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Comparison of the Sorption IsothermCharacteristics of Two Cassava ProductsD.I.O. Ikhu-Omoregbe aa School of Chemical Engineering, University of KwaZulu-Natal ,South AfricaPublished online: 06 Feb 2007.

To cite this article: D.I.O. Ikhu-Omoregbe (2006) Comparison of the Sorption IsothermCharacteristics of Two Cassava Products, International Journal of Food Properties, 9:2, 167-177, DOI:10.1080/10942910600592026

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International Journal of Food Properties, 9: 167–177, 2006Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print / 1532-2386 onlineDOI: 10.1080/10942910600592026

167

COMPARISON OF THE SORPTION ISOTHERM CHARACTERISTICS OF TWO CASSAVA PRODUCTS

D.I.O. Ikhu-OmoregbeSchool of Chemical Engineering, University of KwaZulu-Natal, South Africa

The sorption characteristics of two cassava products were obtained using the gravimetricmethod at three temperatures, 20, 30, and 40°C. The GAB, BET, and the modified BETmodels were found to describe the data reasonably well. The heat of sorption, which variedfrom 39.47 kJ/mol at 1% moisture content to 4.92 kJ.mol−1 at 20% moisture content drybasis, was calculated for the garified product. Similar values for the ungarified productwere found to vary from 26.98 kJ/mol at 1% moisture content to 4.97 kJ/mol at 20% mois-ture content. The shelf-life for both products were estimated from sorption data and recom-mendations on their packaging and storage conditions given. The garified product wasfound to have a higher capacity for moisture adsorption and also potentially stores longerthan the ungarified product.

Keywords: Cassava products, Excess heat, Isother, Monolayer moisture, Shelf-Life, Sorption.

INTRODUCTION

Cassava is a tropical root crop that can be processed into different types of foodproducts in tropical regions especially in West African countries like Nigeria, Benin,Togo, Ghana, and Cameroon. One of the two products of cassava studied in this paper isobtained by grating the peeled roots, fermenting the mash, dewatering, and garifica-tion.[1,2] Garification is a simultaneous process of cooking and drying dewatered cassavato obtain a crisp product called gari and may be milled to size as required. Dewateringcould be done simultaneously as the fermentation process. Another product, which is onlysuitable for animal consumption because it is not cooked, is obtained by spreading thedewatered and fermented product on a suitable surface under the sun for a number of daysuntil dried. In this paper, we refer to the first product as garified and the second as ungari-fied. While improvements have been made in the processing of these products, a lot moreis necessary with regard to their packaging and storage.

The shelf-life of packaged cassava (food) products is influenced by the storage tem-perature, relative humidity, and moisture content and hence the water activity (aw) of theproduct. It is known that the rate of microbial and physico-chemical deterioration is influ-enced by water activity of stored cassava (food) products.[3–5] Chuzel and Zakhia[6]

observed that at high water activities, the moisture content increased with increasing

Received 24 October 2003; accepted 23, May 2004.Address correspondence to D.I.O. Ikhu-Omoregbe, School of Chemical Engineering, University of

KwaZulu-Natal, Durban 4041, South Africa. E-mail: ikhuomoregbe@ukzn.ac.za

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168 IKHU-OMOREGBE

temperature, resulted in a crossing of the isotherms at aw between 0.5 and 0.7. This phe-nomenon was explained due to changes that occurred in the starch structure and modifica-tion of the available water sorption sites.

Oyeniran[7] reported growth of moulds during storage of white and yellow gari at18.6 and 18.1% initial moisture content, respectively, or dried to 11.2 and 10.8% moisturecontent, respectively. He also reported that the discoloration and complete deterioration ofgari stored at above 18% moisture content in polythene bags after 16 weeks was as a resultof the growth of moulds on the products. However gari at less than 12% moisture can bestored safely in polythene bags for 2 months (8 weeks). Ekundayo[8] also observedchanges in colour and odour resulting from microbial spoilage of gari stored in polythenebags after 10 weeks. It is thus evident that gari could not be stored well due to its hygro-scopic nature.

