 1988), and controlled pectolytic enzyme
treatment (Gierschner and Baumann, 1988).
Montgomery and Petropakis (1980) found that the amount of ascorbic acid required to
prevent EB in pear juice is dependent on the length of time between milling and heating.
8.2.3. Effect of the Ascorbic Acid (AA) Content in Color Change
As previously described ascorbic acid (AA) does not inhibit polyphenol oxidase directly but
acts as a reducing compound and reduces the orthoquinones to dehydroxyphenols. This
action will continue as long as the concentration of ascorbic acid is sufficient to maintain a
low concentration of quinones. As the concentration of AA is decreased, the quinone
concentration increases and causes the formation of the brown pigments. Sapers and Douglas
(1987) studied the effectiveness of ascorbic acid (AA) in cut surfaces and apple and pear
juices, finding that 40 ppm AA inhibited approximately 60% in raw Granny Smith juice, but
only ffi20% in Red Delicious juice, after 90 min at 208C.
Sapers and Douglas (1987) also evaluated the effectiveness of sodium bisulfite (NaHSO3 )
and ascorbic acid (AA) in cut surfaces and apple and pear juices. The authors found that EB
in apple juice was completely inhibited by the addition of 10 ppm SO2 . The effectiveness of
ascorbic acid (AA) and erythorbic acid (EA) in inhibiting EB at cut surfaces of apple and in
raw apple juice was determined by tristimulus colorimetry by Sapers and Ziolkowski (1987).
Lozano et al. (1995) studied the color changes of apple pulp treated with the various AA
concentrations at 188C (Fig. 8.8). They found three linear regions in the Hunter L versus log
t plot, and a very well-defined breaking point was observed at the point where the AA loses its
inhibitory properties after which browning proceeds at the usual rate. Similar behavior was
70
65
AA ppm
60
100 ppm
300 ppm
600 ppm
900 ppm
1200 ppm
CIE L*
55
50
45
40
35
30
1
10
100
Time (min)
1000
Figure 8.8. Influence of ascorbic acid on enzymatic browning in apple pulp (Lozano et al., 1994) with permission.
8
.
Inhibition and Control of Browning
200
(MA)
175
18ⴗC
Breaking point
t at L*max
150
Time (min)
193
125
100
75
50
25
0
0
2
4
6
8
10
AA levels (ppm)
12
14
Figure 8.9. Time to reach L ¼ 55 and breaking point (end of browning inhibition) for MA sample as related to AA
concentration, at 188C (Lozano et al., 1994) with permission.
reported by Matsui et al. (1957), studying the oxidative darkening during the processing of
‘‘natural apple juices.’’ Experimental breaking points as a function of AA levels are plotted in
Fig. 8.9.
The data in Fig. 8.9 indicate that the addition of AA at levels greater than 600 ppm
results in a linear increase of the browning breaking point. Visual observation of apple pulp
samples treated with AA by trained judges indicated that for L values greater than 55 the
browning can be considered unacceptable. Time required at 188C to attain this level at
various AA amounts is also plotted in Fig. 8.9.
These values indicate that overtreatment with AA (>600 ppm) will not proportionally
increase the time required to reach a maximum level of browning for commercial acceptability. To make some contribution to processing of cloudy apple juice, experiments regarding
prevention of EB by adding ascorbic acid were also carried out.
When retention of apple pulp in maceration tanks is required, as in the case of cloudy
juice production, the amount of ascorbic acid necessary to inhibit EB must be estimated in
accordance with the maceration time and temperature.
8.2.4. Nonconventional Chemical Inhibition of EB
Table 8.5 lists a selection of chemical methods commonly used to inhibit EB. However, many
other nonconventional methods have been developed simultaneously.
Vacuum and pressure infiltration were investigated (Sapers et al., 1989) as a means of
applying ascorbate- or erythorbate-based EB inhibitors to apple cut surfaces. Apple plugs
infiltrated at 34 kPa pressure showed more uniform uptake of treatment solution and less
extensive water logging than plugs vacuum infiltrated at 169–980 mbar. Delicious and Winesap plugs and dice gained 3–7 days of storage life at 48C when treated by pressure infiltration,
compared to dipping. However, infiltrated dice required dewatering by centrifugation or
partial dehydration to prevent water logging. Red Delicious and Winesap plugs, dipped for
90 s in 0.8–1.6% solutions of AA or EA, showed longer lags before the onset of browning with
the former compound. AA and EA were similar in the effectiveness in apple juice. Because the
relative effectiveness of AA and EA depends on the system in which they are compared, the
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Fruit Manufacturing
Table 8.5. Classical chemical methods used for the inhibition of EB in selected fruits and fruit products.
Fruit/product
Inhibition method
Apple (GD)
Apple (McItosh)
Apple (GS)
N-acetyl-l-cysteine (25 mM).
Reduced glutathione (50 mM)
Chitosan
Ascorbyl-6-fatty acid esters
Apple
Ascorbic acid derivatives
Apple juice
and dices
Apple pulp
0.5% amylose sulfate,
0.5% xylen sulfate
Ascorbic acid
Apple slices
Ascorbic acid
and 4-hexylresorcinol
Avocado
Banana
Model apple
juice solution
Carambola slices
High pressure 800 MPa; 258C
l-cysteine
Citric acid þ ascorbic acid
Pear juice
(D’Anjou)
l-cysteine Ascorbic acid (AA)
Plum juice
0.5 mM cysteine,
1.0 Na metabisulfite
Comments
Treatment with 200 ppm chitosan
Use of 1.14 mM inhibited
EB for 6 h (equivalent
to 0.02% ascorbic acid)
0.250% erythorbic acid
The inhibitory effect could be
enhanced by 0.5% citric acid
100 –1000 ppm, depending
on the required breaking point
Mixture of 0.01% 4-HR
and 0.5% AA reduces
EB as 0.1% SO2
5 mM gives 100% inhibition
Found oxidation depends
on total phenols
Treating slices with 1.0 or 2.5%
citric acid 1 0.25% ascorbic
acid (in water) prior to
packaging was very
effective in limiting
browning
10 mM inhibits 100% PPO,
1 mM AA inhibits 25% PPO,
1 mM l-cysteine inhibits 57%
PPO
Substrate is phenolics
and chlorogenic acids
Reference
Molnar-Perl and
Friedman (1990)
Sapers (1991)
Sapers et al. (1989)
Sapers and
Ziolkowski (1987)
Vámos-Vigyázó
(1995)
Lozano et al. (1994)
Luo and
Barbosa-Canovas
(1996)
Weemaes (1998)
Kahn (1985)
Goupy et al. (1995)
Weller et al. (1997)
Siddiq et al. (1994)
authors indicated they should not be used interchangeably as sulfite alternatives without
experimental verification of equivalence.
Özoglu and Bayindirli (2002), using the response surface methodology, found that the
ascorbic acid, l-cysteine, and cinnamic acid combination provided better results as EB
inhibitor than as individual compounds. The authors found 0.49 mM AA, 0.42 mM
l-cysteine, and 0.05 mM cinnamic acid in cloudy apple juice inhibited browning for 2 h at
258C.
8.2.4.1. Honey
The use of honey as a natural browning inhibitor was demonstrated in apple slices, grape
juice, and model systems (Oszmianski and Lee, 1990). The browning of apple slices was
inhibited to a greater extent by using 10% honey, than by a sucrose solution containing an
equivalent sugar concentration. Analysis of honey revealed that a small peptide is responsible
for the inhibition of polyphenol oxidase. The efficacy of honey in inhibiting polyphenol
oxidase activity varied in accordance with the variety of honey (Chen et al., 1998).
8
.
Inhibition and Control of Browning
195
8.2.4.2. Aromatic Carboxylic Acids
Cinnamic acid and its analogs, p-coumaric, ferulic, and sinapic acids, were found to be potent
inhibitors of apple polyphenol oxidases (Pifferi et al., 1974; Walker and Wilson, 1975).
Cinnamic acid at levels of 0.01% was observed to be effective in providing long-term
inhibition of polyphenol oxidase in apple juice (Walker, 1976).
8.2.4.3. Proteases
Some plant proteases like ficin, papain, and bromelain are sulfydryl enzymes (Labuza et al.,
1992; Taoukis et al., 1990), which are very effective as browning inhibitors. Pineapple juice
was found to be effective in inhibiting browning in apple rings (Lozano-de-Gonzalez et al.,
1993). Bromelain, organic acids, sulfydryl compounds, and certain metallic constituents of
pineapple juice are believed to be responsible for this inhibitory effect. Polyphenol oxidase
activity in plum juice was significantly reduced when the juice was treated in a column
containing immobilized proteases (Arnold et al., 1992).
8.2.5. Miscellaneous Methods
The market for lightly processed apples has increased rapidly (Schlimme, 1995). The development of retail and institutional precut apple products has been limited by browning, which can be
controlled or minimized by using modified atmosphere packaging (MAP) with selected chemical
treatments (El-Shimi, 1993). Lakakul et al. (1999) studied the use of plastic films to control
moisture loss and respiration rate of cut apples. Raghavan et al. (1996) reported that damage to
apple tissue texture could be reduced by calcium treatment and proper storage temperature.
Figure 8.10 shows different treatments that have been experimented during the last few
years to inhibit EB in fruit products. Although some of these methods have been impractical
or expensive up to now, they are sufficiently innovative and safe for use in foods.
Ultrafiltration and
nanofiltration
Sonication
irradiation
Supercritical
CO2
Miscellaneous
and non
conventional
EB treatments
High
pressure
treatment
Blanching
during size
reduction
Figure 8.10. Miscellaneous and nonconventional methods for the treatment of enzymatic browning.
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Fruit Manufacturing
Zemel et al. (1990) showed that PPO activity could be irreversibly inhibited by temporarily lowering the pH to 2.0 with HCl. However, the pH must to be adjusted to its initial value
by the addition of NaOH solution. This treatment inhibited EB and stabilized the apple juice,
but was unfavorable as it affected the flavor.
Tronc et al. (1997) used electrodialysis (ED) to prevent EB in cloudy apple juice, without
employing additives, by temporarily acidifying the juice and then readjusting its pH to the
initial value.
ED is a membrane technique that results in the separation of ions (Lopez-Leiva, 1988).
This method has been used to regenerate mineral acids and bases (Mani, l991). The application of ED with bipolar membranes proved to be a convincing approach for achieving
changes in pH of cloudy apple juice. The authors indicated that in this way, the pH could
be varied by the gradual incorporation of protons and hydroxyl ions derived from the
dissociation of water, without affecting juice flavor by salt formation.
Sapers et al. (1989) have shown that b-cyclodextrins inhibit browning of raw apple juice,
and also found that ultrafiltration (UF) reduces EB in fruit products (Goodwin and Morris,
1991). Recent studies have shown that the simultaneous application of heat and ultrasonic
waves had a synergistic effect on PPO inactivation.
The process called mano-thermosonication needs special equipment and can be used with
foods that are damaged by drastic heat treatment (Chen et al., 1992a,b). Supercritical carbon
dioxide (SC-CO2 ) treatment was tried for inactivation of PPO in vegetables (Chen et al.,
1992). In fruits SC-CO2 was used for the inactivation of pectinesterase (Arreola et al., 1991).
