International Conference on Agricultural and Food Engineering (Cafei2016)
23-25 August 2016
CAFEi2016-228
Moisture sorption isotherm characteristics of instant
cassava (Manihot Esculenta Crantz) at various fitting
models
I.M.A.S. Wijaya1,a, P.P.E. Fitriani2, I.B.W. Gunam3 and L.P. Wrasiati3
1 Department of Agricultural Engineering, Faculty of Agricultural Technology, Udayana University; 2 Food
Science and Technology Magister Program, Udayana University; 3Department of Agro-industrial Technology,
Faculty of Agricultural Technology, Udayana University
Abstract
Instant cassava is dried cassava that processed by soaking the fresh cassava into
salt brine at 50º C for three hours, cooked in pressure cooker for 12 minutes, then
freezed at -15 ±2°C for 72 hours before dried at 60ºC until moisture content of 3%.
This study aims to study the characteristics of moisture sorption isotherm of instant
cassava at various fitting models and to determine the best fitting model. Samples
were analyzed at 28±2°C using standard gravimetric static method over a range of
equilibrium relative humidity (ERH) from 6.90%-97.90%. The experimental data
were fitted into seven equations commonly applied in food e.g. Brunauer-EmmetTeller (BET) model, Oswin model, Halsey model, Henderson model, Caurie model,
Chen-Clayton model and Guggenheim-Anderson-de Boer (GAB) model. The best fitting
model was analyzed using Mean Relative Determination (MRD) analysis. The results
indicated that the moisture sorption isotherm of instant cassava followed type II
moisture sorption isotherm behavior. The Henderson model was the best fitting
model for moisture sorption isotherm characteristic of instant cassava, since it has
the lowest value of MRD coefficient (MRD coefficient= 4.47)
Keywords: instant cassava, equilibrium moisture content, moisture sorption isotherm
INTRODUCTION
In order for food diversification, local food may be expected to be all-ages food
consumption. Lifestyle and high mobility, encouraging the development of various instant
food, including the local one. Fresh cassava undergoes postharvest physiological
deterioration needed postharvest handling to keep the quality, as make it into instant
cassava. Instantiation of any food product will affect the equilibrium moisture content which
changes the nature of itself (Moreno et al., 2003).
The knowledge of the moisture sorption isotherm is critically important in food
science and technology. Moisture sorption isotherm describes the relationship between
water activity (aw) and moisture content of food material in a storage condition at certain
relative humidity (RH). Syarief and Halid (1993) stated that residual from various chemical
reactions of the material of food are accumulate and reversible during storage and caused
unacceptable way of customers. Unacceptable that came from customers due to the
accumulation chemicals reaction is called expiry of date. However the appearance of its food
product, if it have reached the optimum of the end of time, in general the food product has
experienced the change of quality, like nutrient value and taste.
Most of instant foods have low aw, where the condition will not let the bacteria and
leavened growing, except several kind of mold/mildew which needed a very low aw. The
level of quality can decrease caused by the storage condition (Arpah 1998). Labuza (1984)
a
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declared the dried instant food or snack losses the crispiness over the range of a w of 0,2-0,4
and the sugar/glucose becomes sticky at the range aw of 0,4. The lowest of aw where the
spoiled bacteria could be growing is in the range aw of 0,90. The rotten/spoiled named S.
Aureus could hampered by the way of anaerobe at the range aw of 0,91, but in the anaerobe
way at 0,86. Meanwhile for the growing of the fungi and yeast are at the aw of 0,61 and for
the mythoxigenic fungi is at the range aw of 0,78. At this condition of aw, it caused many
reactions and reductions such as enzyme and non-enzyme browning, vitamins degradation,
proteins denaturation, lipid oxidation and scratch retrogression.
The most used method to study on sorption adsorption phenomena is sorption
isotherms, which are important to analyze and design the food processes and can be used to
predict changes in food stability (Zhang et al., 1996). Mathematical models are available in
the literature for describing moisture sorption isotherms such as BET (Brunauer-EmmetTeller) model, Oswin model, Halsey model, Henderson model, Caurie model, Chen-Clayton
model and Guggenheim-Anderson-de Boer (GAB) model, even though no single model is
fully enough to represent all the sorption isotherms of foods.
BET model is widely used to model the food system that first proposed by Brunaurer,
Emmet and Teller (Brunaurer et al., 1938). The BET model represents a basis in the
interpretation of moisture sorption isotherm that has been applied in various food products,
like yam (Montes at al. 2009), dried tomato (Goula et all. 2008) and Blueberry (Vega et al.,
2009). Oswin model (Oswin, 1946) is empirical model that has been applied in Mango pulp.
Halsey model provides a good representation of adsorption data that was found to be
suitable for starchy food product like corn flour (Vega et al., 2006). Just like Oswin model,
the Henderson model (Henderson, 1952) also fitted for starchy food, protein, meats and
fruits (Chirife et al., 1978). On the other hand, the Caurie model could be used for various
dried product. GAB model that comes from Guggenheim, Anderson and De Boer (McKenna.