A moisture sorption isotherm equation is used to describe mathematically the rela-tionship between the activity (aw), and the equilibrium moisture content of a food product.Moisture sorption isotherms are used for a number of purposes in food research.[9] Theseinclude calculations for drying time, ingredient mixing predictions, packaging predictionsand modeling moisture changes which occur during storage and prediction of shelf-lifestability.[10] It also supplies fundamental information about specific interaction betweenwater and the product since it directly relates the thermodynamic potential (or Gibbs freeenergy) of water in the system to its mass fraction. Related thermodynamic propertiessuch as the enthalpy of sorption as well as information on the structure of the product (e.g.specific surface area, pore volume, and crystallinity in some cases) can be derived.[11]

A number of empirical, semi-empirical, or theoretical models have been derived forthe correlation of water sorption in food substances. Among these, the three-parameterGuggenheim-Anderson-de Boer (GAB), Brunauer, Emmet and Teller, (BET) and modi-fied BET equations have been found to be more popular and reliable.[11–14] They arewidely used in determining monolayer sorption values and specific surface areas of sor-bent material. Whereas the GAB equation is found applicable over a wide range of aw, val-ues, the BET is applicable at low aw (aw < 0.43) and the modified BET gives a good fit ataw less than 0.75.

This paper presents the data for the sorption characteristics of two cassava products andthe results of the application of these equations to their sorption isotherms. Theoretical shelf-life is deduced for both materials from their sorption characteristics. Furthermore the bindingenergy or excess heat of sorption will be calculated using the data obtained in this work.

MATERIALS AND METHOD

The two cassava products were obtained from local markets in Edo State of Nigeria,packaged in sealed polythene bags and transported to Zimbabwe where the author wasresiding within two days. The composition of the two materials is given in Table 1. Thesamples were then stored at 2–4°C until the sorption isotherms were determined. Beforeeach experimental run, the products were milled into powder, sieved to obtain a size rangeof +700 μm to −1400 μm that was used for the sorption isotherm determination. Sorptionisotherms of these materials were measured at 20, 30, and 40°C using a standard gravimet-ric method as recommended by COST 90 project with thermally stabilized desiccators.[15]

The temperature of the desiccators was maintained by placing them in a thermostaticallycontrolled water bath set at the required temperature. The desiccators contained saturatedsalt solutions that create known relative humidity in the surrounding atmosphere.[16] The

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COMPARISON OF THE SORPTION ISOTHERM 169

saturated salt solutions used in this study are LiCl, CH3COOK, MgCl2, K2CO3, NaBr,CuCl2, NaCl, (NH4)2SO4, and KNO3 with corresponding water activity of 0.11, 0.23, 0.33,0.43, 0.57, 0.67,0.75, 0.79, and 0.93, respectively, at temperatures from 20 to 40°C.[17]

The materials were first dried over P2O5 for a period of about 2 weeks to ensure that theywere very dry and at the same initial moisture content. About 2 gm portions of the material intriplicates placed on watch glasses were exposed to the different humidity in the desiccators. Inorder to prevent mould growth at high aw, 0.25% of sodium azide was applied to the samples.The weights of the samples were determined on 2-day intervals until constant weights wereobtained. The length of each experimental run was about 2 weeks. The time interval for theremoval, weighing, and replacing the sample in the desiccators was less than one minute tominimize any effect on opening and closing on the results. The moisture content of the equili-brated samples was determined by drying each sample in an oven at 110°C for between 16 and24 hours. The samples are cooled over silica gel before the final weights were taken.

Adsorption Models

The GAB equation is claimed to provide the best equation for the description offood isotherms up to aw 0.9[18] and also adopted by the EEC-COST 90 Group on wateractivity.[19] The transformed GAB is given as:

where X is percentage water content, dry basis, Xm is water content corresponding tooccupation of all primary adsorption sites by one water molecule (Xm is also called the“monolayer moisture”). C and K are GAB constants that are temperature dependent; C is aconstant (also referred to as Guggenheim constant) and K is correction factor for the mul-tilayer molecules. The coefficient α, β and γ of the GAB model were determined for eachtemperature using non-linear regression as recommended by Schar and Ruegg[13] and thevalues of the GAB constants (Xm, C, and K) were also calculated.