With the exception of UF the other nonconventional methods involve heat treatment.
8.2.5.1. Irradiation
Food irradiation is increasingly recognized as a method for reducing postharvest food losses
and ensuring hygiene. Food irradiation is effective in securing the long-term preservation of
foods through the inactivation of microorganisms. Fruits can be preserved by irradiation,
thereby delaying their maturation or sprouting.
Browning reduction in tropical fruits by gamma irradiation was reviewed by Thomas
(1984, 1986). Browning was minimized by controlling the dosage level of the applied radiation. However, ionizing radiation at doses exceeding 1 kGy can introduce various types of
physiological disorders in food products. Free radicals produced during the treatment of food
with ionizing radiation, are capable of reacting with various food constituents and inducing
undesirable side effects, such as tissue darkening, lipid oxidation, and decreased vitamin
content. Nonenzymatic browning (NEB) reactions of free amino acids and proteins with
reducing sugars, such as glucose, may be responsible for this discoloration.
The sensitivity of enzymes to ionizing radiation is defined as the dose required to
inactivate 63% of the original activity of the enzyme, D37 . Table 8.6 lists D37 values of some
enzymes in model systems measured in the dry state (Roozen and Pilnik, 1971).
Combined treatments using both irradiation and heat or other methods have demonstrated a synergistic effect.
8.2.5.2. Ultrafiltration (UF)
UF has shown to be effective in stabilizing the color of fruit juices (Flores et al., 1988; Sims
et al., 1989). Moreover, Goodwin and Morris (1991) studied UF as an alternative to sulfiting
for the control of EB. UF is believed to remove polyphenol oxidase, but not lower molecular
8
.
Inhibition and Control of Browning
197
Table 8.6. Ionizing radiation doses to
inactivate 63% (D37 ) of some selected enzymes.
Enzyme
Alkaline phosphatase
Pectin esterase
Peroxidase
D37 (kGy)
40 –50
60
30 –70
weight polyphenols or Maillard-reaction precursors, which could undergo NEB during
storage. Galeazzi et al. (1981) found that banana PPO fractions had molecular weights >30
kDa, within the range of molecular weight cut-offs for UF membranes.
8.2.5.3. High-pressure treatments
High-pressure treatments reduce microbial counts and enzyme activity, and affect product
functionality (Farr, 1990; Hoover et al., 1989; Cheftel, 1991). This provides a good basis for
development of new processes for food preservation or product modifications (Mertens and
Knorr, 1992). The first commercial products made, using high-pressure treatments, have been
almost exclusively plants or product-containing plants (Knorr, 1995).
Effects of high-pressure treatments on enzymes may be related to reversible or irreversible
changes in protein structure (Cheftel, 1992). However, loss of catalytic activity can differ
depending on type of enzyme, the nature of substrates, and the length and temperature of
processing (Cheftel, 1992; Kunugi, 1992). Ogawa et al. (1990) reported the effect of high-pressure
treatments on pectinesterase and peroxidase activity in model systems from mandarin juice.
Cano et al. (1997) determined the effects of high-pressure treatments up to 400 MPa
combined with mild heat treatments up to 6078C on different enzymes, including polyphenol
oxidase (PPO) in strawberry purée and orange juice. Pressurization/depressurization
treatments caused a significant loss of strawberry PPO (60%) up to 250 MPa. Although
neither enzyme, including PPO, was completely inactivated after pressurization from 100 to
400 MPa, no recovery of enzyme activity was observed during storage. Degree of inactivation
varied depending on the type of fruit and vegetable products studied (Knorr, 1995), and strong
enzyme activation could be observed in cell-free extracts (Anese et al., 1995).
Weemaes et al. (1998) found PPO from apples, avocados, grapes, pears, and plums was
rather pressure stable. While inactivation of PPO from apple became noticeable at 600 MPa
(258C), for pear PPO, pressures as high as 900 MPa were required (Fig. 8.11). Simultaneous
application of mild heat increased the PPO inactivation rate constant. Table 8.7 lists some
high-pressure treatments of fruit, which were positive for PPO inactivation.
The inactivation of a pure enzyme by pressure is dependent on the immersion medium, the
pH, as well as the temperature and duration of the treatment. Moreover, food constituents may
show protective effects on enzymes during high-pressure treatment (Fig. 8.12). Mushroom
polyphenol oxidase shows very high-pressure stability, although it is a thermosensitive enzyme
that is readily inactivated by temperatures exceeding 508C (Weemaes et al., 1997).
8.3. INHIBITION AND CONTROL OF NONENZYMATIC BROWNING (NEB)
The extent of NEB on fruit products depends on product composition, water activity, storage
time, and temperature, as previously discussed (Chapter 7). NEB in fruit products may be
198
Fruit Manufacturing
0
−0.1
25ⴗC
Log relative activity
−0.2
−0.3
−0.4
−0.5
−0.6
−0.7
Apple
pear
−0.8
−0.9
−1
0
25
50
75
100 125
Time. min.
150
175
200
225
Figure 8.11. Inactivation of PPO from apple and pear, at 800 and 900 MPa, respectively
(adapted from Weemaes et al., 1998).
inhibited or reduced by refrigeration, control of water activity in dehydrated fruits (Labuza
and Saltmarch, 1981), reduction of amino nitrogen in juices (Prı́ncipe and Lozano, 1991), and
use of different chemical inhibitors. Two basic treatments have been used for the control or
reduction of nonenzymatic reactions (Fig. 8.13):
.
.
Preventive methods, which are those that limit the advance of NEB reactions; while
Restorative methods, which basically let the NEB to develop, reducing later the
product of the deteriorative reactions.
8.3.1. Preventive Methods
8.3.1.1. Temperature Control
It is well known (Reynolds, 1965) that NEB reaction is retarded by reducing the temperature.
Figure 8.14 shows a t–T plot of the several conditions apple juice undergoes from milling
to distribution. In the particular case of processing apples to obtain juice concentrate,
it may be expected that color, taste, and flavor would be undesirably modified and the
questions are:
Table 8.7. Effective high-pressure treatments for fruit PPO inactivation.
Fruit
Treatment
Reference
Apple (pH 4.5)
Avocado
Pear in slices
White grapes
>500 MPa/258C/1 min
800 MPa/258C
900 MPa/258C (slight inactivation)
700 MPa/258C
Anese et al. (1995)
Weemaes et al. (1998)
Weemaes et al. (1998)
Weemaes et al. (1998)
8
Inhibition and Control of Browning
.
199
1
600 MPa
0.9
0.8
Activity (relative)
0.7
0.6
700 MPa
0.5
0.4
0.3
800 MPa
pH=7
0.2
45ⴗC
0.1
900 MPa
0
0
5
10
15
20
Time (min)
25
30
35
Figure 8.12. Effect of high hydrostatic pressure on relative activity of polyphenol oxidase at pH ¼ 7 (458C)
(adapted from Seyderhelm et al., 1996).
.
.
How much, in any suitable unit, is the damage introduced by a particular operation?
Moreover, can it be quantitatively related to the magnitude of treatment, that magnitude being also measured in any conventional units, such as temperature, time, concentration, and the like?
In the case of juices, more precisely apple juice concentrate, out of the those sensorial
properties mentioned above, color is the one that can be submitted to a more objective
measurement by well-known techniques. Flavor and taste, though perhaps more relevant
from the hedonic point of view, are more subjective in nature.
Temperature control
Process optimization
Preventive
Ion exchange treatment
Use of chemical inhibitors
Neb treatment
Synthetic adsorbers
Restorative
Nanofiltration
Activated charcoal
Figure 8.13. Basic nonenzymatic browning treatments.
200
Fruit Manufacturing
10000000
Transportation
1 month
Time (s)
1000000
1 day
30%
100000
10000
10%
Storage
1h
1000
100
Clarification
Evaporation
10
1 0
1 min
20
40
60
80
100
120
Temperature (ⴗC)
Figure 8.14. Time–temperature plot during the processing of clarified apple juice concentrate
(broken lines indicate percentage of color increase).
Hence it is common to consider the browning expressed as the absorbance or optical
density at a given light wavelength, namely 420 nm, when measured on the pure or singlestrength juice. Less common, but still well known, is the qualification of the color through
their three-tristimulus parameters (e.g., Hunter L, a, and b).
8.3.1.2. Process Optimization
Concentration by evaporation is a very common practice in the fruit juice and pulp industry.
Multiple-effect evaporators used in the fruit juice processing plants were designed to eliminate
water under vacuum at relatively low temperatures. However, it is not unusual in practice to
find very high temperatures in the first stage of processing. This can lead to changes both in
color and flavor of the juice, mainly due to NEB.
Process control has been increasingly adopted in the food industry during the last 30 years
both to improve quality and reduce energy costs (Frost, 1977; Lozano et al., 1984). In order to
properly adopt control strategies it is necessary to obtain either the empirical or the simulated
dynamic model of the process by itself, without considering any control loop. Tonelli et al.
(1990) presented a versatile computer package useful for the simulation of the open-loop
dynamic response of a triple-effect evaporator for the concentration of fruit juice.
In addition to fluid dynamics and thermal considerations some attention should be paid
to the potential damage, which could be induced during concentration. However, selection of
the pair values t–T to perform a given process is not a trivial task, but a trade-off between
opposite considerations.
For instance, low temperatures are more expensive because of the demand for a larger
area, but high temperatures introduce the risk of scaling. Morgan (1967) showed that process
side heat transfer coefficient dropped 10 times after 1 h operation in a tomato paste evaporator as a consequence of fouling. It is also known that heat transfer coefficient increases with
the temperature difference between the heating medium and the solution, which makes
advisable to work at high vacuums, low temperatures. But that means high vapor volumes,
and larger piping and related pieces of equipment to maintain pressure drop within practical
limits, which in turn means larger investment capital. If, alternatively, higher-pressure steam
is used, the process is more expensive.
8
.
Inhibition and Control of Browning
201
The formation of 5-hydroxy-methyl-2-furfuraldehyde (5-HMF), an intermediate of
browning reactions in apple juice, has been directly related to the severity of heating in
fruits and honey (Chapter 7). Toribio and Lozano (1986) found that this reaction follows a
zero-order kinetics, after an induction period where no buildup of 5-HMF is detected. Babsky
et al. (1986) also found that accumulation of 5-HMF achieves maximum accumulation after a
long period of storage. These results may indicate that rate of formation of 5-HMF is similar
that of a second-order autocatalytic reaction. However, no further advances were made to
develop a complete and realistic mechanism of reaction based on the theory available.
From an operational point of view and damage introduced by the thermal treatment, it is
apparent that time is more critical than temperature: residence times of several minutes are
common in evaporators for which relatively small changes in temperature may produce a
dramatic increase in HMF concentration. Alternatively, if time can be kept within small
values, the same temperature step does not provoke a significant HMF growth (Toribio and
Lozano, 1986). This confirms the accepted practice of moving to lower time–higher temperature combination whenever foods are to be thermally processed.