1984) has been widely used of various food products and recommended by the European
project Group COST 90 on Physical Properties of Food (Wolf et al., 1985). The objectives of
this research were to study the characteristics of moisture sorption isotherm of instant
cassava at various fitting models and to determine the best fitting model.
MATERIALS AND METHODS
Materials
Samples were made from fresh cassava which purchased from local supply, Tiara
Dewata Super Market, Denpasar. The utensils used including chef knife, cutting board, tray,
tongs, gloves, wooden spoon, masks, pressure cooker, gas stove, freezer, oven and incubator.
Other utensils used were chambers, hygrometer, glass petri dishes, and analytical balance.
The chemicals used for generating saturated salt solutions, included NaOH, MgCl2, K2CO3, KI,
NaCl, KCl, BaCl2, K2CrO4, (NH4)H2PO4 and K2SO4.
Sample Preparation
Fresh cassava was made into dice form with the dimension of 0,5 cm x 0,5 cm x 0,5 cm
and soaked in salt brine at 50º C for three hours. The samples were cooked in pressurized
cooker for 12 minutes. Then the samples were cooled after cooked process and kept in the
freezer at -15 ±2ºC for 72 hours. Samples were then thawed before dried at 60ºC. The
sampleswere dried until moisture content of 3% (Wrasiati et al., 2013).
Moisture Sorption Isotherm Characteristic Analysis
Saturated salt solutions were made in each chamber by adding distilled water
covering various equilibrium relative humidity (ERH) from 6.90%-97.90% (validated by
using hygrometer and temperature meter) and kept it for period of 24 hours at 28±2ºC
before it could be used. The equilibrium relative humidity of saturated salt solutions of
NaOH, MgCl2, K2CO3, KI, NaCl, KCl, BaCl2, K2CrO4, (NH4)H2PO4 and K2SO4 was 6.9, 32.4, 43.0,
69.0, 75.5, 84.0, 90.3, 92.7, 97.0, and 97.9%, respectively. The experimental sorption curves
were then fitted by plotting constant weight of experimental instant cassava moisture
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International Conference on Agricultural and Food Engineering (Cafei2016)
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content as Y axis and aw as X axis. Water activities values were derived from the ERH divided
by 100 (Labuza, 1980).
The Fitting of Various Models
Seven models, BET model, Oswin model, Henderson model, Halsey model, Caurie
model, Chen-Clayton and GAB model have been proposed to fit the equilibrium moisture
content of the sample to its water activity. The various models were fitted on the
experimental data and evaluated statically by calculating MRD at the probability value of <5.
Addition to BET and GAB model, the others models have been modified into linear forms as
seen at Table 1.
Table 1. Linearization Forms of Various Models
Models
Linearization Forms
Oswin*
ln me=ln P(1)+P(2)ln[aw/(1-aw)]
Hasley*
log [ln(1/ aw)]=log P(1)-P(2)log me
Henderson*
log(ln(1/1- aw))=log K+n log me
Caurie*
ln me= ln P(1)-P2 aw
ChenClayton*
ln (ln(1/ aw))=ln P(1)-P(2) me
y = ax + b
y= ln me
x= ln (aw /(1- aw)
a=ln P(1)
b= P(2)
y=log[ln(1- aw)]
x= log me
a= log P(1)
b= -P(2)
y= log(ln(1/(1- aw))
x= log me
a= log K
b= n
y= ln me
x= aw
a= ln P(1)
b=-P2
y= ln (ln(1/ aw))
x= me
a= ln P(1)
b= -P2
* Source: Rahayu et all. (2005)
MRD =
∑
References
Chirife and
Iglesias (1978)
Isse et al (1993)
Lamauro (1984)
Lamauro (1984)
Lamauro (1984)
(1)
Where:
mi
= Experimental moisture content data (%)
mpi
= Calculated moisture content data (%)
n
= Data quantity
MRD < 5
= Precise model
MRD < 10
= Almost precise model
MRD > 10
= Imprecise model (Isse and others. 1993)
RESULTS AND DISCUSSION
Equilibrium Moisture Content
Instant cassava is a hygroscopic food product that could adsorb the water vapor from
the environment to its material. The adsorption happened during the study from the
saturated salt solution into product to the state of constant. It has been known the minimum
of instant cassava moisture content using BET and GAB models to keep its stability, so it is
able to be the basic directors to select appropriate packaging materials. The experimental
data showed that the moisture sorption followed type II behavior (Labuza 1984). The
relation between instant cassava equilibrium moisture content and the water activity can be
seen at Figure 1.
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Figure 1. Moisture sorption isotherm of instan sassava
Figure 1 showed that the moisture content of instant cassava increased by the
increasing of water activity at the storage condition. Based on the experimental data, m e
value at the lowest aw of 0.069 was 0.041 g/H20/g dry solid and at the highest aw of 0.979
was 0.294 g H20/g dry solid.
