Microwave Pasteurization of Shell Eggs - A Prelude
Satyanarayandev Rajalakshmi Sivaramakrishnan
Department of Bioresource Engineering
Faculty of Agricultural & Environmental Sciences
McGill University
Ste Anne de Bellevue, Quebec, Canada
Jan 2007
A thesis submitted to the McGill University in partial fulfillment of the
requirements for the degree of Master of Science
© Satyanarayandev Rajalakshmi Sivaramakrishnan 2007
ACKNOWLEDGMENTS
It gives a lot of pleasure and pride to work under the supervision of Dr.
G.S.V. Raghavan. His intelligence, support and guidance played a vital
role in completing this study. I am grateful to him for giving me this
opportunity. My sincere thanks to Dr. C.Ruiz-Feria for his interest and
support.
I am indebted to Mr. Yvan Gariépy for all the technical assistance and
guidance in every part of this work. His sense of direction and scrupulous
planning, organization of work and excellent criticism are adorable.
My special thanks to Dr. Valérie Orsat for her friendly encouragement and
professional guidance, both in the academic and personal segments of my
period of study.
I appreciate the help of Dr. M.Ngadi in giving access to the Differential
scanning calorimeter.
I am grateful my friends Miss. Padmini Thumula, for the help rendered by
her in conducting some of the experiments and Mr. Guillaume Pilon, for
his help with the French translation of the abstract . I extend my gratitude
to all my friends who helped making my stay comfortable and gave me
good moral support.
My hearty thanks to my parents for the constant support offered by them
both morally and financially.
I acknowledge the financial support rendered by the Canadian
International Development Agency and the Natural Sciences and
Engineering Research Council.
ii
CONTRIBUTIONS OF THE AUTHORS
The authorship for the first paper in this thesis is S.R. Sivaramakrishnan,
G.S.V. Raghavan and Y. Gariépy; and for the second paper in this thesis is S.R.
Sivaramakrishnan, G.S.V. Raghavan, Y. Gariépy, V. Orsat and C. Ruiz-Feria.
This study was performed by the candidate and supervised by Dr. G. S. V.
Raghavan of the Department of Bioresource Engineering, Macdonald Campus,
McGill University, Montreal. The entire research work was done at the
Postharvest Technology laboratory, McGill University. Mr. Y. Gariépy of the
Department of Bioresource Engineering was technically involved in the
instrumentation and control of all the experiments in this study, giving expert
guidance in the usage of equipment and had a major contribution reviewing and
revising the papers. Dr. V. Orsat of the Department of Bioresource Engineering
was personally involved in giving valid suggestions for improvement in every
stage of the study and made a great contribution in reviewing and improving the
writing of the papers. Dr. C.Ruiz-Feria, professor of the Department of Animal
Science, was a member of the supervisory committee and gave a good support
and technical advice in handling eggs.
iii
TABLE OF CONTENTS
CONTRIBUTIONS OF THE AUTHORS ............................................................................. iii TABLE OF CONTENTS ......................................................................................................iv LIST OF TABLES ................................................................................................................vi LIST OF FIGURES............................................................................................................. vii Chapter 1 GENERAL INTRODUCTION ............................................................................. 1 1.1 Facts and Figures ..................................................................................................... 2 1.2 Problem Statement ................................................................................................... 3 1.3 Proposed Solutions (Hypothesis) ............................................................................. 4 1.4 Objectives ................................................................................................................. 5 Chapter 2 GENERAL REVIEW OF LITERATURE ............................................................. 7 2.1The Incredible Egg .................................................................................................... 7 2.2 Composition of Eggs ................................................................................................ 7 2.3 Nutrient Value of Hen’s Egg ..................................................................................... 9 2.3.1 Dietary Contribution and Affordability of Eggs. ............................................... 11 2.4 Microbial Safety of Eggs ......................................................................................... 12 2.5 Pasteurization of Eggs ............................................................................................ 13 2.6 Microwaves and Their Properties ........................................................................... 14 2.7 Generation of Microwaves ...................................................................................... 15 2.8 Applications of Microwaves .................................................................................... 18 2.8.1 Thermal Application of Microwaves................................................................. 19 2.9 Microwave In-Shell Pasteurization of Eggs ............................................................ 21 2.10 Speciality Eggs ..................................................................................................... 23 Chapter 3 DIELECTRIC PROPERTIES OF EGG COMPONENTS AND MICROWAVE
HEATING FOR IN-SHELL PASTEURIZATION OF EGGS ............................ 25 3.1 Abstract................................................................................................................... 25 3.2 Introduction ............................................................................................................. 27 3.3 Materials and Methods ........................................................................................... 29 3.3.1 Measurement of Dielectric Properties of the Egg Components ...................... 30 3.3.2 Comparing Microwave Heating Rates of Egg White and Yolk In and Out of the
Shell ................................................................................................................ 33 3.4 Results and Discussion .......................................................................................... 37 3.5 Conclusions ............................................................................................................ 45 3.6 References ............................................................................................................. 46 iv
Chapter 4 PHYSICAL PROPERTIES OF EGG WHITE AFTER IN-SHELL
PASTEURIZATION BY USING MICROWAVE OR BY IMMERSION IN HOT
WATER ........................................................................................................... 50 4.1 Abstract................................................................................................................... 50 4.2 Introduction ............................................................................................................. 51 4.3 Materials and Methods ........................................................................................... 53 4.4 Results and Discussion .......................................................................................... 59 4.5 Conclusions ............................................................................................................ 66 4.6 References ............................................................................................................. 67 Chapter 5 SUMMARY AND CONCLUSION ..................................................................... 70 List of References ............................................................................................................. 72 Appendix ........................................................................................................................... 77 v
LIST OF TABLES
Table 2.1 Nutritive value of a large chicken egg (weighing 65g) ...................................... 10 vi
LIST OF FIGURES
Figure 2.1 Composition of eggs .......................................................................................... 8 Figure 2.3 Microwave propagation.................................................................................... 15 Figure 2.4 Longitudinal and cross sectional diagram of a resonant cavity magnetron ..... 16 Figure 2.5 Working of a magnetron (Modified from (Gallawa 1989)) ............................... 17 Figure 2.6 A dipolar water molecule with its polar energy field (Source: Wikipedia.org).. 20 Figure 3.1: Experimental setup for the measurement of dielectric properties .................. 31 Figure 3.2: Experimental setup for microwave heating ..................................................... 34 Figure 3.2.1 Experimental setup for Microwave pasteurization ........................................ 35 Figure 3.2.2 Shell egg with fibre optic probes in the microwave cavity ............................ 35 Figure 3.3 Change in Dielectric properties of water, egg white and egg yolk with varying
temperatures at 2450 MHz.............................................................................. 38 Figure 3.4 Dielectric properties of water, egg white and yolk with varying frequencies at
20ºC ................................................................................................................ 38 Figure 3.5. Measured and predicted ε’ values for egg white ............................................ 40 Figure 3.6. Measured and predicted ε” values for egg white ............................................ 41 Figure 3.7. Measured and predicted ε’ values for egg yolk .............................................. 41 Figure 3.8. Measured and predicted ε” values for egg yolk .............................................. 42 Figure 3.9. Heating curves of egg white and yolk in a beaker at different microwave
power levels. ................................................................................................... 43 Figure 3.10. Heating curves of egg white and yolk in in-shell eggs at different microwave
power levels. ................................................................................................. 44 Figure 4.1. Experimental setup for measurement of foam stability .................................. 57 Figure 4.2. Enthalpy of denaturation of the egg white of untreated and in-shell heated
eggs................................................................................................................. 60 Figure 4.3. Viscosity of the egg white of untreated and in-shell heated eggs. ................ 61 Figure 4.4. Foam density of the egg white of untreated and in-shell heated eggs. .......... 62 Figure 4.5. Foam stability of the egg white of untreated and in-shell heated eggs. ......... 62 Figure 4.6. % Turbidity (650nm) of untreated and in-shell heated egg white .................. 63 Figure 4.6.1 Turbidity of treated and untreated egg white ................................................ 64 Figure 4.7. Dielectric properties of the egg white of untreated and in-shell heated eggs. 65 vii
ABSTRACT
Eggs are potential hosts and carriers for pathogenic microbes like
Salmonella enteritidis, due to their rich nutritive value. Heat pasteurization is a
best solution for controlling these pathogens. Egg is used as a vital ingredient in
several foods, especially for their exceptional functional properties. These
properties mainly depend on the protein quality of the eggs and are severely
affected when heated, due to protein denaturation. Thus a heat pasteurization
technique with minimal changes to these proteins needs consideration.
In this study, microwave heating has been considered for in-shell egg
pasteurization. First of all, the effects of temperature (0-62oC) and frequency (200
MHz to 10 GHz) on the dielectric properties of egg components were
investigated. Then, individual egg components as well as intact in-shell eggs
were brought to pasteurization temperature in a laboratory scale microwave oven
working at 2450 MHz using different power densities (0.75, 1 and 2 W/g) and
heating curve was analyzed to determine the heating time required for different
power levels.
Laboratory trials on microwave heating of in-shell eggs indicated that the
heating rates of both albumen and yolk were similar. Therefore, microwave
heating appeared perfectly suited for in-shell egg pasteurization. Combination of
egg geometry, dielectric properties, and size were the main factors responsible
viii
for the enhanced interior heating. Models for calculating the ε’ and ε” at a given
frequency and temperature for shell egg components were also presented.
Heat pasteurization affects the functional quality of the eggs. In the final
part of this study, the heat induced changes with respect to physical properties
like enthalpy of denaturation, viscosity, foam density, foam stability and turbidity
and also the dielectric properties, brought about by microwave and water bath
heating for in-shell pasteurization of the egg white were compared with that of the
raw egg white. Microwave heated in-shell egg white showed limited changes in
all these properties when compared to the water bath heated ones for the same
pasteurization time and temperatures.
ix
ABRÉGÉ
Les œufs
peuvent être hôtes et porteurs de microbes pathogènes
comme Salmonella enteritidis, étant donné leur haute valeur nutritive.
La
pasteurisation par échauffement est une excellente solution pour contrôler les
éléments pathogènes.
plusieurs
préparations
L’œuf est utilisé comme un ingrédient essentiel dans
culinaires,
spécialement
pour
ses
propriétés
fonctionnelles exceptionnelles. Ces propriétés dépendent principalement sur la
qualité de la protéine présente dans l’œuf et sont sévèrement affectées lorsque
chauffé, dénaturant ainsi la protéine. Ainsi, une technique de pasteurisation par
échauffement ayant des effets minimaux sur la modification des protéines
nécessite considération.
Dans cette étude, l’échauffement par micro-onde fut considéré pour la
pasteurisation de l’œuf à l’intérieur de sa coquille. Premièrement, les effets de la
température (0-62 °C) et de la fréquence (200 MHz à 10 GHz) sur les propriétés
diélectriques des parties de l’œuf furent investiguées. Ensuite, les composantes
individuelles de l’œuf ainsi que des œufs complets avec leurs coquilles furent
amenés à des températures de pasteurisation à l’aide d’un four micro-onde de
laboratoire fonctionnant à 2450 MHz, utilisant différentes densités de puissance
(0.75, 1 et 2 W/g).