It is experimentally known that actual residence times are several times longer than those
calculated on the assumption of piston flow. For instance, in recirculating equipment, Moore
and Pinkel (1968) showed that the actual holding time for 97% replacement of the liquid
volume is 3.6 times greater than the average calculated as plug flow. There are, in addition,
dead times in the distributors, pipes, pumps, and vapor separators, in which the juice is at the
same temperature, which should be taken into account, as the damage is also in progress.
However, this analysis is restricted to the liquid transit along the tubes since those times
cannot be accurately estimated, and can be assumed to be the same in both configurations for
comparison purposes.
EXAMPLE 8.1
Application to multiple-effect evaporator design: evaporator selection
Selection of a given type of evaporator is a task governed by many considerations. When
fruit juice damage is considered, the falling film evaporator is an attractive alternative,
since it imposes only short residence times to the solution, allowing for relatively low
temperatures.
Tonelli et al. (1990) simulated an actual 3-effect falling film unit, by means of a
program specially formulated for that purpose, which provides mass flow rates and
concentrations, at the entrance and discharge of each effect, as well as temperatures live
steam consumption, and mass and enthalpy balances, for both backward and forward
feed. It takes into account the rise in boiling point of solutions, and allows for feed
preheating and vapor thermal recompression. The computer package simulates a tripleeffect horizontal flash concentrator (Fig. 8.15), with a capability to concentrate about
7,600 kg/h of a 16.38Brix clarified apple juice (Table 8.8). A more complete description of
the simulation model and the industrial unit was given previously (Tonelli et al., 1990).
Once flow rates and temperatures were known, physical properties and Reynolds
numbers in the tubes were easily computed. Film widths were calculated by means of the
equation presented by Sideman (1981), at entrance, discharge, and arithmetic average.
Holdup was calculated as a single value for average conditions. Finally, velocities and
residence times were calculated for average conditions. All data are presented in Table 8.9.
202
Fruit Manufacturing
Tap water
Vapors
Steam
Barometric
foot
1st effect
Preheating
2nd effect
Concentrate
Feed
3rd effect
Figure 8.15. Sketch of a triple-effect flash concentrator.
By applying the correlation for damage as a function of time, temperature, and concentration to each effect for both backward and forward flow arrangements, the values of HMF
formation shown in Table 8.8 are obtained. It is apparent that forwarded flow is much less
harmful, the concentration being one order of magnitude lower than that produced in the
countercurrent or backward arrangement.
This is proof of the generalized practice in favor of the parallel flow for heat-sensitive
materials. In both cases the first effect becomes the most damaging one, while in the countercurrent one, the reduction in viscosity of the more concentrated product due to the higher
temperature, is not enough as to reduce the residence time significantly, so the juice is
submitted to the most unfavorable combination of the relevant variables. It is seen that
while in the forward flow the first effect provokes 78% of the damage, in the backward one
it is 99.3%, the absolute figure being ten times greater. Regarding the aroma-stripping
operation the calculation on the commercial unit installed with the 3-effect evaporator did
not indicate HMF formation for the prevailing conditions.
Lozano et al. (1995) studied the open-loop dynamic response of an apple juice evaporator, based on the kinetics of 5-HMF formation. Results indicated that 5-HMF content of
concentrated juice is strongly dependent on the temperature at the 1st effect. Level of 5-HMF
was below 30 mg/L, a proposed reasonable limit (Toribio and Lozano, 1987) for clarified
apple juice.
Table 8.9 also lists the estimated increase in 5-HMF as affected by a variation in the
temperature at the first effect of the simulated evaporator and with the set of industrial
Table 8.8. Experimental industrial
operating conditions.
Feed flow rate (kg/h)
Feed concentration (8Brix)
Feed temperature (8C)
Steam pressure (kPa)
Thirst effect pressure (kPa)
7650.0
16.3
45.0
182.0
10.0
8
.
Inhibition and Control of Browning
203
Table 8.9. Formation of 5-HMF as affected by temperature at the first effect.
Temperature,8C
1st effect
Soluble solids,8C
3rd effect
5-HMF, mg/L
60
60
60
70
70
71
75
75
75
5.02
7.80
12.63
6.6
10.8
16.8
7.4
12.5
18.6
100.1
104.4
109.1
100.1
104.4
109.2
100.0
105.1
109.1
operation conditions given in Table 8.8. The authors concluded that the knowledge of the
dynamic response of heat-exchange equipment, like multiple-effect evaporators, together with
the appropriate kinetics equations of deteriorative reactions is important to estimate and
reduce the heat damage. This reduction can be achieved by the implementation of an
appropriate control configuration.
8.3.1.3. Ion Exchange Treatment
Ion exchange resins have been used in the industry for discoloration of syrups (Harris, 1986),
the hydrolysis of lactose (Guerin and Lancrenon, 1982), and the anthocyanin recovery from
fruit bagasses (Chiriboga and Francis, 1973) among other applications. Ion exchange treatments of liquid foods are legally permitted in several countries (Rankine, 1986; Johnson and
Chandler, 1986). Ion exchange resins, as well as different types of adsorbers, have also been
used in fruit juices (Withy et al., 1978) to elucidate the role of amino acids and polyphenols in
the formation of brown color polymers (Cornwell and Wrolstad, 1981) and for deacidification and debittering (Johnson and Chandler, 1986). Prı́ncipe and Lozano (1990) studied the
effect of such treatments on the quality of the juices, as well as the operative applications of
these processes.
The exchange process is largely confined to a narrow region in the resin bed, which,
within a short time after the liquid to be treated is flowing, moves down the bed at a constant
rate and leaves the column at a point called the break through point. At that point the
absorbed compound suddenly increases its concentration in the effluent, resulting in a typical
S-curve. In order to calculate bed capacities the following equation can be used:
ð Ve
Ct ¼
(X X0 )dV=Vra
(8:1)
0
where Ct is the total capacity of the column, Vra the average volume occupied by resin in the
bed, X and X0 are the concentrations of compound to be absorbed in effluent and influent,
respectively, V the volume eluted at any time, and Ve the volume at which exhaustion of
column results. If integration takes place up to the breakthrough point (Vt ) only, the resulting
column capacity is called effective capacity.
Figure 8.16 shows the reduction in the total amino acids (AA) when apple juice (158 Brix;
pH ¼ 3:8) is passed through a cation exchange column (DOWEX 50 8), as a function of the
volume of effluent collected per gram of dry resin (Prı́ncipe and Lozano, 1990).
204
Fruit Manufacturing
Amino acids, mg/100ml
1.6
0.9
3rd
2nd
0.2
1st
Vt
50
100
150
Effluent, ml/g
Figure 8.16. Effluent amino compound concentration versus effluent volume per gram of cation resin, with the
number of column regenerations as a parameter. Vt is the volume at which break through point occurred (Reprinted
from Lebens. Wiss. Technol., 24, Prı́ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in
clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).
In general, successive regenerations did not reduce the column capacity and the resin
may be used many times. Progress of the ion exchange may be monitored by pH readings in
the outlet juice variable (Fig. 8.17). In order to recover the original juice pH, the cationexchanged juice may be passed through an anion-exchange resin. The anion-exchange column
also reduced the color of the juice.
PH
4.0
3.0
Anion exchange
2.0
Cation exchange
20
40
60
Effluent, ml/g
80
100
Figure 8.17. pH of column effluent as a function of effluent volume per gram of resin (Reprinted from Lebens. Wiss.
Technol., 24, Prı́ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice
by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).
8
.
Inhibition and Control of Browning
205
8.3.2. Restorative Methods
During fruit juice discoloration, chromophoric components are eliminated, without modifying if possible, the other components of the product. While in the case of apple juice
polyphenols are to be removed, red coloration in orange juice is caused by anthocyanins.
The adsorption capacity of certain substances eliminating coloring matter by adsorption is
exploited for discoloration. Properties such as grain size, surface area, and porosity define
adsorbers’ capacity. This could be attributed to polyphenol adsorption.
Adsorption forces are ruled by weak Van der Waals’ forces, which are temperature dependent. Adsorption of color is usually performed by adding activated carbon (AC) as a slurry to the
juice, since this gave better dispersion than the addition of dry carbon. Table 8.10 lists the average
characteristics of a typical AC used for apple juice discoloration (Prı́ncipe and Lozano, 1990).
The ACs listed in Table 8.10 had practically the same adsorption capacity and kinetics,
which was in accordance with the similarity in their characteristics. Absorption of solutes
from a dilute solution, as in the case of brown compounds in apple juice, can be described by
an empirical isotherm similar to that attributed to Freundlich:
Y ¼ m Xn
(8:2)
where Y ¼ C=C0 , color at the equilibrium/initial color; X is the units of color adsorbed (L
juice/g AC), and m and n are constants experimentally determined. Discoloration is basically
a batch operation where the amount of insoluble adsorbent (activated carbon) is very small
with respect to the amount of product treated and the highly colored compounds removed are
much more strongly adsorbed than the other juice constituents (sugars, acids, etc.). A solute,
or color, balance is:
A=J ¼ (Y0 Yf )=(Xf X0 )
(8:3)
where A is the mass of activated carbon, J the volume of juice to be treated; X0 and Xf are the
initial and final color adsorbed/mass of carbon, and Y0 and Yf are the initial and final color of
the treated juice. Since the AC used ordinarily is fresh (X0 ¼ 0), substitution of (8.3) in (8.2)
gives:
A=J ¼ (Y0 Yf )=(Yllm)l=n
(8:4)
This equation permits the calculation of the carbon to juice ratio, for a given change in the
juice color from Y0 to Yf .
Table 8.10. Properties of activated carbons.
Appearance
Black powder
pH
Activity (blue methylene test)
Water content (%)
Particle size (%):
mesh 200
mesh 325
Density (apparent, kg/l)
Ash (%)
Sulfates (%)
Iron (ppm)
4.5–5.5
20(minimum)
10 –15
5
10
0.34 – 0.38
7 (maximum)
0.6 (maximum)
100
206
Fruit Manufacturing
Example 8.2
Application of activated charcoal for apple juice discoloration
Consider the need to reduce by 78% the color of a juice with activated charcoal.
Adsorption of color by AC is fast and strongly dependent on the amount of AC
used. On the other hand, the influence of temperature between acceptable working
values (40 –808C) was practically negligible. The application of the Freundlich-type
equation (8.2) to the equilibrium data resulted in:
Y (C=C0 ) ¼ 44:38X 4:42 (C=gr:C)
(8:5)
This equation was plotted in Fig. 8.18, which represents a typical equilibrium curve for
a single-stage discoloration process. Figure 8.18 also shows the operating line between
the initial relative color (Y0 ¼ 1) and the coordinates of point (Xl , Yl ). If sufficient
contact time is allowed and equilibrium is reached, the operating line intersects the
Freundlich isotherm at Xl . The operating line in the example was a slope A=J ¼ 2:75,
which directly determined the necessary amount of AC.
8.3.2.1. Effect of Storage
Figure 8.19 shows the effect of prolonged storage on clarified apple juice color, after deamino acid treatment. Sample ‘‘a’’ browned at approximately the same rate as the control
juice, while ‘‘b’’ showed a reduced rate of browning.
This behavior can be explained by considering that free amino acids remained in juice.
Therefore, the heated juice had practically the same amino acid content as the fresh juice,
which was demonstrated to directly enhance the rate of NEB (Babsky et al., 1986).