The curve pattern of instant cassava during storage followed the sigmoid type II
behavior, which demonstrated its capability in absorb a little water vapor that neither
descend nor ascend to achieve a lapse aw of 0.7-0.8 (Labuza, 1984). This is a type of dried
food product, namely sigmoid shape. The pattern sigmoid formed due to stock exchange
colligative, capillaries and interaction between the surfaces (Labuza, 1984). The sigmoid
type II behavior was reported from several research that also contained starch, such as
dried cassava flour (Septianingrum, 2008), cassava rice (Widowati et al. 2010), cassava flour
(Famurewa et al. 2012), instant corn flour (Aini et al. 2014) and grain cereal (Zapata et al.
2014). In this type, there are 2 arches that demonstrate its existence of physic-chemical
changes of instant cassava’s water binding. First one, which is at aw of 0,069 and the second
one is at aw of 0.69.
The Fitting of Various Models to Moisture Sorption Isotherm of Instant Cassava
The experimental moisture sorption data produced curve that less than perfect, thus
the data were fitted to 7 models to get a smooth curve (see Figure 2). Seven models that
were used in this research, namely BET, Oswin, Halsey, Henderson, Caurie, Chen-Clayton
and GAB models. The models were chosen due to their suitability to describing the moisture
sorption curve in large range of aw (Isse et al, 1983). These mathematical equations are
converted to linear form to determine coefficient of drag coefficient that is needed in order
to make it easier for the count. The equation of each model are presented at Table 2.
(a)
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(c)
(d)
(e)
(f)
(g)
Figure 2. Isotherm moisture sorption curve of (a) Brunauer-Emmet-Teller (BET) model, (b)
Oswin model, (c) Halsey model, (d) Henderson model, (e) Caurie model, (f) ChenClayton model, and (g) Guggenheim-Anderson-de Boer (GAB) model
Cassava is composed of 0.30 per 100 g lipid content (Anon, 1981), where the
monolayer moisture content of instant cassava have been predicted by using BET and GAB
model, which were 0,072 g H2O/g solid and 0,081 g H2O/g solid, respectively. This amount
of moisture content has significance effect in storage and product distribution, where this is
the minimum amount of water that are bound to not be able to be used for the growth of
microorganism, chemical reaction, and biological reactions to make the longer shelf life
(Septianingrum, 2008). The more fat content in a product, the more decrease the number of
water that can be bound by a food in the upper layers monolayer.
Guggenheim-Anderson-de Boer model has been used based on its theoretical
background, which reflects the absorption pattern in the range of large aw from aw of 0 to aw
of 0.90. This model is a model that suitable to be used in food product of fruits, meats and
vegetables. However, in this research GAB model is a model that quite accurately describes
characteristics of instant cassava, with the value of MRD less than 10 and coefficient of
determination (R2) is 0.9127 from polynomial regression.
In most models, the constant values are only numbers in a product characteristic and
cannot be explained by theoretical backgrounds, but the BET and GAB models produces
constant values which can be explained. As well as the BET model, the GAB model also been
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used to calculate the monolayer moisture content, which produced the similar result, which
is 0,081 g H2O/g solid. The monolayer moisture content prediction of GAB model is higher
than the BET model. It might be caused the GAB model consider there is a multilayer on the
monolayer one. This is similar to a study by Wariyah and Supriyadi (2010) in rich calcium
rice that has moisture content of 0.0721-0.0867 g H2O/g solid. And also by Liendo-Cardenas
et al. (2000) in cereal products cassava with moisture content of 0.061-0.097 g H2O/g solid
in four different temperatures. In a study by Houssein (2007) in pasta dates was getting
moisture content of 0,084 g H2O/g solid using a model GAB. The value of the monolayer
moisture content (mo), indicates the amount of water that is strongly adsorbed to specific
sites at the food surface, and this is a value that must be reached in order to assure its
stability.
The suitability of these seven models were evaluated statically by using Mean Relative
Determination (MRD) method with the value of < 5 describes that the model was precise, <
10 describes the model was almost precise and > 10 describes the model was imprecise. The
counted result of MRD can be seen in the Table 2.
Table 2. The MRD value of various models
Model
BET
Equations
=(
)(
Oswin
Halsey
Henderson
Caurie
ChenClayton
GAB
(
[
(
)
MRD Value
0.46
)
9.93
]
)
= exp( - 0.00237 /
(
ln me=-3.140+1886 aw
(-2.453)
(
19.03
4.47
10.46
5.37
)
)
6.36
(
)(
)
Out of the seven models, the Henderson one presented the best behavior to the MRD
of 4.47. Models that almost accurately reflect the adsorption pattern of instant cassava with
the MRD less than 10 are the Chen-Clayton, GAB and Oswin models. While, models which
imprecise to describe the adsorption pattern of instant cassava with the MRD more than 10
are the Halsey and Caurie models. Although BET model produced MRD value of 0.46, the
compatibility of BET model is only able to predict with good only until aw 0.43.
CONCLUSIONS
The following conclusions can be drawn from the study:
- Moisture Sorption isotherms of instant cassava followed type II behavior which
frequently found in dry food product.
- The behavior of instant cassava equilibrium moisture at 28°C is satisfactorily
described by means of the moisture sorption isotherm of Henderson model.
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