De plus, une courbe d’analyse thermique simple à
l’échauffement fut analysée pour déterminer le temps d’échauffement nécessaire
pour différents niveaux de puissance.
x
Les essais de laboratoire sur l’échauffement par micro-onde sur les œufs
avec coquille indiquèrent que les taux d’échauffement pour l’albumine et le jaune
d’œuf étaient similaires.
Donc, l’échauffement par micro-onde apparaît
parfaitement appliqué pour la pasteurisation de l’œuf en coquille.
La
combinaison de la géométrie et dimensions de l’œuf et des propriétés
diélectriques
furent
les
principaux
facteurs
responsables
favorisant
l’échauffement intérieur de l’œuf. Des modèles pour calculer les paramètres є’ et
є” à une fréquence et une température données pour les composantes de la
coquille d’œuf furent aussi présentés.
La pasteurisation par échauffement affecte la qualité fonctionnelle des
œufs. Dans la dernière section de cette recherche, l’échauffement induisit des
changements relativement au propriétés physiques telles l’enthalpie de
dénaturation, la viscosité, la compressibilité, la stabilité du blanc en neige, la
turbidité et les propriétés diélectriques.
Ces changements créés par
l’échauffement furent provoqués par micro-onde et bain-marie pour la
pasteurisation du blanc d’œuf à l’intérieur de la coquille et furent comparés avec
le blanc d’œuf cru. L’échauffement par micro-onde du blanc d’œuf en coquille
démontra des changements limités pour chaque propriétés en comparaison aux
spécimens chauffés par bain-marie pour les mêmes temps et températures de
pasteurisation.
xi
Chapter 1
GENERAL INTRODUCTION
Today, the consumer’s primary concern is the quality and safety of the food
he/she consumes. Providing a high quality and safe food at an affordable price for
everyone is an important area to be targeted for research. Improving food safety and
quality is inevitable for a healthy future generation. Health comes primarily from food and
is potentially spoiled by the same, if considerable priority is not given for the destruction
of food borne pathogens.
Eggs are one of the most commonly consumed food products. Many of the
dishes like Caesar salad, hollandaise or béarnaise sauce, mayonnaise, eggnog, ice
creams, egg fortified beverages etc. which form an important part of the everyday
Canadian meals contain raw eggs as an essential ingredient. These dishes are not
heated up to the FDA recommended temperatures of 155ºF for at least 15 seconds
(Mermelstein 2001). Although Canadian eggs are considered among the safest in the
world, the poultry industry must command higher safety to address the increasing
consumer concerns for egg salmonellosis and avian flu.
Salmonella enterica serovar Enteritidis is a leading cause of food borne
Salmonellosis. Salmonella infection and its epidemiology have been studied extensively
for several years. To reduce the risk of salmonella infection in liquid eggs, all the liquid
eggs are pasteurized before further use. Pasteurization of liquid eggs is done using
highly efficient methods of heat transfer and to ensure safety of the egg products. But
shell eggs appear to be one of most important sources of Salmonella infection.
1
1.1 Facts and Figures
On an average, Canadians eat around 15.6 dozen eggs per person per annum
(CEMA 2004). From 1993 to 1995, there were more than 20,000 laboratory-confirmed
human cases of Salmonellosis in Canada (Woodward, Khakhria, and Johnson 1997).
Food borne salmonellae are estimated to cause ≈1.3 million illnesses, 15,000
hospitalizations, and 500 deaths per year in the United States (Schroeder et al. 2005).
The economic losses due to Salmonella enteritidis (SE) food borne illness ranges from
$200 million to $1 billion annually (Morales and McDowell 1999). The probability of fresh
eggs having Salmonella varies from 0.005 % (Mermelstein 2001) to 1 %(Griffiths 2005)
depending on various factors involved in the egg production.
The average annual per capita egg consumption has come down from as high as
25 dozen in 1957 to 15 dozen in 2000. Consumer anxiety about Salmonella is
considered one of the important reasons for this (AAFC 2005). More than 85 percent of
Canadians are ready to pay a premium for a safe and high quality Canadian food
(CEMA 2002). Also it is evident that the percentage of total eggs broken has increased
from 5% in 1952 to more than 20% in 1998 (AAFC 2005). Though this leads to the
growth of processed egg industry, the poultry farmers lose a major part of their profit, as
they are paid only the minimum cost of production (COP) for breaking stock.
Pasteurized eggs exhibit a better keeping quality and hence a longer shelf life.
Also the consumer’s safety is assured. Though pasteurization will increase the COP by a
few cents per dozen of eggs, the returns that the farmers get out of this is much more.
2
Also this will help to safe guard the interest of the farmers and to provide safe eggs to
the world. This may increase the export market thereby generating millions of dollars as
revenue.
The SE incidence has increased considerably in two-thirds of the 35
countries reporting to World Health Organization in the last decade (Barbour et al. 2000).
1.2 Problem Statement
Eggs are potential hosts and carriers for pathogenic microbes like Salmonella
enteritidis, due to their rich nutritive value. Heat Pasteurization is a best solution for
controlling these pathogens. More than 90 percent of food borne Salmonellosis is
through the shell eggs (Schroeder et al. 2005; Woodward, Khakhria, and Johnson 1997).
Most of the SE outbreaks generally involved Grade A eggs that are washed and
disinfected and also met the requirements of the state for shell quality (St. Louis, Morse,
and Potter 1988). Although there are several methods of microbial destruction like rapid
chilling and ultrasonic treatments to destroy Salmonella, they are not effective on the
Salmonella present inside shell eggs (Hou et al. 1996). Also it is clear from the thermal
conductivity values of the albumen and yolk that the amount of energy and temperature
gradient required to setup convection currents inside the eggs is much higher than those
used for the pasteurization process. Therefore a major portion of heat transfer occurs
only through conduction and is very slow.
Pasteurization is considered the best solution to the SE problem in eggs.
Presently, there is no efficient and satisfactory technology available for pasteurizing shell
3
eggs. The current technology uses batch hot water immersion or moistened hot air or
both combined, which requires a very long time ranging in hours to complete and are not
really energy efficient due to the poor thermal properties of the shell and the shell
membrane though they are not really the focus of pasteurization and as a result they are
not cost effective (Mermelstein 2001). Also these treatments affect the functional
properties of the egg components.
Proteins are highly heat sensitive components of the egg. The functional
properties like whipability, foamability, foam stability etc. which make the eggs an
inevitable ingredient of various food products are severely affected by high
temperatures. Also experimentally it is found that the egg yolk needs to be heated to a
higher temperature than the albumen. This is possible by conventional heating only if the
yolk and albumen are separated as the yolk is concentric within the albumen in a shell
egg (i.e. only if the shell is broken). The existing method of pasteurizing the shell eggs
results in overheating of the albumen and partially cooked eggs along the shell
membrane (Hank et al. 2001).
1.3 Proposed Solutions (Hypothesis)
Microwaves can be used to raise the temperature of in-shell eggs to the required
pasteurization temperature in minutes. It is also a proven fact that microwave enhances
the thermal destruction of microbes. (Tajchakavit 1997). Microwaves are not ionising
radiations but the dielectric properties of the microorganism (i.e. heat generated within
the microorganism) itself enhance its destruction in a microwave environment. The
microwave power distribution inside the shell eggs also seems to be well suited for
4
uniform pasteurization (Datta et al, 2005). There is very little work done on making
microwave pasteurization viable for industrial use and there is very limited literature
available.
Microwave pasteurization of the eggs can make the process faster and
continuous and the total operation can be completed in a few minutes. The shell egg
appears ideally suited for pasteurization in a microwave environment (Fleischman 2004;
Rehkopf 2005). Though heating uniformity can be an issue in microwave heating, it can
be overcome with the proper orientation of the egg and a specially designed waveguide,
which is an engineering issue (Fleischman 2004) and also by the precise design of the
container (equipped with microwave susceptors) taking the eggs into the microwave
chamber (Yakovlev 2001).
1.4 Objectives
There is a need for a lot of work to be done to make microwave pasteurization of
shell eggs viable for industrial use and there is very limited literature available. Taking
this into consideration, this work was done to demonstrate the potential of microwave
pasteurization of shell eggs for adoption by the industry with the following objectives.
i) Measure the dielectric properties of egg white and yolk in the frequency range
of 200 MHz to 10 GHz and in the temperature range of 0°C to the
pasteurization temperatures (57.5 °C for egg white and 61.1°C for yolk).
ii) Study the heating rates and time taken by the egg white and yolk to reach the
above mentioned pasteurization temperatures from ambient temperature
5
(come-up time) for a given power level in the laboratory microwave applicator at
2450 MHz for albumen and yolk, in and out of the egg shell were determined
and compared.
iii) Compare the effect of microwave and water bath heating of egg white for inshell pasteurization with the raw egg white for the heat induced changes with
respect to physical properties like enthalpy of denaturation, viscosity, foam
density, foam stability, turbidity and dielectric properties.
6
Chapter 2
GENERAL REVIEW OF LITERATURE
2.1The Incredible Egg
The egg is one of nature's marvels, designed to have self sustainability and
excellent defence mechanisms to bring a fertilized cell into life as a chick. It is exquisitely
simple, yet enormously complex. The eggs remain a focus of research and development
of products for centuries. It has enthused several scientists and researchers in terms of
its incredible functionality and functional properties, both as an individual entity by itself
and as an ingredient in several foods.
2.2 Composition of Eggs
The composition of a typical hen’s egg is discussed in figure 2.1. The intelligent
design of nature gives the eggs the best protection against most of the biological
hazards. The egg has many natural, built-in barriers to help prevent bacteria from
entering and growing. These protect the egg on its way from the hen to chick or to our
diet (American egg board: www.aeb.org).
But, although it does help, the porous shell itself is not a foolproof bacterial
barrier. For further safety, government regulations require that eggs be carefully washed
with special detergent and sanitized. Then, the hen’s original protective shell coating is
generally replaced by a thin spray coating of a tasteless, odorless, harmless, natural
mineral oil. A shiny shell indicates oiling, rather than an unsafe or old egg.
7
Other protective barriers include the shell and yolk membranes and layers of the
white which fight bacteria in several ways. The structure of the shell membranes helps
prevent the passage of bacteria. The shell membranes also contain lysozyme, a
substance that helps prevent bacterial infection. The yolk membrane separates the
nutrient-rich yolk from the white.
Figure 2.1 Composition of eggs
(Source: American egg board: www.aeb.org)
8
In addition to containing antibacterial compounds such as lysozyme, layers of the
white discourage bacterial growth because they are alkaline, bind nutrients bacteria
need and/or don’t provide nutrients in a form that bacteria can use. The thick white
discourages the movement of bacteria. The last layer of white is composed of thick
ropey strands which have little of the water that bacteria need but a high concentration of
the white’s protective materials. This layer holds the yolk centered in the egg where it
receives the maximum protection from all the other layers.
2.3 Nutrient Value of Hen’s Egg
Eggs provide a significant amount of protein to one's diet, as well as various
nutrients. Chicken eggs are the most commonly eaten eggs, and are highly nutritious.
They supply a large amount of complete, high-quality (readily absorbed) protein (which
contains all essential amino acids for humans), and provide significant amounts of
almost all vitamins (except vitamin C) and minerals, including vitamin A, vitamin D,
vitamin E riboflavin, folic acid, vitamin B6, vitamin B12, choline, iron, calcium,
phosphorus and potassium (Li-Chan, Powrie, and Nakai 1995).