1.0
n = 4.52
Y , C/Co
.8
.6
.4
Operating
line.
slope = A/J
.2
(X1 , Y1)
.3
.1
X , color / g c.a
Figure 8.18. Nonenzymatic browning of apple juice concentrate as a function of time of storage, at 378C. Control:
untreated juice. Sample ‘‘a’’: Fresh processed juice discolored with AC. Sample ‘‘b’’: Long-term storage, highly
colored, AJC rediluted and treated with AC (Reprinted from Lebens. Wiss. Technol., 24, Prı́ncipe, L. and Lozano,
J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange,
pp. 34–38 (copyright) 1991, with permission from Elsevier).
8
.
Inhibition and Control of Browning
Red Del.: 75 Brix
37⬚C
1.8
Absorbance 420 Nm
207
Sample “a”
1.4
1.0
Sample “b”
o.6
o.2
0
40
120
80
Storage time, days
Figure 8.19. Freundlich equilibrium curve for a single-stage discoloration of apple juice (Reprinted from Lebens.
Wiss. Technol., 24, Prı́ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified
apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).
On the other hand, in sample ‘‘b’’ most of the amino compounds, already reacted to form
melanoidins, were adsorbed by the ACs, and NEB reactions scarcely developed. Figure 8.20
shows a typical browning curve during storage of apple juice with different levels of total
amino acid content.
RED DELICIOUS
37⬚C
1.8
Control
Abs., 420 nm
1.4
1.0
Sample “a”
0.6
Sample “b”
0.2
0
20
40
60
80
Time of storage, days
Figure 8.20. Color of ion exchange-treated AC as a function of storage time, at 378C. Sample ‘‘a’’: Initial amino acid
content, 57 mg/L. Sample ‘‘b’’: Initial amino acid control, 38 mg/l (Reprinted from Lebens. Wiss. Technol., 24,
Prı́ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by
absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).
208
Fruit Manufacturing
Table 8.11. Effect of ion-exchange treatment on apple juice.
Total amino acids (meq/l)
Acidity (g/l)
Malic acid, L (g/l)
Total phenols (ppm)
Calcium (mg/100 g)
Sodium (mg/100 g)
Iron (ppm)
Soluble solids (8Brix)
Initial color (Abs4z()
Before
After
Values determined in natural apple juice in natural
7.0
3.7
4.1
39.3
30.1
23.s
12.4
15.4
0.337
5.8
4.5
4.0
15.4
0.7
0.1
1.2
15.3
0.124
3–30
2.4–7.6
—
< 300
>3
—
< 18
—
< 0:5
*Babsky et al. (1986); Prı́ncipe and Lozano (1991).
Partially treated juice suffered some compositional changes, with the major components
retaining acceptable values, but significant reductions in concentrations of calcium and iron
were observed.
Results of the experimental treatment of apple juice with ion-exchange resins are listed
in Table 8.11. Calcium, amino acids, and other nutrients have been shown to affect the growth
of microorganisms. Treated juices became much less susceptible to microbiological spoilage.
On the other hand, Fe levels greater than 8 ppm are practically unacceptable in clarified
apple juice. Adsorption, or ion exchange treatment, can modify the color attributes of
the apple juice. Treated juices were more purple than fresh juice, and displayed a more
natural chromaticity after several days of storage (Fig. 8.21).
Prı́ncipe and Lozano (1990) concluded that AC treatment should only be used to reduce
the color of fruit juice concentrates subjected to prolonged and/or high temperature conditions of storage. Carbon adsorption of fresh juices will not reduce the rate of NEB.
Yellow
Db
Orange
20
78 days
10
15 days
Green
–5
0
0
5
10
15
Da red
7 days
–10
Purple
–20
blue
Figure 8.21. Hunter Da and Db parameters with time of storage at 378C. (&) Untreated juice, (O) AC-treated juice,
(x) ion-exchange treated juice (Reprinted from Lebens. Wiss. Technol., 24, Prı́ncipe, L. and Lozano, J.E., Reduction
and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright)
1991, with permission from Elsevier).
8
.
Inhibition and Control of Browning
209
The process of discoloration with AC included tedious steps like clarification with
bentonite and gelatin, filtration with a filterpress or vacuum filter, and reconcentration to
original soluble solids’ content.
On the other hand, fresh clarified fruit juice can be treated with cation þ anion exchange
resins in order to reduce the amino compound content to levels low enough to satisfactorily
reduce the rate of Maillard-type browning. Additionally, ion exchange treatment can also
adjust the pH values and reduce the amount of micronutrients, which could make the juice
more stable from a microbiological standpoint. AC and resins are readily available, and resins
can be reactivated economically and simply, and used over and over again without significant
working capacity reduction.
8.3.2.2. Use of PVPP (Polyvinyl Polypyrrolidone)
The synthetic polymer polyvinyl polypyrrolidone (PVPP), which has been used in the beverage industry since several years, is an absorbent, which shows selective affinity to polyphenols
and tannins (Binnig, 1992).
PVPP is used not only for discoloration of fruit juices, but also to prevent haze formation
after processing. The regenerability of PVPP is the advantage of this product compared with
the AC treatment. The application of PVPP can be realized either by batch process or with
continuous dosage using a precoat filter for the elimination. For a significant discoloration as
much as 3 g/L must be added (Hoffsommer and Cook, 1991). Regeneration of PVPP is done
by alkaline solution followed by an acid neutralization. Polyvinyl polypyrrolidone (PVPP)
shows a high selectivity for adsorption of polyphenols and has been established as a final
stabilization treatment after UF (Günther and Stocké, 1995).
8.3.3. Miscellaneous Methods for Inhibition and Control of Nonenzymatic Browning
8.3.3.1. Color Reduction by Combined Methods
Various pre- and post-treatments are available to avoid post-turbidity and discoloration of fruit
juices. Stabilization of beverages by gelatin, bentonite, and silica gel is a widespread conventional treatment. Pretreatment techniques, including hyperoxidation of raw juice with PPO
prior to UF, have been used as an alternative (Giovanelli and Ravasini, 1993; Maier et al., 1994).
The use of combined adsorbent resins for clear apple juice stabilization has gained increasing
importance as a final treatment after clarification (Schobinger et al., 1995; Weinand, 1995).
However, such treatments imply an additional cost in existing juice processing lines. Polyphenols that are responsible for haze formation and browning during storage of clear apple juice
and concentrate, could be selectively removed by an UF process using membranes of polyethersulfone and polyvinylpyrrolidone (Gökmen et al., 1998). The authors compared their
results with those from commercial UF membranes made out of regenerated cellulose acetate.
The effects of laccase treatment on removal of polyphenols and color in apple juice were also
investigated. Custom membranes were effective in reducing the amount of polyphenols. A
remarkable desired color removal of apple juice could also be achieved using these membranes.
Resulting products were stable in color and had clarity at 508C for up to 6 weeks. Laccase
treatment increased the percentage removal of polyphenols from apple juices. However, laccasetreated samples were more susceptible to coloration and haze formation during storage.
Kacem et al. (1987) studied the NEB during the storage of single-strength orange juice and
synthetic orange drinks under aerobic and anaerobic conditions. The effect of free amino acids
on browning was linear, with concentration being more pronounced in the presence of high
210
Fruit Manufacturing
0.7
0.6
4.2 mg/100mL
38 mg/100mL
0.5
71.8 mg/100mL
0.4
0.3
0.2
0.1
0
0
4
8
12
16
20
Figure 8.22. Effect of ascorbic acid concentration on browning of orange drinks with 0.66% amino acids.
Solid line indicates juice stored in retort pouch, while dashed line represents juice stored in polyethylene pouch
(Kacem et al., 1987 with permission).
levels of ascorbic acid. Ascorbic acid was found to be the most reactive constituent of orange
juice (Fig. 8.22). Packaging in polyethylene pouch greatly accelerates loss of ascorbic acid.
8.3.3.2. Use of Chemical Inhibitors
Bolin and Steele (1987) investigated the effect of various treatments on NEB of dried apples,
and determined that cysteine incorporation did not reduce browning during storage. The
same was valid for manganese and tin addition.
Many nonsulfite compounds have been shown to exhibit NEB protection in a variety of
foods. Trehalose has been found to retard reaction between dry proteins and reducing sugars
(Loomis et al., 1979). Bolin et al. (1976) used packaging with nitrogen headspace, to reduce
the darkening rate in sulfured dried peaches.
The authors attributed only 20% of the NEB to Maillard-type reactions. Tamaoka et al.
(1991) studied the effect of high pressure (up to 500 MPa at 508C) on Maillard reaction
between amino compounds with carbonyl compounds. Results indicate that the high pressure
may suppress the browning process.
8.4. CONCLUSIONS
The heating of the fruit mash or juice immediately after crushing the fruit appears to be the
most effective way to control EB in many fruit products. The addition of sulfur dioxide,
ascorbic acid, or cysteine has been used to retard browning during the heating period. The
effect of a definite amount of AA in apple fruit pulp showed a very well-defined breaking
point after which browning proceeds at the usual rate. Nontraditional methods, like UF of
liquid fruit products, use of supercritical carbon dioxide, or sonication, in combination with
heat treatments have been used by researchers since the last decade.
8
.
Inhibition and Control of Browning
211
Although foreign additives are considered ever more undesirable in foods in general (for
health reasons), the inhibition of chemicals will play primary role in the prevention of EB, at
least in the very near future. The search for effective safe inhibitors of the NEB is stimulated
by the need to: (1) prevent undesirable Maillard reactions, and (2) find alternatives to the use
of sulfites. Organoleptic considerations are the major barrier to the use of some Maillard
inhibitors (O’Brien and Labuza, 1994). Cysteine appears to be a good alternative to the use of
sulfite in foods at present.