The BET sorption isotherm given below is applicable at low aw was also tested.

Table 1 Initial Composition of the two materials.

Garifiedwt %

Ungarified wt %

Moisture 13.62 20.50Carbohydrates 84.27 77.41Crude Protein 1.03 1.03Fat 0.25 0.24Total phosphorus 0.82 0.81Others < 0.01 < 0.01

a

Xa a

C C

ww w= + +

= −⎡⎣⎢

⎤⎦⎥

= −⎡⎣⎢

⎤⎦

α β γ

α β

2

11 1

2where

K

X

1

Xm m

; ⎥⎥ =; 1

Xm

γCK

(1)

XX Ca

a C am w

w w

=− + −( ) ( )[ ]1 1 1

(2)

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170 IKHU-OMOREGBE

The BET equation can be modified to give another equation, which is applicable for aw upto 0.75 can be written as:

The fitting confidence was determined using the relative root mean square error (% RMS)estimate.[13]

Moisture-Binding Energy

The water activity of a food substance, which is made up of non ideal mixtures, isknown to be a function of temperature; hence it is important to know the effect of temper-ature on its sorption isotherm. Temperature affects the mobility of water molecules, andthe equilibrium between the vapor and adsorbed phases. An increase in temperature, atconstant water activity results in a decrease in the amount of adsorbed water.[20] However,for certain sugars and low molecular weight food constituents, which dissolve in waterand become more hygroscopic at higher temperatures are exception to this rule.

The level of moisture content at which the differential heat of sorption approachesthe heat of vapourisation of pure water is often taken as indicative of the amount of“bound” water existing in the food.[21] Knowledge of the differential heat of sorption isvery important for drying process equipment design. This is because heat of vaporizationof sorbed water may increase to values above the heat of vapourisation of pure water asfood is dehydrated to low moisture levels.[22]

The binding energy is defined as the difference between the isosteric heat of watersorption by the solid substrate and the condensation heat of water vapor at the sametemperature. The relationship between activity and temperature can be described by theClausius-Clapeyron equation:

where ΔHS is the binding energy or excess heat of sorption.

RESULTS AND DISCUSSION

Adsorption Isotherms

The results of the sorption isotherm measurements are shown in Table 2. The sig-moid characteristic curves of the isotherms were obtained for the two products, shown inFig. 1, have a fairly similar pattern for both materials. The results suggest that the garifiedproduct has a stronger affinity for moisture at a given water activity. This implies that thegarified product is more hygroscopic compared to the ungarified product. However, thisdoes not suggest at this stage that the ungarified product is more stable and hence can storebetter. The crossing of adsorption isotherm described by Chuzel and Zakhia[6] was not

XX Ca

a C am w

w w

=− − −( ) ln( )[ ]1 1 1

(3)

d a

dT

H

RTw

X cons t

S(ln )

1 2⎛⎝⎜

⎞⎠⎟

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

=−

= tan

Δ (4)

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COMPARISON OF THE SORPTION ISOTHERM 171

observed in this study for either products, hence the stability of both cassava-based prod-ucts could be reduced with increase in storage temperature.

Tables 3 and 4 show the estimated parameters by nonlinear regression for GAB,BET, and Modified BET models for both products. The results show that these equationsdescribe the moisture sorption isotherms of both materials reasonably well as shown bythe values of correlation coefficients and the root mean square error (%RMS). The mono-layer moisture content (Xm) values obtained from the GAB analysis (Tables 3 and 4) at thethree temperatures and for both materials do not appear to be significantly different andthey are similar to values obtained for starchy foods.[23] While the monolayer values donot appear to have definite relationship to temperature within the temperature range cov-ered, they appear to depend on the particular model used. It can be observed that the Mod-ified BET tend to correlate the garified products better, whereas the GAB correlates theungarified product better within the water activity range for which the particular model issaid to be valid.

For the garified product the monolayer values ranged from 5.21 to 5.54 g. H2O/100g solid for the GAB model, 3.37 to 3.62 g . H2O/100 g solid for the BET model and 11.25to 11.98 g . H2O/100 g solid for the Modified BET. The corresponding monolayer values

Table 2 Equilibrium moisture contents, g H2O/100 g solids at different water activities for the two materials.