Table 2.1 gives a comprehensive overview of the nutrient content of eggs. Eggs
are also one of the least expensive single-food sources of complete protein. One large
chicken egg contains approximately 7 grams of protein. In fact, egg protein is of such
high quality that it is used as the standard by which other proteins are compared. Eggs
have a biological value (efficacy with which protein is used for growth) of 93.7%.
Comparable values are 84.5% for milk, 76% for fish, and 74.3% for beef. Eggs really are
9
Table 2.1 Nutritive value of a large chicken egg (weighing 65g)
Nutrient (unit)
Whole Egg
Egg White
Egg Yolk
Calories (kcal)
75
17
59
Protein (g)
6.25
3.52
2.78
Total lipid (g)
5.01
0
5.12
Total carbohydrate (g)
0.6
0.3
0 .3
Fatty acids (g)
4.33
0
4.33
Saturated fat (g)
1.55
0
1.55
Monounsaturated fat (g)
1.91
0
1.91
Polyunsaturated fat (g)
0.68
0
0.68
Cholesterol (mg)
213
0
213
Thiamin (mg)
0.031
0.002
0.028
Riboflavin (mg)
0.254
0.151
0.103
Niacin (mg)
0.036
0.031
0.005
Vitamin B6 (mg)
0.070
0.001
0.0069
Folate (µg)
23.5
1.0
22.5
Vitamin B12 (µg)
0.50
0.07
0.43
Vitamin A (IU)
317.5
0
317
Vitamin E (mg)
0.70
0
0.70
Vitamin D (IU)
24.5
0
24.5
Choline (mg)
215.1
0.42
214.6
Biotin (µg)
9.98
2.34
7.58
25
2
23
0.72
0.01
0.59
5
4
1
Copper, Cu (mg)
0.007
0.002
0.004
Iodine, I (mg)
0.024
0.001
0.022
Zinc, Zn (mg)
0.55
0
0.52
63
55
7
0.012
0.001
0.012
Calcium, Ca (mg)
Iron, Fe (mg)
Magnesium, Mg (mg)
Sodium, Na (mg)
Manganese, Mn (mg)
Source: (Li-Chan, Powrie, and Nakai 1995)
10
the best protein money can buy, and it has all those other valuable vitamins and
minerals too.
All of the egg's vitamin A, D and E is in the yolk. The egg is one of the few foods
which naturally contain vitamin D. A large egg yolk contains approximately 60-75
calories; the egg white contains about 15-17 calories. A large yolk contains more than
two-thirds of the recommended daily intake of 300 mg of cholesterol (although one study
shows that your body does not absorb much cholesterol from eggs).
The yolk makes up about 33% of the liquid weight of the egg. It contains all of the
fat in the egg and slightly less than half of the protein and much of the nutrients. It also
contains all of the choline, and one yolk contains approximately half of the
recommended daily intake. Choline is an important nutrient for development of the brain,
and is said to be important for pregnant and nursing women to ensure healthy fetal brain
development.
2.3.1 Dietary Contribution and Affordability of Eggs.
Eggs are an important contributor to the nutritional quality of the Canadian diet.
While eggs provide only 1.3% of the average caloric intake, they are so nutrient dense
that they contribute to a great extent for the: 6% of the RDA (recommended dietary
allowance) for riboflavin, 5% of the folate, 4% of the vitamin E and vitamin A, and almost
4% of the protein (CEMA 2004).
11
Eggs not only make a contribution to the nutrient value of the Canadian diet, they
also make a major contribution to the affordability of the diet. At $2.20CAD per dozen
large eggs, the consumer pays only $1.35CAD per pound for a nutrient rich source of
highest quality protein available (CEMA 2004).
2.4 Microbial Safety of Eggs
The risk of getting a food borne illness from eggs is very low. However, the
nutrients that make eggs a high-quality food for humans are also a good growth medium
for bacteria. In addition to food, bacteria also need moisture, a favorable temperature
and time in order to multiply and increase the risk of illness.
The bacterium Salmonella enteritidis (SE) has been found inside a small number
of eggs over the recent years. Many of these were Grade A eggs, certified good for
human consumption (St. Louis, Morse, and Potter 1988). Other types of microorganisms
could be deposited along with dirt on the outside of an egg. So, in Canada, the eggshells
are washed and sanitized to remove possible hazards and are further protected by
discarding eggs that are unclean, cracked, broken or leaking and ensuring good hygiene
practices in eggs handling.
Bacteria are most likely to be in the white and will be unable to grow, mostly due
to lack of nutrients and the antimicrobial activity of the white. As the egg ages, however,
the white thins and the yolk membrane weakens. This makes it possible for bacteria to
reach the nutrient-dense yolk where they can grow over time if the egg is kept at warm
12
temperatures (Fleischman et al. 2003). But, in a clean, uncracked, fresh shell egg,
internal contamination occurs only rarely.
If not properly handled, Salmonella bacteria can double every 20 minutes and a
single bacterium can multiply into more than a million in 6 hours. To block SE from
multiplying in the egg, eggs must be held at cool temperatures (5ºC) following packing
and throughout transportation. Important, too, are industry education programs which
encourage food preparers to use safe food-handling practices (FSIS-USDA 2006).
2.5 Pasteurization of Eggs
Pasteurization is defined as “a process of heating food for the purpose of killing
harmful organisms such as bacteria, viruses, protozoa, molds, and yeasts.” (Lewis and
Heppell 2000). The process was named after its inventor, French scientist Louis
Pasteur.
Pasteurization does not completely kill or eliminate all the microorganisms
present in the food. It is described as a mild process because the amount of chemical
damage caused is small and the changes to the sensory characteristics are minimal.It
aims to achieve a certain number "log reductions" in the number of viable organisms.
This renders the microorganisms ineffective to in causing disease.
Once pasteurized, it is also crucial to prevent the product from becoming
recontaminated. Such contamination is referred to in general terms as post-processing
contamination,
but
more
specifically
in
this
instance
as
post-pasteurization
contamination. To ensure this, care and attention should be paid to hygiene and general
13
aspects of cleanliness. After pasteurization, if the food is not refrigerated till consumed
and/or not consumed within the recommended period, then the pasteurized cannot be
considered safe for consumption any more.
Keeping quality is perhaps the most important commercial quality consideration.
Since pasteurization only inactivates vegetative spores, the keeping quality will be
influenced by a number of factors and may vary considerably. The important control
factors
are
raw
material
quality,
time/temperature
conditions,
reducing
post
pasteurization processing, and storage temperatures. It will be demonstrated that
keeping quality can be extended by understanding and controlling the overall
pasteurization process.
Food Safety and Inspection Service (FSIS) of the United States Department of
Agriculture (USDA) recommends heating the egg white and the egg yolk to 57.5°C and
61.1°C respectively for atleast 2.5 minutes to ensure egg safety against Salmonella and
other food borne pathogens (FSIS-USDA 2006).
2.6 Microwaves and Their Properties
Microwaves are very short waves of electromagnetic energy that travel at the
speed of light. They have all the basic properties of any electromagnetic radiation. They
have excellent penetrating power, which is inversely proportional to their frequency.
Figure 2.2 shows the position of the microwaves in the electromagnetic spectrum.
Microwave spectrum has wavelengths ranging from millimetres to centimetres. Hence, a
portion of the microwaves spectrum is also termed as centimetre waves (Pozar 2005).
14
Wavelength
in metres
Figure 2.2 Electromagnetic spectrum
The propagation of one complete cycle in the waveform of the microwaves or
any electromagnetic radiation is shown in the Figure 2.3. Thus microwaves create an
alternating electric field and an alternating magnetic field perpendicular to each other.
This property is being exploited for the thermal applications of microwaves.
Figure 2.3 Microwave propagation
(E – Electric field, M- Magnetic field)
As microwaves are electromagnetic radiation similar to visible light, they follow all
the basic laws of physics like reflection, refraction, interference, diffraction and
polarization.
2.7 Generation of Microwaves
Microwaves are generated in a microwave oven by a high voltage system. The heart
of this high voltage system is the magnetron tube. The magnetron is a diode-type
15
electron tube which is used to produce the required frequency of microwave energy. A
magnetic field imposed on the space between the anode (plate) and the cathode serves
as the grid. While the external configurations of different magnetrons will vary, the basic
internal structures are the same (Gallawa 1989). These include the anode, the
filament/cathode, the antenna, and the magnets. Figure 2.4 shows a longitudinal and
cross sectional diagram of a magnetron.
The magnetron operation is based on the motion of electrons under the
combined influence of electric and magnetic fields, (i.e.) electrons must flow from the
cathode to the anode. There are two fundamental laws that governing this
1. When force is exerted by an electric field on an electron, it tends to move from a
point of negative potential toward a positive potential. Figure 2.5 A shows the
uniform and direct movement of the electrons in an electric field with no magnetic
field present, from the negative cathode to the positive anode.
Figure 2.4 Longitudinal and cross sectional diagram of a resonant cavity magnetron
(Modified from diagram in (Gallawa 1989) & (Morgan 1960))
16
2. When force exerted on an electron by a magnetic field, which is at right angles to
the electric field itself, and to the path of the electron, the direction of the force is
such that the electron proceeds to the anode in a curve rather than a direct path
(Figure 2.5 B and C)
Figure 2.5 Working of a magnetron (Modified from (Gallawa 1989))
Electrons, being negative charges, are strongly repelled by other negative
charges. So this floating cloud of electrons would be repelled away from a negatively
charged cathode. The distance and velocity of their travel would increase with the
intensity of the applied negative charge. Momentum is thus provided by a high negative
DC voltage, which is produced by means of the high-voltage transformer and the double
action of the high-voltage diode and capacitor (Pozar 2005).
17
A high negative potential on the cathode puts a corresponding high positive
potential on the anode. This makes the electrons blast off from the cathode. They
accelerate towards the positive anode. This is when they encounter the powerful
magnetic field of two permanent magnets. These are positioned so that their magnetic
fields are applied parallel to the cathode. The effect of the magnetic fields tends to
deflect the speeding electrons away from the anode. They curve to a path at almost right
angles to their previous direction, resulting in an expanding circular orbit around the
cathode, which eventually reaches the anode (Figure 2.5 D)
The interaction of this rotating space-charge with the configuration of the surface
of the anode produces an alternating electromagnetic flow in the resonant cavities of the
anode as the physical structure of the anode forms the equivalent of a series of high-Q
resonant inductive-capacitive (LC) circuits. The effect of the strapping of alternate
segments is to connect the LC circuits in parallel (Gallawa 1989). Thus microwaves are
generated.
The generated microwaves are then transmitted into the microwave chamber
with a series of wave guides.
2.8 Applications of Microwaves
Microwaves are good for transmitting information from one place to another
because microwave energy can penetrate haze, light rain and snow, clouds, and smoke.
Shorter microwaves are used in remote sensing. These microwaves are used for radar
like the doppler radar used in weather forecasts. Microwaves, used for radar, are just a
18
few inches long. Microwave applications are quite extensive and they are used in almost
any form of wireless communication from military communication to personal
communication and networking of computers and peripherals.
Another important application of microwaves is its thermal application. The longer
microwaves, those closer to 15 cm, are the waves which heat the food in a microwave
oven.