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INDEX
A
Absorbance spectrophotometry, 99–104
Acetic acid, as preservative, 8
Acetoin, 153
N-Acetyl-L-cysteine, as browning inhibitor, 192
Acidity, 157
measurement of, 7
ripening-related decrease in, 21
as taste component, 136
Activated carbon, use in browning control, 203–207
Activity coefficients, of aroma components,
118–125
experimental values, 124, 125
NRTL model, 121, 122
UNIFAC model, 121, 123
UNIQUAC model, 121–123
Wilson equation, 121, 122
Agglomeration, 67–71
selective (spherical), 71
Air, chemical composition of, 12, 13
Aligning, as fruit sorting method, 29
Alkaline phosphatase, radiation-related inactivation of,
195
Aluminum cans, 6
Amino acids, 136. See also names of specific amino acids
in browning, 164, 165, 199, 200
effect of ascorbic acid on, 207, 208
in fruit juice, storage-related loss of, 153–155
g-Aminobutyric acid, 153, 170
Amylase, 40
Amyloglucosidase, 35
AnalySIS 2.1, 113
Anions, as browning inhibitors, 187, 189
Anthocyanidins, 149, 150
Anthocyanins, 99, 148, 149, 157
recovery from fruit bagasses, 199
storage-related loss of, 158
Anthoxanthins, 148
Antibrowning agents, 187–193
nonconventional, 187, 191–193
Antoine equation, 95, 121
Appearance, as food quality indicator, 99, 161
Apple butter, pH of, 138
Apple juice
amino acid content of, 153
as browning cause, 204
Apple juice (cont.)
storage-related loss of, 153, 154
browning in, 162, 204
canned, 24
clarification of, 35–37
clarified
boiling point rise in, 94, 95
comparison with cloudy apple juice, 110
concentration of, 198–200
effect of storage on, 204–207
5-hydroxymethylfurfural in, 175, 176
thermophysical properties of, 93–95
cloudy, 109
browning inhibition in, 186, 194
comparison with clarified apple juice, 110
pH of, 194
starch granules in, 40, 41
viscosity of, 116–119
color difference development in, 186
concentration of, 36
density of, 93, 94
enzymatic browning in, 193
enzymatic browning inhibition in, 188–192
enzymatic processing of, 36–38
frozen, 24
nonenzymatic browning in
5-hydroxymethylfurfural in, 173, 175–178
kinetics of, 169–171
Maillard-type reactions in, 167
storage-related, 204–207
nonenzymatic browning inhibition in, 196–198
with activated carbon, 203–205
with ion-exchange resins, 201, 202, 205–207
organic acid content of, 157
pasteurization of, 56
pH of, 35, 40
effect of ion exchange on, 202
reducing sugars in, 156
specific heat of, 81
starch content of, 40, 41
sucrose hydrolysis in, 156
sugar content of, as natural preservative, 36
viscosity of, 91, 92
Apple juice concentrate, nonenzymatic browning in, 171,
172
Apple pulp
enzymatic browning inhibition in, 192
217
218
Apple pulp (cont.)
enzymatic processing of, 36, 37
light-colored, 165
Apples
aroma/aroma components of, 119, 146, 147
browning in, 99
bulk density of, 78
canned, 22
chemical composition of, 134, 135
cooling methods for, 12
dried, 22, 23
enzymatic browning in, 162
inhibition of, 192, 195, 196
luminosity in, 165
measurement of, 164, 165
in unripe fruit, 164, 165
frozen, 22
harvest time for, 157
lightly processed, 193
major commercial applications of, 4
milling processing of, 30, 31
pectin content of, 140
pH of, 138
pigment content of, 134
polyphenol oxidase content of
heat-inactivation kinetics of, 183
substrates for, 163
protein content of, 136
red skin color of, 157
scientific name of, 4
specific heat of, 81
starch content of, 35, 139
starch granules, 139, 140
storage life of, 12
storage temperature for, 9, 157
unripe, starch content of, 35
washing of, 27
water content of, 134
world production of, 2, 4
Apple sauce, 22
pH of, 138
specific heat of, 81
viscosity of, 93
Apricot products, enzymatic browning inhibition in, 186
Apricots
dried, 22
frozen, 22
major commercial applications of, 4
pectin content of, 140
pH of, 138
polyphenol oxidase phenolic substrates in, 163
protein content of, 136
scientific name of, 4
world production of, 4
Arabanase, 37, 38
Arabans, 40
Arabinose, 139
Index
Arginine, 135, 136, 170
L-Arginine, 153
Aroma
formation during ripening, 21
properties of, 123
Aroma compounds, 144, 146, 147
activity coefficients of, 118–125
experimental values, 124, 125
NRTL model, 121, 122
UNIFAC model, 121, 123
UNIQUAC model, 121–123
Wilson equation, 121, 122
effect of storage on, 152, 153
volatile, 119–120
infinite dilution coefficients of, 120, 121
relative volativity of, 119
vapor pressure of, 121
Aroma stripping and recovery, 26, 35, 119–125, 146
by flash condensation, 124–126
Arrhenius relationship, 183, 184
Ascorbic acid
in browning, 162, 206, 207
Maillard reactions in, 171
as browning inhibitor, 187–192
fruit content of, 144, 145
processing and storage-related destruction of, 150–152
vegetable content of, 144
Ascorbic acid derivatives, as browning inhibitors, 192
Ascorbic acid-6-fatty acid esters, as browning inhibitors,
188, 189
Ascorbic acid-2-phosphate esters, as browning
inhibitors, 188, 189
Ascorbic acid-2-triphosphate esters, as browning
inhibitors, 188, 189
Ascorbyl fatty acid esters, as browning inhibitors,
188–189
Ascorbyl-6-fatty acid esters, as browning inhibitors, 192
Ascorbyl phosphate esters, as browning inhibitors, 188
Asparagine, 153–155, 170
Aspartic acid, 153, 154
Aspergillus niger, 38
Atomization, in spray drying, 65
Avocados
enzymatic browning inhibition in, 192, 195, 196
fat content of, 140
major commercial applications of, 4
polyphenol oxidase phenolic substrates in, 163
protein content of, 136
scientific name of, 4
storage temperature for, 186–176
world production of, 4
B
Bag filters, for powder recovery, 65, 66
Bananas
browning in, 162
Index
Bananas (cont.)
chemical composition of, 135
enzymatic browning inhibition in, 192, 195
esters content of, 144, 146
major commercial applications of, 4
polyphenol oxidase content of
heat-inactivation kinetics of, 183
phenolic substrates of, 163
protein content of, 136
scientific name of, 4
specific heat of, 81
starch content of, 139
storage temperature for, 186–176
world production of, 2, 4
Basic Four food guide, 6
Beans, protein content of, 136
Beer-Lambert law, 101
Beer’s law, 100–102
Belt conveyors, 29
Bentonite
as browning inhibitor, 207
as clarification agent, 35, 36
Benzaldehyde, flash condensation recovery of, 126
Benzoic acid, as browning inhibitor, 164
Berries
frozen, 22
refrigeration of, 186
Beta-carotene, 135
Binder media, in agglomeration, 68, 70
Bin food driers, 63
Bins, use in food processing, 27
Birdseye, Charles, 6
Blackberries, 4
Blanching, as enzymatic browning control
method, 182, 183
Blueberries
cooling methods for, 12
pectin content of, 140
storage life of, 12
Boiling point rise, 94, 95
Boysenberries, 4
Bread, protein content of, 136
Breadfruit, 4
Brix-to-acid ratio, 7
Bromelain, as browning inhibitor, 187, 193
Brown extractors, 30
Browning
as deterioration cause, 161–179
enzymatic. See Enzymatic browning
inhibition and control of, 181–212
nonenzymatic. See Nonenzymatic browning
reducing sugars in, 156
Bruising, in harvested fruits, 7
Butanol, as aroma component, 125
flash condensation recovery of, 126
Butyl acetate, as aroma component, 124
flash condensation recovery of, 126
219
C
Caffeic acid, 148
Cage presses, 32, 33
Calcium
as apple preservative, 193
fruit content of, 140, 141
Calcium chloride treatment, of harvested
fruits, 7, 8
Calorimetry, differential scanning, 80
Candy, 3
Canned fruits, 21, 22
Carambola juice, sugar content of, 139
Carambolas
enzymatic browning inhibition in, 192
oxalic acid content of, 137
protein content of, 136
starch content of, 139
Caramel, 150
Caramelization, in nonenzymatic browning,
165–167
Carbohydrates. See also Sugars
fruit content of, 134, 135, 137–140
production and function of, 137, 138
Carbon dioxide
diffusion through packaging film, 14–17
effect on plant respiration, 13
in fruit storage facilities, 14
in modified atmosphere packaging, 15
supercritical, 194
Carbon dioxide processing, 56
b-Carotene, 148
Carotenoids, 135, 147–148
degradation of, 157
Catechins, 148–150, 163, 164
Catecholase. See Polyphenol oxidase
Cellars/caves, as fruit storage areas, 11
Cellulases, 37
Cellulose, 21, 78, 135, 137, 138
Centrifugation
as enzymatic browning control
method, 185
as extraction process, 34
Cereals, protein content of, 136
Champagne, 3
Chelating agents, as browning inhibitors, 187
Chemical composition, of
fruits and fruit products, 133–160
amino acids, 136
carbohydrates, 137–140
effects of processing or storage on, 150–158
organic acids, 136, 137
pectin, 140
proteins, 136
proximate, 133–150
starch, 138–140
Chemical peeling, 29
220
Chemical preservatives, See also names of specific
preservatives 7,8
definition of, 7, 8
for semiprocessed fruit products, 17, 18
Chemical treatment, of harvested fruits, 7, 8
Cherries
frozen, 22
major commercial applications of, 4
mineral content of, 141
pectin content of, 140
pH of, 138
protein content of, 136
scientific name of, 4
world production of, 4
Chill injury, 186, 187
Chitosan, as browning inhibitor, 187, 192
Chlorophyll, 147
breakdown during ripening, 21
CIELAB method, for surface color quantification,
106–108
Cinnamate, as browning inhibitor, 189, 192
Cinnamic acid, as browning
inhibitor, 164, 192
Cinnamic acid esters, as polyphenol oxidase
substrates, 163, 164
Citric acid, 21, 136, 157
as browning inhibitor, 192
in peaches, 171
as preservative, 8
Citronellon, 153
Citrus fruit. See also Lemons; Limes; Oranges
aroma components of, 118, 119
Citrus juice. See also Lemon juice; Orange juice
nonenzymatic browning in, 167
Citrus juice extractors, 30
Clarification processes, 26, 35, 36
centrifugation, 35
concentration, 26, 35
of partial concentrates, 35, 36
temperature for, 40
Clausius-Clapeytron equation, 95
Clostridium botulinum, 13
Coagulation, 109
Coconuts, 4
Cold wall forced-air cooling, 10
Colloidal particles
as fruit juice viscosity cause, 115–118
size, shape, and distribution measurement of
with electron microscopy, 112, 115
with photon correlation, 115
with sedimentation methods, 113, 114
Colloidal stability, 109, 111
DVLO theory of, 111
Colloidal systems, types of, 110
Colloids, 109
Color, of fruits and fruit products, 161. See also
Browning; Pigments
Index
Color, of fruits and fruit products. See also Browning;
Pigments (cont.)
attributes of, 106
effect of processing and storage on, 147
measurement of, 7, 99–108
absorbance spectrophotometry-based, 99–104
tristimulus colorimetry-based, 99–104
visual systems-based, 100
relationship to pigment concentration, 108
Color compounds. See Pigments
Colorimeters/colorimetry
applications of, 108
definition of, 102
tristimulus, 99, 104–108
CIELAB method, 106–108
Commission Internationale de L’Eclerage (CIE)
system, 106–108
Compaction, as size enlargement process, 67
Concentration, of fruit juices, 26, 35–36
through evaporation, 175, 198
effect of boiling point rise on, 94, 95
Concentrators, triple-effect, 198–201
Condensation, formation on stored fruit, 8
Conduction, 74, 75
Laplace equation of, 83
Continuous belt presses, 32, 33
Continuous pressure filters, 42
Convection, 74, 75
Cooling, of harvested fruits, 8–12
methods, 9–12
alternative methods, 11
forced-air cooling, 9, 10, 12
hydrocooling, 9–12
package icing, 9
room cooling, 9, 10, 12
top icing, 9, 11
vacuum cooling, 9
purpose of, 9
Copper, 141, 144, 152, 157
Coring, of harvested fruit, 29
Coumaric acid, 148
p-Coumaric acid, as browning inhibitor, 164, 192
Crab apples, 4
pectin content of, 140
Cranberries, 4
pectin content of, 140
Cranberry sauce, 22
Cream of tartar (potassium hydrogen
tartrate), 137
Cucumbers, pH of, 138
Currants, 4
pectin content of, 140
pH of, 138
Cutting, of harvested fruit, 29
b-Cyclodextrans, as browning inhibitors, 194
Cyclodextrin, as browning inhibitor, 187
Cyclone collectors, for powder recovery, 65, 66
Index
221
Cysteine, as browning inhibitor, 187, 189, 192
Cystine, 136
D
Deacidification, 199, 200
Debittering, 199, 200
Debye length, 111
Deep freezing, invention of, 6
Dehydrated products, definition of, 62
Dehydration, as fruit processing
method, 62–67
critical moisture content (Xcr) in, 63
definition of, 62
driers for, 63–67
belt, 64
bin, 63
cabinet, 64
continuous belt or conveyor, 64
drum, 64
explosion puffing, 65
fluidized bed, 63, 65
microwave, 65
osmotic bed, 65
spray, 62, 65–67
sun or solar, 64
three-stage, 67
tunnel, 64
two-stage, 67
vibrofluidizers, 63
drying curve in, 63
fruit shrinkage during, 125–128
shrinkage coefficient, 127, 128
Dehydroascorbic acid, in browning, 162
Dehydroascorbic acid (DHAA), 165
Density, 73, 74, 77–79
bulk, 77–79
definition of, 77
of fruit juice, 93, 94
measurement of, 77–79
particle, 77
substance, 77
Depectinization, 35, 36, 40
Destarching, of fruit juices, 36
Deterioration, of fruits and fruit products
browning-related, 161–179
color measurement of, 99
mechanisms of, 161, 162
postharvest, 6, 7
DHAA (dehydroascorbic acid), 165
Diffusion, as extraction process, 34
Dihydroquercetin, 163
3,4-Dihydroxy phenylalanine, as polyphenol oxidase
substrate, 163, 164
Diphenols, transformation to melanins, 164
Dispersibility, 68
Dispersions. See Food dispersions
Dodecanal, 153
Dried fruits, 21, 23
Dried products, definition of, 62
Drying, as preservation method
dehydration-related shrinkage during, 125–128
historical development of, 3
E
Eggplants, polyphenol oxidase phenolic
substrates in, 163
Egyptians, 3
Elderberries, 4
Electrodialysis, as enzymatic browning inhibition
method, 194
Electromagnetic waves, 73, 74, 100
Electron microscopy, of colloidal particles, 112, 113, 115
Electrostatic (charge) repulsion, in food dispersions, 111
Electrostatic forces, 69
Encapsulation, as size enlargement process, 67
Endocarp, 1, 2
Enthalpy, 73, 74, 80
Enzymatic browning, 161–165
effect of temperature on, 165
inhibition and control of, 163, 181–195
with chemical treatments, 183, 187–190
with color measurement, 163–165
with miscellaneous methods, 193–195
with nonconventional chemical treatments, 183,
191–193
with thermal treatments, 182–187
kinetics of, 163–165
phenolic compounds and oxidases in, 161–163
susceptibility to, 162
Enzymes. See also specific enzymes
apple content of, 134
in fruit and fruit juice processing, 36–41
inactivation of
Arrhenius relationship in, 183, 184
with chemical agents, 183, 187–190
for enzymatic browning control, 182–187
with miscellaneous methods, 193–195
with refrigeration, 186, 187
with thermal methods, 182–187
Erythorbic acid, as browning inhibitor, 188, 190–192
Essential amino acids, 136
Essential oils, in apples, 134
Esters, 144, 146
Ethanol, as aroma component, 125
flash condensation recovery of, 126
Ethyl acetate, as aroma component, 124
flash condensation recovery of, 126
Ethyl butyrate, as aroma component, 124, 153
flash condensation recovery of, 126
Ethylene, 13, 21
Ethylenediamine tetraacetic acid (EDTA), as browning
inhibitor, 187, 189
222
Index
Ethyl valerate, as aroma component, 124
flash condensation recovery of, 126
Evaporated products, definition of, 62
Evaporation, 58–62
as concentration method, 175, 198
effect of boiling point rise on, 94, 95
condensers for, 58
definition of, 58
heat exchangers for, 58–60
vacuum system for, 58
vapor separator for, 58–60
Evaporators, 58–62
batch pan (calandria), 58, 59
falling film, 60
multiple-effects, 61, 62, 198–200
with aroma recovery, 124–126
mechanical vapor recompression, 61, 62
thermocompression, 61, 62
rising film, 59
scraped-surface, 60, 61
Exocarp, 1, 2
Extraction processes, 30–34
centrifugation, 34
for citrus fruits, 30
diffusion, 34
for pome fruits, 30, 31
pressing, 32, 33
F
Fats
bulk density of, 78
fruit content of, 140
Feret diameter, of colloidal particles, 112, 113
Fermentation, 3
Ferulic acid, 148
as browning inhibitor, 164, 192
Fiber, dietary, 133
Ficin, as browning inhibitor, 187, 193
Figs, 4
dried, 22
protein content of, 136
Filter presses, 32–33
Filtration methods and filters, 42–53
driving force-type, 42
filter aid and processing, 42, 43
filtrate in, 42
membrane-type, 45–53
hollow fiber membranes, 47
microfiltration membranes (MF), 46, 47
module structures of, 47, 48
nanofiltration membranes (NF), 46
reverse osmosis (RO) membranes, 46
ultrafiltration membranes (UF), 46–53
operating cycle-type, 42
pressure-type, 42–45
candle filters, 44, 45
Filtration methods and filters (cont.)
filter press (plate and frame), 43
vacuum filters, 45
vertical and horizontal pressure leaf, 43, 44
Fining, 26, 35, 36
Fining agents, 35
Flavan, 148
Flavones, 148, 149
Flavonoids, 162
Flavonols, 148, 149
Flocculating agents, 35
Flocculation, 109
FMC citrus juice extractors, 30
Folic acid, vegetable content of, 144
Folin-Ciocalteau reagent, 157
Food dispersions, 109–118
characterization of, 111, 112
Debye length in, 111
definitions of, 109–111
electrostatic (charge) repulsion in, 111
particle size, shape, and size distribution in, 112–114
stability of, 109, 111
steric repulsion in, 111
Z-potentials in, 111
Food guides
Basic Four, 6
Food Guide Pyramid, 6, 133–135
Forced-air cooling, 9, 10, 12
Formic acid, as preservative, 17, 18
Free radicals, ionizing radiation-related
production of, 194
Freeze-drying, invention of, 6
Freeze-thawing peeling, 29
Freezing, of fruits and fruit products
of semiprocessed fruit products, 17, 18
structural damage during, 129
thermophysical properties during, 75
Freundlich equilibrium curve, 203, 204
Frozen fruits, 21–23
Fructose, 138
D-Fructose, 155
Fructose/glucose ratio, 156
Fruit products. See also specific fruit products
pH of, 138
Fruits. See also specific fruits
biology of, 1, 2
classification of, 1, 3
definition of, 1
recommended daily servings of, 133
world production of, 1
Fruit salads, 22
Fruor, 1
G
Galacturonic acid
in browning, 171, 172
Index
223
Galacturonic acid (cont.)
methoxylation of, 140
Gelatin
as browning inhibitor, 207
as clarification agent, 36
as fining agent, 35
Genetics, development of, 6
Geometric method, of bulk density measurement, 77, 78
Glass transition, effect on nonenzymatic
browning, 177, 178
Glucose, 138
bulk density of, 78
D-Glucose, 21, 155
Glucose oxidase, 38
Glutamic acid, 153, 154
Glutamine, 155
Glutathione, as browning inhibitor, 187, 192
Glyceric acid, 137
Glycine, 136
Gooseberries, 4
pectin content of, 140
Granulation, as size enlargement process, 67
Grapefruit
frozen, 22
major commercial applications of, 4
pH of, 138
protein content of, 136
world production of, 4
Grape juice
canned, 24
density of, 93, 94
frozen, 24
5-hydroxymethylfurfural content of, 173
tartartrates content of, 137
viscosity of, 91
Grapes
enzymatic browning in, 162
inhibition of, 195, 196
major commercial applications of, 4
pectin content of, 140
polyphenol oxidase content of
heat-inactivation kinetics of, 183
phenolic substrates for, 163
protein content of, 136
world production of, 2, 4
Grater mills, 31
Greeks, ancient, 3
Grinding mills, 31
Guavas, 4
protein content of, 136
starch content of, 139
H
Hammer mills, 31
Harvesting, of fruits, 6–8
Harvest time, 11
Heat capacity, prediction of, 81, 82
Heating, as juice haziness cause, 36
Heat transport, 73, 74
Heat transport properties
calculation of, 75
definition of, 73
thermal conductivity, 73
definition of, 82
measurement and prediction of, 82–86
thermal diffusivity, 73–75
definition of, 83
measurement and prediction of, 85, 88
Heavy metals, 152
Heptulose, 139
Hexanal, as aroma component, 124
flash condensation recovery of, 126
Hexanol, as aroma component, 125
flash condensation recovery of, 126
Hexylresorcinol, as browning inhibitor, 187, 192
High-intensity pulsed light, 56, 57
High-pressure methods, for browning inhibition, 195–197
High-pressure sterilization, 56–58
Histidine, 136
Honey
as browning inhibitor, 187, 192
as preservative, 3
Horizontal pack presses, 32
Hot gas peeling, 29
Hue, 106, 107
Hue angle, 184, 185
Humidity
in fruit storage facilities, 8, 9
during postharvest cooling, 9, 10
Hunter color values, 107, 108
Hydraulic presses, 33, 34
Hydrocooling, 9–12
Hydrometric method, of bulk density
measurement, 77, 78
Hydroxybenzoic acid, 148
Hydroxycinnamic acid, 148
5-Hydroxymethylfurfural, 199–201
formation of, 172–178
during nonenzymatic browning, 168, 172–178
during processing, 175–178
during storage, 172–176
I
Ion exchange resins, as browning inhibitors, 199–202,
205–207
Iron
effect on carotenoid degradation, 157
fruit content of, 141, 144
Irradiation
as browning inhibition method, 193–195
as food preservation method, 6, 56
Irrigation, invention of, 3
224
Index
Isinglass, as clarification agent, 36
Isobutyrate, flash condensation recovery of, 126
Isocitric acid, 137
J
Jackfruits
protein content of, 136
starch content of, 139
Juice, 24. See also Apple juice; Grape juice; Lemon juice;
Orange juice; Peach juice; Pineapple juice; Plum
juice; Prune juice; Tangerine juice
amino acid content of, effect of storage on, 153–155
appearance of, 99
aroma of, effect of storage on, 152, 153
aroma recovery in, 119
canned, 24
categorization of, 21
clarification temperature for, 40
cloudy, 109
processing of, 25, 26
viscosity of, 115–119
enzymatic browning in, 162
effect on luminosity, 184, 185
inhibition of, 184–186
enzymatic hydrolysis of starches in, 40, 41
frozen concentrated, 24
‘‘natural,’’ 157
nonenzymatic browning inhibition in, 196–208
pasteurization of, 55
powdered, instantizing of, 67, 68
pressurized, 56
processing of, 21–54
centrifugation method, 34
diffusion method, 34
extraction processes, 30–34
final grading, inspection, and sorting, 28, 29
front-end operations, 27–29
nonthermal, 56
pressing method, 32, 33
reception procedures, 27, 28
stages of, 25, 26
semiprocessed, 17
storage-related vitamin loss in, 150
world trade of, 21
K
Kayleigh scattering, 103
Kieselsol, 36
Kiwifruits, 5
protein content of, 136
Kumquats, 5
L
Laccase, 207
Lactic acid, 137
Lactose, hydrolysis of, 199
Lambert’s law, 100, 101
Laplace equation, of heat conduction, 83
Lead, 152
Lemonade, 3
Lemon juice, canned, 24
Lemons
frozen, 22
major commercial applications of, 5
pH of, 138
protein content of, 136
world production of, 5
Leucoanthocyanins, 148–150
Lightness. See Luminosity
Limes, 5
protein content of, 136
Limonene, 118, 119
Linalool, 153
Liquefaction, enzymatic, 37
Longans, 5
Loquats, 5
Lovibond Tintometer, 108
Luminosity, 106
effect of enzymatic browning on, 165, 184, 185
Lychees
fat content of, 140
major commercial applications of, 5
protein content of, 136
world production of, 5
Lycopene, 147, 148
M
Maceration, enzymatic, 37
Magnesium, fruit content of, 141, 143
Magnus-Taylor pressure tester, 27
Maillard reactions, 162
cysteine-related inhibition of, 189
in nonenzymatic browning, 165–172
basic reactions, 167, 168
effect of amino acids on, 168, 170, 172
effect of fructose-to-glucose ratio on, 169, 170
effect of organic acids on, 171
effect of reducing sugars on, 168, 169, 172
effect of soluble solids on, 168, 169
effect of temperature on, 171, 172
kinetics of, 168
Maleic acid, 137
Malic acid, 21, 136, 137, 157
in nonenzymatic browning, 171
in peaches, 171
Maltodextran, 177, 178
Maltose, 138, 139
Manganese, effect on carotenoid degradation, 157
Mangoes
major commercial applications of, 5
polyphenol oxidase phenolic substrates in, 163
protein content of, 136
Index
225
Mangoes (cont.)