2.8.1 Thermal Application of Microwaves
The phenomenon of microwave heating of foods was discovered accidentally. In
the late 1940s, a candy bar in the shirt pocket of an engineer softened considerably
when the engineer stood in front of a microwave transmitter. It didn’t take long for this
phenomenon to be capitalized upon and in just a few years microwave ovens began to
appear. Although microwave heating has been successfully applied at the industrial
level in other fields, in food processing it has met with limited success.
A microwave oven uses microwave radiation, usually at a frequency of 2450 MHz
(a wavelength of 12.24 cm). These waves are passed through the food in order to heat
it. Water, fat, and sugar molecules in the food absorb microwave energy in a process
called dielectric heating.
Dielectric Heating
Many molecules (such as those of water) are electric dipoles, meaning that they
have a positive charge at one end and a negative charge at the other, and therefore
rotate as they try to align themselves with the alternating electric field induced by the
19
microwave beam. This molecular movement creates heat as the rotating molecules hit
other molecules and put them into motion. Figure 2.6 shows the dipolar nature of a water
molecule with its polar energy field.
O2H+
H+
Figure 2.6 A dipolar water molecule with its polar energy field (Source: Wikipedia.org)
Microwave heating is most efficient on liquid water, much lesser on fats and sugars
(which have less molecular dipole moment), and frozen water (where the molecules are
not free to rotate). Microwave heating sometimes occurs due to rotational resonance of
water molecules, which happens only at much higher frequencies, in the tens of
Gigahertz.
In reality, microwaves are absorbed in the outer layers of food in a manner
somewhat similar to heat from other methods. Microwaves penetrate dry substances at
the surfaces of many common foods, and thus often deposit initial heat more deeply
than other methods. Also the amount of heat lost to the surrounding is very high on the
surface of the food than the interior. This gives an appearance that microwaves are
20
heating the food from inside out, though they heat up almost every part of the food
equally.
Depending on water content, the depth of initial heat deposition may be several
centimeters or more with microwave ovens, in contrast to convection heating, which
deposit heat shallowly at the food surface. Depth of penetration of microwaves is
dependent on food composition and the frequency, with lower microwave frequencies
being more penetrating.
At the consumer level, cheap ovens and fast heating (along with a tolerance of
unevenly heated food) has led to the near saturation of microwave ovens. However,
poor economics and complex heating patterns have led to its low industrial acceptance.
Nevertheless, industrial application is possible if certain conditions are met. Special
design of the microwave oven to address the complex heat distribution problem is
possible if the food is fairly uniform in shape and composition. Furthermore, if the added
quality is tangible to the point where the added expense of microwave processing can be
passed along to the consumer, then an industrial process becomes more viable.
2.9 Microwave In-Shell Pasteurization of Eggs
Microwaves are energy rich electromagnetic radiations, whose energy is readily
absorbed by substances containing dipolar molecules. The best example of a dipolar
molecule is water. The frequently alternating polarity of the electromagnetic radiation
(Microwaves) causes a dipolar molecule to rotate and gets heated up due to molecular
21
friction. Microwaves have the capability to penetrate substances that are opaque for
visible light, thus making it suitable for heating up different food materials (Pozar 2005).
Among the egg white and the yolk, which are the two primary components of the
egg, the albumen is the primary infection site as Salmonella needs only indirect contact
with the yolk for its growth and multiplication and hence the albumen is the primary
target of microwave heating (Fleischman et al. 2003). When thinking of microwave
heating of a shell egg, the first thing that comes to mind is the high risk of great pressure
build-up within the eggs. However this is not inevitable within the pasteurization
temperatures. With proper control of the process parameters, microwave heating can
provide efficient and rapid heating for thermal pasteurization.
The issue of microwave heating uniformity can be overcome with the proper
orientation of the egg and a specially designed waveguide, which is an engineering
issue (Fleischman 2004) and also by the precise design of the container (equipped with
microwave susceptors) taking the eggs into the microwave chamber (Yakovlev 2001).
A complete understanding of the dielectric properties and egg curvature on
power distribution will help design a system highly specific and efficient for this
application. There are several ways of measuring the dielectric properties of different
materials like perturbation technique, transmission line technique, open ended probe
technique, time domain reflectometry, free-space transmission technique, microstrip
transmission line etc. (Venkatesh and Raghavan 2005). Among these the open ended
22
coaxial probe technique was found to be more appropriate and precise for measuring
the dielectric properties of the egg components.
Theoretical mathematical studies have shown that even though albumen exhibits
better dielectric properties than yolk, the egg’s curvature has a focussing effect which
leads to a suitable power distribution. Mermelstein (2001) had written that ““If ever
microwave processing needed a specific type of product it could do better than any other
process it’s this. Microwaves are ideally suited for pasteurization of shell eggs.” Dr.
Fleischman Said”.
2.10 Speciality Eggs
Organic eggs
These are eggs produced by hens that are fed a special feed having ingredients
that were grown without pesticides, herbicides and commercial fertilizer so as to
preserve the integrity of the soil. They have the same nutritional value as any other egg.
Vegetarian eggs
These are speciality produced by hens that are fed a special diet of feed
containing ingredients of plant origin only (No animal by-products).
Omega 3 eggs
Recently, chicken eggs that are especially high in Omega 3 fatty acids have
come on the market. These eggs are made by feeding laying hens a diet containing
23
polyunsaturated fats using flax seeds and kelp meal. Nutrition information on the
packaging is different for each of the brands.
Vitamin enhanced eggs
These eggs are from hens fed a nutritionally-enhanced diet having higher levels
of certain nutrients (eg. vitamin E, folate, vitamin B-6, vitamin B-12). As a result, these
eggs contain slightly higher amounts of nutrients.
In- shell pasteurized eggs
These are eggs recently introduced into the market. As the name implies, the
eggs are heat pasteurized in hot water or hot air or a combination of both. These eggs
are the safest, but do not retain the exceptional functional properties of other raw eggs.
This study is aimed at improving the functional properties of in-shell pasteurized
eggs and thereby the functional quality of the pasteurized eggs by using microwaves to
accomplish the task.
24
Chapter 3
DIELECTRIC PROPERTIES OF EGG COMPONENTS AND
MICROWAVE HEATING FOR IN-SHELL PASTEURIZATION OF
EGGS
3.1 Abstract
Eggs are potential hosts and carriers for pathogenic microbes like Salmonella
enteritidis, due to their rich nutritive value. Heat Pasteurization is a best solution for
controlling these pathogens. Egg is used as a vital ingredient in several foods, especially
for their exceptional functional properties. These properties mainly depend on the protein
quality of the eggs and are severely affected when heated, due to protein denaturation.
Thus a heat pasteurization technique with minimal changes to these proteins needs
consideration.
In this study, microwave heating has been considered for in-shell egg
pasteurization. In the first part, the effects of temperature (0-62°C) and frequency (200
MHz to 10 GHz) on the dielectric properties of egg components were investigated. In the
second part, individual egg components as well as intact in-shell eggs were brought to
pasteurization temperature in a laboratory scale microwave oven working at 2450 MHz
using different (0.75, 1 and 2 W/g) power densities and heating curve was analyzed to
determine the heating time required for different power levels. Under the conditions
studied, it was demonstrated that the albumen had higher dielectric properties and loss
25
factors than the yolk. This implied that albumen was more efficient in converting
microwave energy into heat than the yolk. This was corroborated by the microwave
heating trials performed on individual components where albumen always heated up
faster. These observations suggested that microwave heating was not suitable for inshell egg pasteurization as albumen coagulation was likely to occur before the yolk could
reach the required pasteurization temperature. Laboratory trials on microwave heating of
in-shell eggs indicated that, on the contrary, the heating rates of both albumen and yolk
were similar.
Therefore, microwave heating appeared perfectly suited for in-shell egg
pasteurization. Combination of egg geometry, dielectric properties, and size were the
main factors responsible for the enhanced interior heating. Models for calculating the ε’
and ε” at a given frequency and temperature for shell egg components were also
presented.
Keywords:
pasteurization, shell eggs, microwave, heating patterns, dielectric
properties.
26
3.2 Introduction
Egg is a popular ingredient in many foods and food industries. Also egg is an
excellent food supplement, giving almost every essential amino acid which most of our
regular diet may lack. It is also an excellent source of Vitamin A, B3 & Folate. It also
contains useful amounts of many other vitamins and minerals (Li-Chan, Powrie, and
Nakai 1995). The yolk contains the main reserve of food substances, required for the
development of embryo. The albumen is the chief reservoir of water and it is the most
alkaline of all the natural liquids (Lokhande et al. 1996).
Eggs are one among the major animals foods mostly marketed raw and
frequently consumed raw. Eggs are potential hosts and carriers for pathogenic microbes
like Salmonella enteritidis, due to their rich nutritive value. More than 90 percent of food
borne Salmonellosis, caused by Salmonella enteritidis is through the shell eggs
(Schroeder et al. 2005; Woodward, Khakhria, and Johnson 1997). Most of the
Salmonella enteritidis outbreaks generally involved Grade A eggs that are washed and
disinfected and also met the requirements of the state for shell quality (St. Louis, Morse,
and Potter 1988).
The Food Safety and Inspection Service (FSIS) of United States Department of
Agriculture(USDA) recommends heating the egg white and the egg yolk to 57.5°C and
61.1°C respectively for 2.5 minutes to ensure egg safety against Salmonella and other
food borne pathogens (FSIS-USDA 2006). This is possible by conventional heating
method only if the yolk and egg white are separated before processing. But breaking and
27
repacking them aseptically involves huge additional costs. Therefore In-Shell
pasteurization has gained a great commercial importance in recent times.
Current technique for in-shell pasteurization of egg involves heating the eggs in a
water bath at 60 ºC for about 20-25 minutes, depending on the size of the eggs. This
leads to the overheating of the egg white proteins (i.e the egg white gets heated up more
than the yolk, which is against the recommendations) resulting in denaturation &
coagulation (Hou et al. 1996) This denaturation greatly affects the functional properties
of the eggs. Therefore a process that can heat the shell eggs from inside will be a best
alternative to solve this problem.
Microwaves have the ability to generate heat from within the substance that is
exposed to it. Theoretical mathematical studies have shown that even though albumen
exhibits better dielectric properties than yolk, the egg’s curvature has a focusing effect
which leads to a suitable power distribution (Zhang and Datta, fig11.6, p 508, in
Dielectric properties of food(Datta, Sumnu, and Raghavan 2005). The shell egg appears
ideally suited for pasteurization in a microwave environment (Fleischman 2004; Rehkopf
2005).
The measurements of dielectric properties of materials are finding increasing
application as new electro-technology and microwave processing is adapted for use in
the agriculture and food processing industries. Measurements of the dielectric properties
(dielectric constant and dielectric loss factor) are required for understanding, explaining,
and empirically relating physico-chemical properties of the material microwave energy
(Venkatesh and Raghavan 2005).
28
Dielectric properties vary with the composition, moisture content and temperature
of the food and the frequency of the electric field. Information on the dielectric properties
of albumin and yolk is limited. Studying the dielectric properties will help understand the
behavior of egg components in a microwave environment. Modeling changes in
dielectric properties of the albumen and yolk with temperature and frequency will allow
predicting the same at any prescribed temperature and frequency thereby facilitating
equipment design, process optimization to ensure best end product quality. A complete
understanding of the dielectric properties and egg curvature on power distribution will
help design a system highly specific and efficient for this application (Liao et al. 2003).
Therefore this study was conducted with the following objectives:
i) To measure the dielectric properties of albumen and yolk of eggs in the
frequency range of 200 MHz to 10 GHz and in the temperature range of 0oC to
the pasteurization temperatures (57.5 °C for egg white and 61.1 oC for yolk).
ii) To study the heating rates and time taken to reach the above mentioned
pasteurization temperatures from ambient temperature (come-up time) for a
given power level in the laboratory microwave applicator at 2450 MHz for
albumen and yolk, in and out of the egg shell.
3.3 Materials and Methods
In this study, the potential benefits of using microwave energy for heating fresh
in-shell eggs to pasteurization temperatures were investigated. At first, dielectric
properties of egg albumen and yolk were measured at temperatures ranging from 0ºC to
29
the required pasteurization temperature, and at frequencies ranging from 200 MHz to 10
GHz. Empirical relationships were then obtained to express dielectric properties as a
function of temperature and frequency. In the second part, the egg white and yolk as
well as intact in-shell eggs were heated from 24ºC to the temperature required for
pasteurization in a laboratory scale microwave oven. Effects of power levels on heating
rates and egg quality were measured and compared.
3.3.1 Measurement of Dielectric Properties of the Egg Components
Egg Samples
The fresh whole eggs, within 3 days of grading and packing (identified from the
best before date stamped on the eggs), used in this study were procured from the local
market and kept in a refrigerator set at 5oC until used. They were all of Canadian Grade
A and of large size with an average weight of 60g each.
Equipment
Measurements of the dielectric properties were made with an Agilent 8722 ES sparameter Network Analyzer equipped with a high temperature probe model 85070B
and controlled with the Agilent 85070D Dielectric Probe Kit Software Version E01.02.
According to the manufacturer, the equipment has an accuracy of ±5% for the dielectric
constant (ε’) and ±0.005 for the loss factor (ε”) (HP 1992). A diagram of the experimental
setup used for the measurement of dielectric properties is shown in Figure 3. 1.
30
Figure 3.1: Experimental setup for the measurement of dielectric properties
The dielectric constant and loss factor were calculated from the load admittance
YL (ω, ε) directly by the software.
The expression for calculating YL (ω, ε) is as follows (Venkatesh and Raghavan
2005):
YL (ω , ε ) = Y0
1 − Γ(ω , ε )
1 + Γ(ω , ε )
Where,
ΥL, (ω, ε) is load admittance in S;
ω is the operating angular frequency in Hz;
ε, is the overall permittivity in F/m;
Y0 is the characteristic admittance of probe of 50 Ω; and
Г(ω, ε) is the measured reflection coefficient
31
(1)
Experimental Design & Procedure
Prior to the measurements, the eggs were cracked carefully and the egg white
(35 g) and yolk (20 g) were collected separately in small cylindrical beakers. All
measurements were made in triplicates (from an individual egg for each replicate for the
egg white and Yolk and two eggs’ shell and membrane for each replicate in order to
make up the minimum volume required for measurement).
The shell and shell membrane were separated by soaking in water. The shell
was finely powdered using a mortar and pestle to an average particle size less than 250
μm (U.S.A standard test sieve E-11 specification No.60). The membrane was carefully
folded to the required thickness of measurement (6 mm). All the prepared samples were
refrigerated to 0°C.
The samples to be measured were taken in small cylindrical beakers. The probe
was mounted with the stand, facing downwards. Calibration of the probe was done using
a shorting block, air and water before each experiment and then checked by measuring
distilled water to insure the calibration was stable.
The samples were heated using a water bath set at desired temperatures and
placed right beneath the probe on the platform of the stand used to mount the probe
(Figure 3.1). Measurements were taken every 5 oC interval from 0°C to 57.5 °C for egg
white, 61oC for yolk and 60°C for the shell and shell membrane. The temperatures of the
samples were measured using an alcohol thermometer.
32
The dielectric properties of demineralised water were also measured within the
above mentioned temperature and frequency range for comparison. The dielectric
properties were measured at 100 different frequencies ranging from 200 MHz to 10 GHz
Data Analysis
MATLAB version 7.01 was used to analyze the collected data and to establish
the mathematical relationships for the dielectric constant and loss factor as a function of
frequency and temperature.
3.3.2 Comparing Microwave Heating Rates of Egg White and Yolk In and Out of the
Shell
Egg Samples
Commercially available large size fresh eggs were procured from the local
market in refrigerated conditions and allowed to stay at the room temperature for about
3-4 hours to reach the ambient temperature of about 24oC. Samples of egg white and
yolk were collected in a similar fashion as for the measurement of dielectric properties.
The shell and shell membrane were discarded. For in-shell heating trials shell eggs
weighing 60±1g were used.
Equipment
An instrumented and computer controlled laboratory scale microwave (MW) oven
(Figures 3.2, 3.2.1 & 3.2.2) was used for this part of the study. Its main components
were: a 2450 MHz microwave generator with adjustable power from 0 to 750 kW,
waveguides, a three-port circulator, a manual three-stub tuner to match the load
33
impedance, microwave couplers to measure forward and reflected power, a carbon load
to absorb reflected power and a microwave cavity in which the egg samples were
processed.
The microwave generator (magnetron) produced microwaves with varying power
densities based on the supplied power. The generated microwaves were guided using
the waveguides into the microwave cavity via the above mentioned components in a
sequence. The manual three-stub tuner was used to adjust the reflected power, thereby
keeping it at the minimum possible value (<10% of the incident power). The
temperatures were measured using fiber optic probes. The probes were connected to an
Agilent 34970A data acquisition unit which was again connected to a computer. The
entire setup was monitored and controlled using the HPVEE (Agilent) object oriented
programming language.
Figure 3.2: Experimental setup for microwave heating
34
Figure 3.2.1 Experimental setup for Microwave pasteurization
Figure 3.2.2 Shell egg with fibre optic probes in the microwave cavity
35
Experimental Design & Procedure
All the measurements were taken in triplicates (each replicate obtained from an
individual egg, except for the yolk wherein the yolk from two eggs were combined for
each replicate). The samples of egg white and yolk (40 g each) were placed (one at a
time) inside the microwave chamber in small (50 ml) cylindrical glass beakers (2 cm in
diameter & 5 cm in height) and heated to the pasteurization temperatures of 57.5oC and
61.1oC respectively.
Also, probes were introduced through the shell of the in-shell eggs (one for the
white and one for the yolk), assuming that the yolk was located approximately at the
center of the in-shell egg (later confirmed by breaking the egg and carefully examining
the probe location) and heated in the microwave chamber with the broad end of the egg
facing upwards, till the yolk reached 61.1°C with the same experimental design. In both
of the above mentioned cases, temperature measurements were taken at an interval of
5 seconds each and the experiment was repeated for different power densities (0.75, 1
and 2 W/g). The egg white, yolk and the shell egg were held at the pasteurization
temperatures (±0.5 ºC) with repeated microwave heating cycles controlled by the
computer running HPVEE (Agilent) object oriented programming language.
36
Post-treatment Shell Integrity (Visual observation)
Tests were also conducted without any probe and the eggs were examined for
any visible cracks or deformations in the egg shell, following the same experimental
design and procedure mentioned above. The time of microwave application was
determined by the heating rate obtained from the previous trials with the probe. A scale
ranging from 0 to 5, of which 0 being no cracks and 5 being a broken/leaking egg was
used to measure the visual shell integrity of the egg.
Data Analysis
The temperature measurements obtained for the egg white and yolk for a given
power level were plotted as a function of temperature versus time to observe the heating
curves within the pasteurization temperatures to study the actual heating time required
for in-shell eggs to reach the pasteurization temperatures and the heating rates for
different power densities (0.75,1 and 2 W/g) were compared. The data obtained was
used to calculate the come up time (the time taken to reach the pasteurization
temperatures) for egg white, yolk and the shell egg at different power densities
mentioned above.
3.4 Results and Discussion
Dielectric Properties of the Egg Constituents
As shown in Figures 3.3 and 3.4, both ε’ and ε” for egg white and yolk appeared
linearly related to temperature and frequency in a similar fashion as that of water and
also the dielectric properties of the egg white appeared to be much closer to that of
water.
37
Change in dielectric properties with temperature at 2450 MHz
90
80
70
Water ε'
ε’ and ε”
60
Egg White ε'
50
Egg Yolk ε'
40
Water ε"
Egg White ε"
30
Egg Yolk ε"
20
10
0
0
10
20
30
40
50
60
70
Temperature, °C
Figure 3.3 Change in Dielectric properties of water, egg white and egg yolk with varying
temperatures at 2450 MHz
Change in dielectric properties with varying frequency at 20°C
90
80
70
Water ε'
60
ε’ and ε”
Egg White ε'
50
Egg Yolk ε'
Water ε"
40
Egg White ε"
30
Egg Yolk ε"
20
10
0
0
2
4
6
8
10
12
Frequency, GHz
Figure 3.4 Dielectric properties of water, egg white and yolk with varying frequencies at
20ºC
38
As the egg white has nearly 90% water (Li-Chan, Powrie, and Nakai 1995), the ε’
and ε” values obtained for egg white were much closer to that of water (Collie, Hasted,
and Ritson 1948) than those observed for the yolk.
At any given temperature and
frequency, repeatability of the measurements was excellent and the variances calculated
among replicates were smaller than 0.15.
The ε’ for both egg white and yolk decreased with increasing temperature and
frequency (eq. (3) and (5)) whereas the ε” decreased with increase in temperature and
increases with increase in frequency (eq. (4) and (6)). A linear additive model was used
to relate ε’ or ε” to temperature and frequency. Its general form was:
(ε’ or ε”) = a + b y T ± c y F
(2)
Where,
T is the temperature in °C,
F is the frequency in GHz, and
a, b, c are the model coefficients,
The regression analysis performed on the collected data yielded the following
relationships for egg white:
•
ε’ = 72.38 - 0.17 y T - 1.75 y F
(R2 = 0.925)
(P<0.01)
(3)
•
ε” = 17.22 - 0.19 y T + 1.58 y F
(R2 = 0.948)
(P<0.01)
(4)
and for egg yolk:
•
ε’ = 50.085 - 0.13 y T - 1.72 y F
(R2 = 0.925)
(P<0.01)
(5)
•
ε" = 13.55 - 0.11 y T + 0.65 y F
(R2 =0.905)
(P<0.01)
(6)
39
These models are useful in determining the dielectric properties of egg yolk or
egg white at any given temperature and frequency within the range studied. Graphical
representations of the measured and predicted values of ε’ and ε” for the egg white and
yolk can be found in Figures 3.5 to 3.8.
The yolk samples had lower values for both ε’ and ε”. This was attributed to the
fact that the yolk contains lower moisture of around 47 % (Li-Chan, Powrie, and Nakai
1995) when compared to the egg white. Both ε’ and ε” of the yolk were less sensitive to
temperature and frequency than the egg white (Figures 3.7 and 3.8).