starch content of, 139
storage temperature for, 186–176
world production of, 5
Mango pureé, sugar content, 139
Marmalade, 24
Martin diameter, of colloidal particles, 112, 113
Mature fruit, definition of, 21
Meat Inspection Act, 6
Melanins, 162, 181
Melanoidins, 166, 168, 204
Melons
frozen, 23
honeydew, protein content of, 136
major commercial applications of, 5
world production of, 5
Mendel, Gregor, 6
Mesocarp, 1, 2
Metabisulfite, as browning inhibitor, 192
Metals, fruit content of, 141, 144
2-Methyl-butanol, flash condensation
recovery of, 126
Methyl paraben, as preservative, 8
Microwave food driers, 65
Microwave technology, 6, 56
Milling and millers, 30, 31
Molds (fungi). See also Yeast
in fruit storage rooms, 8
Monochromators, 102
Mulberries, protein content of, 136
Mushrooms, polyphenol oxidase in, 195
N
Nanofiltration, 46
Nano-thermosonication, 194
Nectarines
protein content of, 136
world production of, 5
Nephelometry, 103
Nitrogen
apple content of, 134
as browning inhibitor, 189, 190
in modified atmosphere packaging, 15
Nitrogen compounds, browning reactions of, 161
Nitrogen-containing substances, in fruits, 136
Nonenzymatic browning, 161, 162, 165–178, 194
absorbance in, 168
caramelization in, 165–167
effect of glass transition on, 177, 178
effect of pH on, 171
5-hydroxymethylfurfural in, 168, 172–178
inhibition and control of, 195–208
factors affecting, 195
with ion exchange, 199–202
miscellaneous methods, 207, 208
preventive methods, 196–202
with process optimization, 197–199
Nonenzymatic browning (cont.)
restorative methods, 196, 197, 203–207
with temperature control, 196–198
Maillard reactions in, 165–172
basic reactions, 167, 168
effect of amino acids on, 168, 170, 172
effect of fructose-to-glucose ratio
on, 169, 170
effect of organic acids on, 171
effect of reducing sugars on, 168, 169, 172
effect of soluble solids on, 168, 169
effect of temperature on, 171, 172
kinetics of, 168
phases in, 166
pyrolysis in, 165, 166
tristimulus parameters for, 168
Nuts
fat content of, 140
protein content of, 136
O
Octanal, 153
Oils, fruit content of, 140
Olives
fat content of, 140
major commercial applications of, 5
pH of, 138
protein content of, 136
world production of, 5
Opacity, 100, 103
Orange juice
aroma of, effect of storage on, 152, 153
dehydrated, amino acid content of, 170
density of, 93, 94
5-hydroxymethylfurfural in, 176
nonenzymatic browning in, 207, 208
pH of, 138
specific heat of, 81
viscosity of, 91
Orange marmalade, 24
Oranges
chemical composition of, 135
ester content of, 144, 146
major commercial applications of, 5
mandarin, protein content of, 136
navel, 6
pH of, 138
storage-related vitamin loss in, 150, 151
world production of, 2, 5
Organic acids. See also Citric acid; Malic acid; Quinic
acid
in browning, 157
as browning inhibitors, 187
fruit content of, 133, 135–137
processing and storage-related
changes in, 157
Orthodiphenol oxidase. See Polyphenol oxidase
226
Index
Oxygen
diffusion through packaging film, 14–17
effect on plant respiration,
in fruit storage facilities, 14
in modified atmosphere packaging, 15
P
Package icing, 9
Packaging, modified atmosphere (MAP), 13–17
advantages of, 14
browning inhibition inside, 193
disadvantages of, 15
effect on respiration in fruits, 13–16
Packaging materials, for high-pressure
food processing, 58
Palletizing (tabletting), as size enlargement
process, 67
Panthothenic acid, 151
Papain, as browning inhibitor, 187, 193
Papayas, 5
irradiation of, 6
Particles. See Colloidal particles
Passion fruits, 5
protein content of, 136
Pasteur, Louis, 6, 55
Pasteurization, 6, 55–57
batch, 55
high-temperature, short-time (HTST), 55
invention of, 6
nonthermal, 56, 57
of semiprocessed fruit products, 17, 18
UHT (ultra-high temperature), 55–56
Pasteurized products, 55
Peaches
browning in, 162
cooling methods for, 12
dried, 23
frozen, 23
major commercial applications of, 5
mineral content of, 141
organic acid content of, 171
pectin content of, 140
pH of, 138
polyphenol oxidase phenolic substrates in, 163
protein content of, 136
storage life of, 12
world production of, 5
Peach juice
acidity of, 171
amino acid content of, 155, 157
nonenzymatic browning in, 170, 171
Pear juice
enzymatic browning inhibition in, 189, 192
nonenzymatic browning in, 167
reducing sugars in, 156
viscosity of, 91, 93
Pears
bulk density of, 78
dried, 23
enzymatic browning in, 162
inhibition in, 195, 196
major commercial applications of, 5
milling processing of, 30, 31
polyphenol oxidase content of
heat-inactivation kinetics of, 183
phenolic substrates for, 163
protein content of, 136
world production of, 5
Peas
pH of, 138
protein content of, 136
Pectic enzymes. See Pectinases
Pectic substances, ripening-related hydrolysis of, 21
Pectin
chemical structure of, 140
fruit content of, 140
viscosity of, 93
Pectinases, 36, 37
activity determination of, 38, 39
as browning inhibitors, 189, 190
as clarification agents, 36, 171
Pectinesterase, 36, 38–40
Pectin esterase, 56
radiation-related inactivation of, 195
Pectinlyase, 36, 38, 39
Peeling methods, 29
Peel oil, 118, 119
Penicillum italicum, 39
Penicillum spp., 39
Pentyl acetate, as aroma component, 124
flash condensation recovery of, 126
Pericarp, 1, 2
Permeate flux, in ultrafiltration membrane
filtration, 48–53
effect of volume concentration ratio (VCR)
on, 50–53
as function of time, 50, 51
stationary, 49, 50
Peroxidase, radiation-related inactivation of, 195
Persimmons, 5
fat content of, 140
pH
effect on pectic enzyme activity, 39, 40
for enzymatic browning inhibition, 194
of fruit products, 138
Phenolase, 150
Phenolic acids
in enzymatic browning, 162, 163
subgroups of, 148
Phenolic compounds
in browning, 157, 164
as browning inhibitors, 164
as polyphenol oxidase inhibitors, 164
Index
Phenolic compounds (cont.)