Figure 3.5. Measured and predicted ε’ values for egg white
40
Figure 3.6. Measured and predicted ε” values for egg white
Figure 3.7. Measured and predicted ε’ values for egg yolk
41
Figure 3.8. Measured and predicted ε” values for egg yolk
The ε’ and ε” values of both the shell and shell membrane did not change
significantly with respect to temperature and frequency. The ε’ and ε” values remained
relatively constant around 3.5 and 0.5, respectively. As a result, both the shell and the
shell membrane are relatively transparent and permeable to microwaves. This was
attributed to the low moisture content of both shell and shell membrane which was only
9%w.b and to a lesser extent the composition and structure of the shell proteins, and to
the fiber matrix of shell and shell membrane might be responsible for this good
transparency to microwaves (Calvery 1933; Sasikumar and Vijayaraghavan 2006).
Microwave Heating of Individual Egg Constituents
Egg white and yolk samples were first heated separately in the laboratory
microwave oven. Because of their higher dielectric property values, egg white samples
heated up faster than the yolk samples (Figure 3.9). The egg white took about 4.1, 6.9
and 9 minutes with average heating rates of 14.02, 8.33 and 6.39 ºC/min for the
microwave power densities of 2.0, 1.0 and 0.75 W/g respectively, whereas the yolk
42
required about 9.1, 14.1 and 16.9 minutes with average heating rates of 6.71, 4.33 and
3.61 ºC/min respectively for the same microwave power densities mentioned above.
The heating rates for the power density of 2 W/g was significantly different
(p<0.05) from that of both 1 W/g and 0.75 W/g, except that the pasteurization
temperatures were attained a 2-3 min faster using 1W/g than 0.75 W/g. The heating was
more even, without any coagulated protein lumps when heated at low power (0.75 or 1.0
W/g) of microwave. At higher power densities, heating was uneven and lumps of
coagulated egg white were observed.
Heating Pattern of Yolk in a Beaker
70
70
60
60
50
0.75 W/g
40
1 W/g
30
2 W/g
20
T em p eratu re (°C)
T em p eratu re (°C)
Heating Pattern of Egg white in a Beaker
10
50
0.75 W/g
40
1 W/g
30
2 W/g
20
10
0
0
0
3
6
9
12
15
18
0
Time (min)
3
6
9
12
15
18
Time (min)
Figure 3.9. Heating curves of egg white and yolk in a beaker at different microwave
power levels.
Microwave Heating of In-Shell Egg
The heating curves of in-shell egg were studied using the laboratory scale
microwave oven. Two optic fiber sensors, carefully inserted through the egg shell, were
used to monitor the albumen and yolk temperature during processing. There was no
43
significant difference (p<0.05) in the come up time (time taken to reach the
pasteurization temperature) for the egg white heated in beaker and in shell, although it
exhibited a significantly different (p<0.05) heating curve with different power densities.
However, the yolk had heated up faster in-shell than in a beaker. It was further noted
that at any power density studied, the time to reach pasteurization temperatures were
similar for both the yolk and the albumen indicating that microwave heating is definitively
suited for in-shell egg pasteurization The overall time to reach the pasteurization
temperatures during In-shell heating was found to be about 3.5, 7 and 9 minutes for
power densities of 2.0, 1.0 and 0.75 W/g respectively (Figure 3.10.).
Heating Pattern of Egg Yolk InShell
70.00
70.00
60.00
60.00
50.00
50.00
0.75 W/g
40.00
1 W/g
30.00
2 W/g
20.00
Temperature (°C)
Temperature (°C)
Heating Pattern of Egg w hite InShell
0.75 W/g
40.00
1 W/g
30.00
2 W/g
20.00
10.00
10.00
0.00
0.00
0
2
4
6
8
0
10
2
4
6
8
10
Time (min)
Time (min)
Figure 3.10. Heating curves of egg white and yolk in in-shell eggs at different microwave
power levels.
Possible reasons for this phenomenon have been proposed by Datta et al.
(2005). They suggested that the focusing effect of the egg shell curvature, the spherical
geometry and the central yolk position inside a shell egg have resulted in a convergence
of the microwave energy towards the center hence increasing heat dissipation in the yolk
44
(Datta, Sumnu, and Raghavan 2005). In addition, the radial penetration depth and
loss/attenuation of the microwave energy could have contributed to the higher heating
rate of the yolk. The visual examination showed no crack or any deformation on the shell
surface.
The higher yolk heating rates were also observed at higher power density (Figure
3.10.). But higher power densities also led to greater non-uniformity in heat/energy
distribution within the shell resulting in localized overheating. This was evident from the
number of small coagulated lumps increasingly found in both egg white and yolk with
increasing power densities.
Post-treatment Shell Integrity (Visual observation)
All the intact shell eggs (without any probes) heated for the same duration for
different power densities as observed in the previous trials with probes, remained
absolutely intact without any cracks. Thus all the eggs scored ‘0’ without any deviation in
the visual test for shell integrity.
3.5 Conclusions
The study of dielectric properties had provided a good qualitative
understanding of the behaviour of the egg constituents under microwave heating. A
regression model was developed to assist in further investigation and development of a
microwave in-shell egg pasteurization unit. The overall dielectric behaviour of the major
egg components (egg white and yolk) were found similar to that of trend followed by
water for the frequency and temperature limits of this study. The egg shell and shell
45
membrane showed a very good transparency to microwaves in their dielectric properties,
thereby making the egg shell a suitable container for microwave pasteurization.
Also the heating curves of the egg components in and out of the shell gave a
lucid picture of the heating behaviour of the shell eggs in a microwave environment and
a clear idea of the heating time required for different power levels. It was interesting to
note that the yolk, which had poorer dielectric properties, heats up a little faster than the
egg white, when heated within the shell. This information can be useful in further
investigations and in designing an appropriate equipment to accomplish the task of
microwave pasteurization of shell eggs.
Microwaves appeared to be the best of
currently available technologies for In-shell egg pasteurization.
Acknowledgements
The financial support by the Canadian International Development Agency (CIDA)
and Natural Sciences and Engineering Research Council (NSERC) of Canada is greatly
acknowledged.
3.6 References
Calvery, H.O. 1933. Some Analyses of Egg-Shell Keratin. Journal of Biological
Chemistry 100 (1):183-186.
Collie, C. H., J. B. Hasted, and D. M. Ritson. 1948. The Dielectric Properties of Water
and Heavy Water. Proceedings of the Physical Society 60 (2):145.
46
Datta, Ashim, G. Sumnu, and G.S.V. Raghavan. 2005. Dielectric Properties of Foods. In
Engineering Properties of Foods, edited by M. A. Rao and A. Datta. Boca Raton,
Florida: Taylor & Francis Publications.
Fleischman, G.J. . 2004. Microwave pasteurization of shell eggs. In IFT Annual Meeting.
Las Vegas, USA: IFT.
FSIS-USDA. 2006. Risk Assessments for Salmonella enteritidis in Shell Eggs and
Salmonella spp. in Egg Products. Omaha, NE: FSIS.
Hou, H., R. K. Singh, P. M. Muriana, and W. J. Stadelman. 1996. Pasteurization of intact
shell eggs. Food Microbiology 13:93-101.
HP. 1992. Dielectric Probe Kit 85070A. In Test and Measure Measurements, edited by
R. D. Unit. Palo Alto, CA: Hewlett Packard Corporation.
Li-Chan, E.C.Y., W.D. Powrie, and S. Nakai. 1995. The Chemistry of Eggs and Egg
Products. In Egg Science and Technology, edited by W.J.Stadelman and
O.J.Cotterill. New York: Food Products Press.
Liao, Xiangjun, G.S.V. Raghavan, Jianming Dai, and V.A. Yaylayan. 2003. Dielectric
properties of a-d-glucose aqueous solutions at 2450 MHz. Food Research
International 36:485–490.
47
Lokhande, M.P., B.R. Arbad, M.G. Landge, and S.C. Mehrotra. 1996. Dielectric
properties of albumin and yolk of avian egg. Indian Journal of Biochemistry and
Biophysics 33:156-158.
Rehkopf, A. 2005. Quality validation of a microwave-pasteurization process for shelleggs. Paper read at IFT Annual Meeting, at New Orleans, Lousiana.
Sasikumar, S., and R. Vijayaraghavan. 2006. Low Temperature Synthesis of
Nanocrystalline Hydroxyapatite from Egg Shells by Combustion Method. Trends in
Biomaterials & Artificial Organs 19 (2):70-73.
Schroeder, Carl M., Alecia Larew Naugle, Wayne D. Schlosser, Allan T. Hogue,
Frederick J. Angulo, Jonathon S. Rose, Eric D. Ebel, W. Terry Disney, Kristin G.
Holt, and David P. Goldman. 2005. Estimate of Illnesses from Salmonella
enteritidis in Eggs, United States, 2000. Emerging Infectious Diseases 11 (1):113115.
St. Louis, M.E., D.L. Morse, and M.E. Potter. 1988. The Emergence of grade A eggs as
a major source of Salmonella enteritidis infections : new implications for the control
of salmonellosis. Journal of American Medical Association 259:2103–2107.
Venkatesh, M.S., and G.S.V. Raghavan. 2005. An overview of dielectric properties
measuring techniques. Canadian Biosystems Engineering 47 (7):15-30.
Woodward, D. L., R. Khakhria, and W. M. Johnson. 1997. Human Salmonellosis
Associated with Exotic Pets. Journal of Clinical Microbiology 35 (11):2786-2790.
48
Connecting text
After studying the dielectric properties of the egg components and subjecting to
microwave heating, it was found that microwaves are exceptionally suitable for
pasteurizing the eggs. As it is well known that heating causes egg proteins to denature
and thereby affects the physical properties like enthalpy of denaturation, viscosity,
turbidity etc... , further study was conducted to evaluate these changes with respect to
the raw eggs and compare it to the conventional water bath pasteurization technique.
The following chapter will explain this assessment in a detailed manner.
49
Chapter 4
PHYSICAL PROPERTIES OF EGG WHITE AFTER IN-SHELL
PASTEURIZATION BY USING MICROWAVE OR BY IMMERSION
IN HOT WATER
4.1 Abstract
In this study, the functional properties of egg white after in-shell pasteurization
by microwave have been compared to that of egg white after in-shell pasteurization by
immersion in hot water. The effects of the heat treatments on egg white enthalpy of
denaturation, viscosity, turbidity, foam density and stability, and also the dielectric
properties, were investigated and compared to that of untreated egg white. Results have
demonstrated that microwave pasteurized in-shell eggs had very limited changes in all
the properties tested for, when compared to the water bath pasteurized ones.
Keywords: Post-Processing Quality, Microwave in-shell pasteurization, Shell eggs
50
4.2 Introduction
Eggs are popular for the exceptional functional properties of its two major
components: the egg white and the yolk. Egg white is used as a foaming, emulsifying,
gelling and/or binding agent in numerous food preparations. Egg white proteins are the
most heat sensitive components of an egg. The physical properties like whipability,
foamability, foam stability, viscosity etc., affecting the functional properties, which make
the eggs an inevitable ingredient of various food products are severely affected by high
temperatures (Iesel Van der Plancken, Ann Van Loey, and Hendrickx 2006).