as polyphenol oxidase substrates, 163, 164
processing and storage-related changes in, 157, 158
Phosphate-based agents, as browning inhibitors, 189
Photometers, 102
Photon correlation technique, 115
Phytofluene, 148
Pigments, 147–150
apple content of, 134
concentration of, relationship to color, 108
effect of processing and storage on, 157
natural, 147–150
Pineapple juice
as browning inhibitor, 193
pH of, 138
Pineapples
ester content of, 144, 146
frozen, 23
major commercial applications of, 5
pectin content of, 140
protein content of, 136
storage-related vitamin loss in, 150, 151
world production of, 5
a-Pinene, 153
Pitting, of harvested fruit, 29
Plastic, as high-pressure processing packaging
material, 58
Pliny the Elder, 3
Plum juice, enzymatic browning in, 193
inhibition of, 186, 192
Plums
enzymatic browning inhibition in, 195
frozen, 23
major commercial applications of, 5
pectin content of, 140
pH of, 138
polyphenol oxidase phenolic substrates in, 163
protein content of, 136
world production of, 5
Polyethersulfone membranes, 207
Polygalacturonase, 36, 38–40
Polymer films, permeability coefficients of, 16, 17
Polyphenol oxidase (PPO)
as browning catalyst, 161–163
inhibition of, 181
with chemical antibrowning agents, 187–193
with cold temperatures, 186, 187
with miscellaneous methods, 193–195
with thermal inactivation, 183
phenolic compound-oxidizing activity of, 162
phenolic substrates for, 163, 164
Polyphenols, 37
in juice, ion exchange resin control of, 199, 200
Polyvinyl polypyrrolidone (PVPP), 177, 178, 207
Polyvinyl pyrrolidone membranes, 207
Pomegranates, 5
protein content of, 136
227
Porosity, 77, 78
Postharvest handling, of fruits, 8–12
Potassium, fruit content of, 141, 142
Potassium hydrogen tartrate (cream of tartar), 137
Potassium sorbate, 8
Potassium tartrate (Rochelle salt), 137
PPO. See Polyphenol oxidase
Preservation, of fruits and fruit products. See also
Dehydration; Freezing; Storage
with chemical preservatives, 7, 8
of semiprocessed fruit products, 17, 18
Pressing, as extraction method, 32, 33, 37
Pressure, effect on viscosity, 92, 93
Pressure infiltrates, of antibrowning agents, 191, 192
Pressure sterilization, 56–58
Proanthocyanidins, 149, 150
Processing, of fruits and fruit juices, 21–54
clarification and fining processes, 26, 35, 36
centrifugation, 35
concentration, 26, 35
filtration, 42–53
of partial concentrates, 35, 36
enzymatic, 36–41
extraction processes, 30–34
centrifugation, 34
for citrus fruits, 30
diffusion, 34
for pome fruits, 30, 31
pressing, 32, 33
front-end operations, 27–29
final grading, inspection, and sorting, 28, 29
reception procedures, 27, 28
history of, 2–4
overview of, 1–19
Processing facilities, 24
Process optimization, 197–199
Proline, 153, 170
Propranol, as aroma component, 125
flash condensation recovery of, 126
Proteases, as browning inhibitors, 193
Proteins
bulk density of, 78
fruit content of, 135, 136
Proteolytic enzymes, as browning inhibitors, 187
Provitamin A, 148
Prune juice, pH of, 138
Prunes, 5, 22
color of, 162
Pulps, 17
apple
enzymatic browning inhibition in, 192
enzymatic processing of, 36, 37
light-colored, 165
raspberry
color stability of, 158
effect of processing and storage on, 158
storage-related vitamin C loss in, 151, 152
228
Index
Pulsed electric fields (PEF), 56
Pulsed technologies, 56, 57
Pumpkins
pH of, 138
protein content of, 136
starch content of, 139
Puree
processing of, 24–26
Pureé
mango, sugar content of, 139
Purées-marks, 17
Pure Foods Act, 6
Pycnometry, 78
Pyrolysis, in nonenzymatic browning, 165, 166
Pyruvic acid, 137
Q
Q10 (temperature coefficient), 184, 186
Quercetin, 163
Quinces, 5
pectin content of, 140
protein content of, 136
Quinic acid, 157, 171
o-Quinone, 163, 187
Quinones, 158, 162
R
Rack and cloth presses, 32
Radiation, 73, 74
Radiofrequency (RF) energy, 57
Raisins, 23
color of, 162
pH of, 138
Raman scattering, 103
Raspberries
economic importance of, 157
mineral content of, 141
Raspberry pulp
color stability of, 158
effect of processing and storage on, 158
storage-related vitamin C loss in, 151, 152
Rasp mills, 31
Reel washers, 28
Refrigeration, 8-12. See also Freezing
as enzymatic browning control method, 186, 187
Refrigerators, invention of, 6
Rehydration, 67
Relative volatility, 119
Resorcinols, as browning inhibitors, 189
Respiration, in harvested fruits, 7, 8
in controlled atmosphere storage (CAS), 12, 13
in modified atmosphere packaging (MAP), 13–16
Retrograding, of starches, 40, 41
Reynolds’ number, 114
Rhubarb, pectin content of, 140
Ripe fruit, pectin content of, 140
Ripening
in harvested fruits, 7
process of, 21
Rochelle salt (potassium tartrate), 137
Romans, 3
Room cooling, 9, 10, 12
Rotary drum pressure filters, 42
Rotary presses, 30
S
Saccharomyces cerevisae, 57
Sapotes, 5
Saturation, 106
Saturation index, 184, 185
Scalding, 198
Screw presses, 32, 33
Sedimentation methods, for particle size
determination, 113, 114
Seeds, of fruits, 1
Semiprocessed fruit products
categories of, 17
preservation of, 17, 18
Senescence, in fruits, 21
Serpentine forced-air cooling, 10
Shade, 11
Shikimic acid, 137
Shimadzu Centrifugal Particle Size Analyzer, 114
Shrinkage, in dehydrated fruits, 125–128
shrinkage coefficient, 127, 128
Sieve diameter, 112
Silica gel, as browning inhibitor, 207
Silo systems, 27
Sinapic acid, as browning inhibitor, 192
Sintering, as size enlargement process, 67
Size enlargement processes, 67–72
agglomeration, 67–71
selective (spherical), 71
compaction, 67
encapsulation, 67
granulation, 67
instantizing, 67, 68
sintering, 67
tabletting (palletizing), 67
Sodium, fruit content of, 141, 143
Sodium benzoate, as preservative, 8, 17, 18
Sodium bisulfite, as browning inhibitor, 190
Sodium diacetate, as preservative, 8
Sodium nitrate, as preservative, 8
Sodium propionate, as preservative, 8
Soft drinks
invention of, 3
in plastic bottles, 6
Sorbates, as preservatives, 17, 18
Sorbic acid, as preservative, 17, 18
Sorting, of harvested fruit, 28, 29
Sparkolloid, 36
Specific heat, 73, 74, 80, 81
Index
Specific heat (cont.)
measurement of, 80–82
Specific volume, 73
relationship to density, 79
Spectrophotometers, 101–104
components of, 101–103
ultraviolet (UV), 102
visible light, 101, 102
Spectrophotometry, absorbance, 99–104
Spray drying, 65, 66
powder recovery process in, 65–67
Starches
fruit content of, 138–140
fruit juice content of
enzymatic hydrolysis of, 40, 41
as haziness cause, 35, 36
retrograding of, 40, 41
ripening-related decrease in, 21
Steam, use in fruit processing, 56
Steam blanching, as enzymatic browning control
method, 182, 183
Steam peeling, 29
Stefan-Boltzmann law, 74
Steric repulsion, 111
Sterile products, 55
Sterilization techniques, 56, 57
Stokes’ law, 112–114
Storage, 7
controlled atmosphere (CAS), 12, 13
refrigerated, 8–12, 186, 187
relative humidity during, 8, 9
temperature during, 9
effect on ascorbic acid (vitamin C) content,
150–152
Storage life, postharvest, 6, 7
Strawberries
canned, pigment instability in, 158
chemical composition of, 135
mineral content, 141
pectin content, 140
protein content, 136
cooling methods for, 12
major commercial applications of, 5
pH of, 138
storage life of, 12
world production of, 5
Stress crack formation, 126
Succinic acid, 137
Sucrose, 138
hydrolysis of, 155, 156, 169
Sugar/acid ratio, 6, 7, 21, 136
Sugars. See also Fructose; Glucose
effect of storage on, 155, 156
as energy source, 138
fruit content of, 133, 135
hydrolysis products of, 155
ripening-related increase in, 21
specific heat of, 81
229
as taste components, 136
Sulfiting agents, as browning inhibitors, 187
Sulfur dioxide
as browning inhibitor, 187
as preservative, 8, 17
Sulfydryl compounds, as browning inhibitors, 188, 193
Sumerians, 3
Surfactants, 7, 8
Sweetness, of fruit, 21
Syrups, ion exchange treatment of, 199
T
Tabletting (palletizing), as size enlargement process, 67
Tangerine juice, 24
Tangerines, world production of, 5
Tannins, 35, 134, 158
Tartaric acid, 21, 136, 137
Tartness, 21, 136
Taste
acidity-based, 136
sugar-based, 136
Temperature
for agglomeration, 70
in dehydration processes, 64
effect on enzymatic browning, 165
effect on 5-hydroxymethylfurfural formation, 175, 176
effect on modified atmosphere packaged fruit, 16
effect on viscosity, 92, 93
for fruit juice clarification, 40
in fruit storage facilities, 9
in pasteurization, 55, 56
Temperature coefficient (Q10), 184, 186
Terpinene-4-ol, 153
Terpinolene, 153
Texture, measurement of, 7
Thermal conductivity, 73
definition of, 82
measurement and prediction of, 82–86
Thermal diffusivity, 73–75
definition of, 83
measurement and prediction of, 85, 88
Thermal methods
for browning control
D value in, 183
elevated temperatures, 182–186
in enzymatic browning, 182–187
Q10 value in, 184
refrigeration-related methods, 186–187
as scalding cause, 198
Thermal radiation, 73, 74
Thermodynamical properties
definition of, 73
enthalpy, 73, 80
specific heat, 73, 74, 80, 81
measurement of, 80–82
specific volume, 73
relationship to density, 79
230
Index
Thermophysical properties, 73–98
density, 73, 74, 77–79
enthalpy, 73, 74, 80
experimental data and prediction models for, 76–95
during freezing, 75
identification of, 73–75
specific volume, 73, 74
viscosity, 73, 74
of cloudy fruit juices, 115–118
definition of, 85, 86
effect of temperature and pressure on, 92, 93
measurement of, 87–94
Newtonian, 85–87, 89–91
Tin, 152
Tomato concentrate, viscosity of, 93
Tomatoes, protein content of, 136
Tomato juice, pH of, 138
Tomato paste, viscosity of, 93
Top icing, 9, 11
Total anthocyanin (TA), 99
Trans-2-hexenal, as aroma component, 124
flash condensation recovery of, 126
Translucency, 103
Transparency, 103
Trimming, of harvested fruit, 29
Tristimulus colorimeters/colorimetry, 99, 104–108
CIELAB method, 106–108
Trucks, refrigerated, 12
Tryptophan, 136
Turbidimetry, 103
Turbidity, 103
Tyrosine, as polyphenol oxidase substrate, 163, 164
U
Ultrafiltration
as enzymatic browning inhibition method, 193–195
polyvinyl polypyrrolidine-based stabilization of, 207
Ultrafiltration membrane filtration, 46–53
permeate flux in, 48–53
as function of time, 50, 51
influence of volume concentration ratio (VCR) on,
50–53
stationary, 49, 50
Ultrapasteurized products, 55
Ultraviolet light processing, 56
Ultraviolet pressure processing, 56
United Nations Food and Agricultural Organization
(FAO), 1, 2
Unripe fruit
enzymatic browning rate in, 164, 165
pectin content of, 140
V
Vacuum cooling, 9
Vacuum infiltrates, of antibrowning agents, 191, 192
Van der Wall’s (dispersion) forces, 68, 69, 203
Vegetables. See also names of specific vegetables
fruits classified as, 1
vitamin content of, 144
Viscosity
of cloudy fruit juices, 115–118
definition of, 85, 86
effect of temperature and pressure on, 92, 93
measurement of, 87–94
in Newtonian fruit products, 89–91
in non-Newtonian foods, 88, 89
in non-Newtonian fruit products, 91, 92
Newtonian, 85–87, 89–91
Vitamin(s), 141
fruit content of, 135, 144, 145
processing and storage-related destruction of, 150–152
vegetable content of, 144
Vitamin B1, vegetable content of, 144
Vitamin B6
processing and storage-related destruction of, 151
vegetable content of, 144
Vitamin C. See Ascorbic acid
Vitamin E, vegetable content of, 144
Vitamin K, vegetable content of, 144
Volatile compounds, of fruit aroma, 146–147
Volatility of the volatile, 119
Volume fraction of particles (Ø), 116
W
Washing, of harvested fruit, 27, 28
Water
bulk density of, 78
fruit content of, 21, 133–135, 145
Water content, effect on thermophysical properties, 75
Watermelons
cooling methods for, 12
storage life of, 12
Web scrubbers, for powder recovery, 65, 66
Well water, 11
Whiskey distilleries, 3
X
Xanthophyll, 147–148
Xylose, 139
Y
Yeast, removal from fruit juice, 35
Z
Zinc
fruit content of, 144
orange juice content of, 152
Z-potentials, 111

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