Due to its exceptional nutritive value, eggs remain a potential host for pathogens
like Salmonella enteritidis. More than 90 percent of food borne Salmonellosis, caused by
Salmonella enteritidis is through the shell eggs (Schroeder et al. 2005; Woodward,
Khakhria, and Johnson 1997). Most of the Salmonella enteritidis outbreaks generally
involved Grade A eggs that are washed and disinfected and also met the requirements
of the state for shell quality (St. Louis, Morse, and Potter 1988). The probability of fresh
eggs having Salmonella varies from 0.005 % (Mermelstein 2001) to 1 %(Griffiths 2005)
depending on various factors involved in the egg production.
Thermal processing methods are the most widely used technique for destroying
microorganisms and imparting foods with a lasting shelf-life, among which Pasteurization
has got its own prominent and predestined applications. Pasteurized foods are safety
assured for the consumer within the recommended storage period and storage
conditions. Today various techniques are applied for the pasteurization and thermal
processing of foods. The conventional methods of thermal processing of foods result in
51
peripheral over heating before the food in the centre reaches the required temperature.
This is potentially a great problem in pasteurization, especially when it comes to shell
eggs.
The Food Safety and Inspection Service (FSIS) of United States Department of
Agriculture(USDA) recommends heating the egg white and the egg yolk to 57.5°C and
61.1°C respectively for 2.5 minutes to ensure egg safety against salmonella and other
food borne pathogens (FSIS-USDA 2006). The existing method of pasteurizing the shell
eggs uses immersion in hot water at 60 ºC for 20 mins, results in overheating of the egg
white and partially cooked eggs (Mermelstein 2001; Hou et al. 1996). There are different
protein fractions namely conalbumin, ovalbumin, ovotransferin, ovomucoid, ovomucin,
globulins, lyzozyme, etc. that are contributing to the functional properties of the egg
white as a whole (McDonnell et al. 1955; Cunnningham 1995). The denaturation of some
of these proteins start at temperatures as low as 45 ºC.
Studies on the physico-chemical changes due to heat treatment of egg white
revealed that at lower temperatures (< 50°C) these changes were only temperature
dependent, but at higher temperatures (> 50°C) the time factor, plays an important role,
indicating a time-temperature-dependent level of denaturation with an equilibrium
(denaturation-saturation) time (Iesel Van der Plancken, Ann Van Loey, and Hendrickx
2006). Therefore the total time of heating is crucial for a better quality pasteurized egg
white.
52
In the previous chapter (Chapter 3), it was demonstrated that microwave heating
is an inimitable alternative to overcome the problem of peripheral overheating during
shell egg pasteurization. Also the FSIS recommendation of heating up the yolk to a
higher temperature (61.1ºC) was absolutely possible without heating the egg white
beyond its recommended pasteurization temperature (57.5ºC). The risk of great pressure
build-up within the egg shell when heated using microwaves is not inevitable within the
pasteurization temperatures (Fleischman 2004; Rehkopf 2005). A comprehensive
assessment of the functional quality of the microwave heated eggs can be done by
examining the changes in the physical properties responsible for that.
Therefore this paper focuses on comparison of microwave and water bath heated
egg white for in-shell pasteurization with the raw egg white for the heat induced changes
with respect to physical properties like enthalpy of denaturation, viscosity, foam density,
foam stability, turbidity and dielectric properties.
4.3 Materials and Methods
In-shell egg will be pasteurized using a laboratory microwave oven setup and
using hot water bath maintained at 60 ºC. Effects of heat treatments on the physical
properties affecting the functional quality of the egg white recovered from the treated
eggs will be measured and compared to that of fresh untreated egg white.
53
Egg samples
The fresh whole eggs, within 3 days of grading and packing (identified from the
best before date stamped on the eggs, which is usually 35 days from date of packing ),
(CEMA 2004) used in this study were procured from the local market and kept in a
refrigerator set at 5oC until used. They were all of Canadian Grade A eggs and of large
size, each with an average mass of 60±2 g. Prior to pasteurization, the eggs were
brought to room temperature of about 24°C by placing the opened carton on the
laboratory counter for a period of 3 to 4 hours (tested by breaking and measuring inner
temperatures of 3 representative samples) before giving the heat treatments.
Heat treatments for pasteurization
Two heat treatments for the pasteurization of in-shell eggs were investigated and
compared. Each treatment was done in triplicates (i.e) three eggs were used for each
treatment for the measurement of each parameter within the scope of this study. The
first treatment consisted of heating in-shell eggs in a laboratory scale microwave oven
working at 2450 MHz using a power density of 1 W/g. In-shell egg white was heated in 7
minutes to 57.5°C for 2.5 minutes, as per the FSIS-USDA recommendation (FSISUSDA 2006). The temperature was measured using a fiber optic probe inserted into the
egg white through the shell and the microwave operation was controlled by the computer
running HPVEE (Agilent) object oriented programming language to maintain the desired
process temperature.
54
The second treatment consisted of immersing the in-shell egg in a temperature
controlled water bath maintained at 60 °C for a period of 20 minutes (Schuman et al.
1997).
Immediately after both the above mentioned heat treatments the shell eggs were
immersed in cold water tub containing water at 5 ºC for 10 mins.
Measurements of the egg white physical properties
The physical properties attributed to the functional quality of the egg white of inshell heat treated and untreated eggs were measured and compared. Eggs were
cracked carefully and the egg white was collected in small cylindrical beakers. Shells
and yolks were discarded. All measurements were taken in triplicates. Parameters
measured to assess the functional properties of egg white were: enthalpy of protein
denaturation, the viscosity, foam density and foam stability, turbidity, and dielectric
properties.
Enthalpy of protein denaturation
The enthalpy of denaturation is the net value of the combination of endothermic
reactions, such as the disruption of hydrogen bonds, and of exothermic processes, such
as the breakup of hydrophobic interactions and protein aggregations. The resulting
residual enthalpy has been correlated to the remaining content of ordered secondary
structure of a protein (Iesel Van der Plancken, Ann Van Loey, and Hendrickx 2006).
The instrument used to measure the enthalpy of denaturation was a TA
Instruments Q100 Differential Scanning Calorimeter operated with the TA Instruments
55
Q100 DSC 7.0 Build 244 software. Untreated and heat-treated samples were first placed
in aluminum pans (20µl/pan) and then hermetically sealed. The pans were transferred to
the instrument pan holder and heated from 20°C to 120°C at a constant rate of
10°C/min. An empty pan was used as a reference. The sample residual enthalpy was
the recorded at the denaturation temperature of 83°C
Viscosity
An Oswald viscometer and a temperature controlled water bath were used for
measurement and comparison of the egg white viscosity. Measurements were made on
all samples at temperature ranging from 0 ºC to 45 ºC for every 5 ºC increment in
temperature. Measurements of viscosity could not be made above 45 ºC as any further
increase in temperature may lead to further denaturation and clogging of the viscometer
capillary. For each sample, measured viscosity values were plotted against temperature
and analyzed.
Foam density and foam stability
Foam density is a measure of the thickness of the foam, which gives a clear
picture of the quantity of air incorporated in the egg white foam. This is an important
factor that tells about the aerating properties of the egg white in its food applications.
The stability of the egg foam is a crucial parameter for the functional quality of the egg
white. The commercial use of egg white is highly dependent on its foam stability in many
of its applications in the food industry (McDonnell et al. 1955).
56
Each egg white sample was foamed in a cylindrical beaker (500 ml) with a Braun
60 Egg Beater (USA). The foaming process consisted of beating 50g of egg white for 2
minutes at a speed of 2000 rpm. The foam density was then measured and the foam
stability was taken as the quantity of liquid drained as a function of time from the
completion of foaming. Foam stability measurements were taken for 180 minutes after
foaming. Figure 4.1 shows the experimental setup for the determination of foam stability.
Figure 4.1. Experimental setup for measurement of foam stability
57
Turbidity
Turbidity is a direct measure of the extent of protein coagulation, as coagulated
proteins are opaque and reduce the transmittance of light through the egg white. The
amount of light absorbed (absorbance) is a function of the turbidity of a liquid. The
absorbance of the heat treated and untreated egg white samples was measured at 650
nm (Iesel Van der Plancken, Ann Van Loey, and Hendrickx 2006) using a Biochrom
Ultra spec 2100 Pro spectrophotometer at 24 ºC. Plain demineralised water was used
for calibration. An absorbance (turbidity) of 0% corresponded to a totally clear solution.
Dielectric properties
The change in dielectric properties is considered to be a good indicator of the
extent of denaturation of the egg white proteins (Bircan and Barringer 2002).The openended coaxial probe technique was used to measure and compare the dielectric
properties of heat-treated and untreated samples. Measurements of the dielectric
properties were made with an Agilent 8722 ES s-parameter Network Analyzer equipped
with the high temperature probe model 85070B. This instrument was controlled with the
Agilent 85070D Dielectric Probe Kit Software Version E01.02. According to the
manufacturer, the equipment has an accuracy of ±5% for the dielectric constant (ε’) and
±0.005 for the loss factor (ε”) (HP 1992). The dielectric property measurements (ε’ and
ε”) were taken at 2450 MHz for every 5 ºC rise in temperature from 0°C to 60°C.
58
Data analysis
All the data obtained were statistically analyzed using the software MATLAB- 7.0.1
from Mathworks. Analyses of variances followed by Duncan’s multiple range tests were
conducted to locate significant differences among means.
In all comparisons, significant deviations from mean values obtained from
untreated egg white was considered to be an effect on egg white functional properties.
4.4 Results and Discussion
Enthalpy of protein denaturation
As shown in the Figure 4.2, reductions in residual enthalpy indicated that the heat
treatments had partially denatured the ordered secondary structure of the egg white
proteins of all heat treated samples. However, microwave treated samples exhibited less
reductions than samples heated in the hot water bath. This implied that denaturation was
very less in the microwave heated in-shell egg. These results corroborate the work by
Iesel Van der Plancken et al. (2006), for the heat treated egg white.
Statistical analysis performed on the data revealed that the difference in mean
enthalpy of the egg white between microwave heated and the untreated (raw) in-shell
eggs was not significant (P<0.01), whereas the water bath heated in-shell eggs had
significant (P<0.01) lower enthalpy from the others.
59
Enthalpy of denaturation of untreated and in-shell heated
egg white
12
Calc Std Deviation
at the data points
Enthalpy of denaturation, J/g
10
8
6
4
2
0
Untreated
Microw ave Heated
Waterbath Heated
Treatments
Figure 4.2. Enthalpy of denaturation of the egg white of untreated and in-shell heated
eggs.
Viscosity
The egg white viscosity of heat treated eggs were lower than that of untreated
eggs (Figure 4. 3), and decreased with temperature (Pitsilis, Walton, and Cotterill 1975)
and the level of protein denaturation. At higher temperatures, differences among the
treatments were lower than at lower temperatures. An overview of the data in Figure 4.3
depicts that all the three samples had a considerable difference among them at lower
temperatures.
The results of the statistical analysis (ANOVA & Duncan’s Test) were similar to
that of the enthalpy of denaturation. The viscosity of in-shell microwave heated egg
white was not significantly different (P<0.05) from the viscosity of the untreated ones,
60
whereas the water bath heated samples were significantly different (P<0.05) from the
other two.
Viscosity of untreated and in-shell heated egg white
Calc Std Deviation
at the data points
4.5
4
Viscosity, centipoise
3.5
Untreated
Mcrowave heated
3
Waterbath heated
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
Temperature, °C
Figure 4.3. Viscosity of the egg white of untreated and in-shell heated eggs.
Foam density and foam stability
Good foaming property is associated with a low foam density. The analysis performed
on the foam density data indicated that heat treated egg white samples had significantly
higher values than untreated egg white samples (Figure 4.4).
The foam density of the egg white of the microwave heated in-shell egg was lower
than that of water bath heated in-shell eggs making them more suitable for commercial
applications.
The foam stability reported as the amount of drained liquid as a function of time
indicated that microwave heated samples had a foam stability closer to that of the
61
untreated eggs (Figure 4.5), whereas the water bath heated samples had poor foam
stability.
Foam density of untreated and in-shell heated egg white
Calc Std Deviation at
the data points
0.3
0.25
Foam density, g/cc
0.2
0.15
0.1
0.05
0
Untreated
Microwave Heated
Waterbath Heated
Treatments
Figure 4.4. Foam density of the egg white of untreated and in-shell heated eggs.
Foam Stability of Untreated and in-shell heated egg white
Calc Std Dev at the
data points
60
50
Drainage, ml
40
30
20
Untreated
Microwave Heated
10
Waterbath Heated
0
0
15
30
45
60
75
90
105
120
135
150
165
180
195
Time, min
Figure 4.5. Foam stability of the egg white of untreated and in-shell heated eggs.
62
Statistical analysis (ANOVA & Duncan’s Test) revealed that the differences in foam
stability between the microwave heated in-shell egg and the untreated ones was not
significant (P<0.05). The stability of the foam made with the egg white of the eggs
heated in the water bath was significantly lower than the two others.
Turbidity
The analysis performed on the turbidity of the egg white samples measured as
the absorbance at 650 nm indicated that the microwave heated in-shell egg egg white
had better transmittance than the water bath heated ones (Figure 4. 6 and 4.6.1).
% Turbidity (650nm) of Untreated and inshell heated egg white
60
C alc S td D eviation at the data points
Turbidity %
50
40
30
20
10
0
Untreated
Microwave Heated
Waterbath Heated
Treatment
Figure 4.6. % Turbidity (650nm) of untreated and in-shell heated egg white
63
Figure 4.6.1 Turbidity of treated and untreated egg white
Differences in mean turbidity values for the egg white taken from untreated,
microwave heated or water bath heated eggs were all significant at the 0.01 level.
Absorbance values of the microwave heated samples were closer to the untreated ones.
Dielectric properties
The dielectric constants and loss factors of the egg white of untreated eggs and
of eggs heated in a microwave oven or in a hot water bath as a function of temperature
are presented in Figure 4.7. The trends observed in the dielectric properties measured
at 2450 MHz of egg white of microwave heated in-shell eggs were similar to that of the
untreated eggs.
64
ε' (2450 MHz) for untreated and in-shell heated egg white
71
Calc Std Dev at the
data points
69
67
65
ε'
63
61
59
Untreated
Microwave Heated
57
Waterbath Heated
55
0
10
20
30
40
50
60
70
Temperature, °C
ε"(2450 MHz) for untreated and in-shell heated egg white
20
Calc Std Dev at the
data points
19
18
17
16
ε" 15
14
Untreated
13
Microwave Heated
Waterbath Heated
12
11
10
0
10
20
30
40
50
60
70
Temperature, °C
Figure 4.7. Dielectric properties of the egg white of untreated and in-shell heated eggs.
The dielectric properties of the egg white of the eggs treated in hot water
exhibited a complete change in trend as they became directly proportional to the
temperature. This behaviour was associated with a greater denaturation of proteins in
65
these samples. The change in dielectric properties with temperature got reversed with
denaturation (i.e.) the dielectric properties were inversely proportional to the temperature
when raw (untreated) and became directly proportional to temperature when denatured
(Bircan and Barringer 2002).
Statistical analysis (GLM on curves and ANOVA and Duncan’s test on the slopes
and intercepts) revealed significant differences (P<0.05) among all the samples.
4.5 Conclusions
The effects of microwave heating and hot water bath heating of in-shell eggs on
the functional properties of the egg white was assessed and compared to that of
untreated eggs. It was demonstrated that enthalpy denaturation was much higher for the
microwave heated in-shell egg white and also it was more clearer and had a higher
viscosity. The microwave heated egg white produced a more stable foam with low foam
density. Also the dielectric properties gave a lucid idea about the extent of denaturation
in all the three samples.
From the data obtained from all the tests conducted it had been confirmed that
though there was a considerable change in all the above tested parameters in the
microwave heated in-shell egg white, the changes were much less when compared to
that of the water bath heated ones and the microwave heated ones were more closer to
that of the raw (untreated) egg white.
Thus microwave was proven to be a viable and better alternative for the in-shell
heating and pasteurization of shell eggs.
66
Acknowledgements
The financial support by the Canadian International Development Agency (CIDA)
and Natural Sciences and Engineering Research Council (NSERC) of Canada is greatly
acknowledged.
4.6 References
Bircan, C., and S.A. Barringer. 2002. Use of dielectric properties to detect egg
protein denaturation. Journal of Microwave and Electromagnetic Energy
37 (2):89-96.
CEMA. 2004. The Canadian Egg Industry Fact Sheet, edited by CEMA:
Canadian Egg Marketing Agency.
Cunnningham, F.E. 1995. Egg-Product Pasteurization. In Egg Science and
Technology, edited by W.J.Stadelman and O.J.Cotterill. New York: Food
Products Press.
Fleischman, G.J. . 2004. Microwave pasteurization of shell eggs. In IFT Annual
Meeting. Las Vegas, USA: IFT.
FSIS-USDA. 2006. Risk Assessments for Salmonella enteritidis in Shell Eggs
and Salmonella spp. in Egg Products. Omaha, NE: FSIS.
Griffiths, M.W. 2005. Issues Related to the Safety of Eggs and Egg Products.
Chile: University of Chile.
67
Hou, H., R. K. Singh, P. M. Muriana, and W. J. Stadelman. 1996. Pasteurization
of intact shell eggs. Food Microbiology 13:93-101.
HP. 1992. Dielectric Probe Kit 85070A. In Test and Measure Measurements,
edited by R. D. Unit. Palo Alto, CA: Hewlett Packard Corporation.
Iesel Van der Plancken, Ann Van Loey, and Marc E. Hendrickx. 2006. Effect of
heat-treatment on the physico-chemical properties of egg white proteins: A
kinetic study. Journal of Food Engineering 75 (3):316-326.
McDonnell, L.R., R.E. Feeney, H.L. Hanson, A. Campbell, and T.F. Sugihara.
1955. The functional properties of the egg white proteins. Food
Technology 9:49-53.
Mermelstein, Neil H. 2001. Pasteurization of Shell Eggs. Food Technology,
December 2001, 72,73 &79.
Pitsilis, J.G., H.V. Walton, and O.J. Cotterill. 1975. The apparent viscosity of egg
white at various temperatures and pH levels. Transactions of ASABE
18:347-349.
Rehkopf, A. 2005. Quality validation of a microwave-pasteurization process for
shell-eggs. Paper read at IFT Annual Meeting, at New Orleans, Lousiana.
Schroeder, Carl M., Alecia Larew Naugle, Wayne D. Schlosser, Allan T. Hogue,
Frederick J. Angulo, Jonathon S. Rose, Eric D. Ebel, W. Terry Disney,
Kristin G. Holt, and David P. Goldman. 2005. Estimate of Illnesses from
68
Salmonella Enteritidis in Eggs, United States, 2000. Emerging Infectious
Diseases 11 (1):113-115.
Schuman, J.D., B.W. Sheldon, J.M. Vandepopuliere, and H.R. Ball Jr. 1997.
Immersion heat treatments for inactivation of Salmonella enteritidis with
intact eggs. Journal of Applied Microbiology 83:438-444.
St. Louis, M.E., D.L. Morse, and M.E. Potter. 1988. The Emergence of grade A
eggs as a major source of Salmonella enteritidis infections : new
implications for the control of salmonellosis. Journal of American Medical
Association 259:2103–2107.
Woodward, D. L., R. Khakhria, and W. M. Johnson. 1997. Human Salmonellosis
Associated with Exotic Pets. Journal of Clinical Microbiology 35 (11):27862790.
69
Chapter 5
SUMMARY AND CONCLUSION
Eggs play an important role in our everyday meal. Egg infected with Salmonella
enterica serovar Enteritidis (SE) is a leading cause of food borne Salmonellosis. Recent
outbreak of avian influenza has raised the concern of the risk of spread in humans
through consumption of sick birds and eggs. Pasteurization is a heat treatment that is
effective to inactivate food borne pathogens such as SE and the avian flu virus. The
microbial safety of the egg needs prime consideration, especially when it comes to raw
consumption. Enhancing the safety of the eggs through heat pasteurization adversely
affects its functional quality, which is inevitable but minimizable by using appropriate
technology.
The current technology consists of immersing the eggs in hot water bath. It takes
hours to complete the process and some level of thermal degradation of proteins cannot
be avoided. Alternatively, microwave can be used to raise the temperature of in-shell
eggs to the required pasteurization temperature in minutes. The complexity of the
interaction between microwave and food materials makes it difficult to predict heating
patterns. This is why we have chosen to use an experimental and modeling approach.
The study consisted of developing regression models for dielectric behavior of
the egg components, studying the heating rates of the egg white and yolk in and out of
70
the shell and finally comparing the functional quality through certain physical properties
of the egg of the microwave and water bath pasteurized egg white
The regression models developed from the dielectric property measurements
gave a better understanding of the behaviour of the eggs components in a microwave
environment. The results of the study indicated that shell eggs were well suited for
microwave heating and perfect pasteurization with limited changes in quality can be
achieved using microwaves. Further investigation is necessary to explain the anomalous
heating behaviour of the in-shell yolk in the microwave environment.
Microwaves proved to be an inimitable alternative for the conventional water bath
pasteurization technique for in-shell eggs. The results of this study can be used for
further investigations and development of a continuous microwave in-shell egg
pasteurization unit. Microwave pasteurization of in-shell eggs could become a significant
breakthrough for the poultry industry. Successful commercial applications will yield new
and safer products as pasteurized in-shell eggs are not currently sold in Canada. Infants,
pregnant women, sick person, immune-suppressed individuals and the elders stand to
benefit the most from the introduction of this new line of products.
71
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76
Appendix
ANOVA table for ε” of egg white
Source
Model
Sum of
squares
DF
Mean
squares
2
22939.031
11469.515
Error
647
1864.564
2.882
Corrected Total
649
24803.595
F
Pr > F
3979.899
< 0.0001
ANOVA table for ε” of egg white
Source
Sum of
squares
DF
Model
Mean
squares
2
21398.501
10699.251
Error
647
1179.588
1.823
Corrected Total
649
22578.089
F
Pr > F
5868.504
< 0.0001
ANOVA table for ε’ of egg yolk
Source
Sum of
squares
DF
Model
Mean
squares
2
18146.746
9073.373
Error
597
1479.822
2.479
Corrected Total
599
19626.568
F
Pr > F
3660.443
< 0.0001
F
Pr > F
ANOVA table for ε” of egg yolk
Source
Model
Sum of
squares
DF
Mean
squares
2
5224.386
2612.193
Error
647
705.017
1.090
Corrected Total
649
5929.403
77
2397.231
< 0.0001