Water activity aw

Garified Ungarified

20°C 30°C 40°C 20°C 30°C 40°C

0.11 4.6 4.34 4.06 3.32 3.01 2.210.23 6.23 6.30 5.70 4.68 3.97 3.850.33 8.52 8.52 8.30 5.78 5.33 4.960.43 9.01 9.28 8.78 6.42 5.91 5.740.57 9.88 9.64 9.39 7.17 6.94 6.450.67 13.03 12.61 12.42 10.67 10.14 9.550.75 16.89 16.23 16.10 14.81 14.34 14.010.79 21.93 20.24 19.54 19.42 18.78 18.240.93 26.80 25.24 25.02 21.09 20.64 19.40

Figure 1 Comparison of the adsorption isotherms for the two products at 30°C.

0

5

10

15

20

25

30

0.23 0.43 0.67 0.8

Water activity, a

Mo

isru

re c

on

tetn

t, g

m H

2O

/10

0 g

m d

ry

soli

ds

garified

ungarified

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172 IKHU-OMOREGBE

for the ungarified product gave the same value of 12.97 g. At the three temperatures forthe GAB model, ranged from 5.90 to 8.78 g . H2O/100 g solids for the BET model andfrom 2.94 to 7.21 g . H2O/100 g solid for the modified BET. These values are similar tothose in literature.[6,23–29]

Comparison of these three models indicates that the modifield BET model gavethe highest monolayer moisture estimates, followed by the GAB model and then theBET equation for the garified product. On the other hand, the GAB gave the highestmonolayer moisture estimates than the other two models. The values of the root meansquare error (%RMS) for the garified products suggest that whilst both the GAB andBET gave a rather poor fit, the correlation with the Modified BET is reasonable. How-ever, similar comparison for the ungarified products shows that the GAB model gavesmaller values than the other two models. The GAB model can therefore be said to give

Table 3 Estimated parameters by nonlinear regression for GAB, BET, and Modified BET Models—garifiedproduct.

Fitting range, (aw)

XM Monolayer Moisture,

g . H2O/100 g solidC

Constant

K Correction

factor

r2 Correlation coefficient %RMS

GAB, (°C) 0–0.9320 5.21 5.44 0.79 0.978 17.7930 5.26 5.62 0.77 0.978 16.1640 5.54 5.63 0.73 0.979 16.42

BET, (°C) 0–0.4320 3.37 7.19 0.998 26.4830 3.62 8.13 0.994 20.2140 3.45 6.53 0.998 23.65

Modified BET, (°C) 0–0.7520 11.25 2.94 0.984 3.8730 12.25 3.81 0.987 10.6340 11.98 3.00 0.989 6.88

Table 4 Estimated parameters by nonlinear regression for GAB, BET, and Modified BET models—ungarifiedproduct.

Fitting range, (aw)

XM Monolayer Moisture,

g . H2O/100 g solidC

Constant

K Correction

factorr2 Correlation

coefficient %RMS

GAB, (°C) 0–0.9320 12.97 3.55 0.487 0.943 2.7030 12.97 7.29 0.728 0.954 5.2840 12.97 5.08 0.422 0.960 4.46

BET, (°C) 0–0.4320 8.78 1.72 0.971 24.2730 5.90 1.97 0.948 23.6540 7.31 1.81 0.956 23.89

Modified BET, (°C) 0–0.7520 7.21 1.84 0.976 21.0530 2.94 2.94 0.986 16.4640 5.65 1.44 0.991 19.19

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COMPARISON OF THE SORPTION ISOTHERM 173

a better fit compared to the other two models in the water activity range for which theparticular model is applicable.

The initial stages (grating, fermentation and dewatering) of production of theseproducts induce a starch damage of about 3–6%.[6] During the process of garificationcomplete gelatinisation does not take place due to the low initial moisture content of about1 g/g.[30] However, there is a loss of crystallinity and extensive swelling of the starch gran-ules resulting in crisp grain-like products when dry. A complex metastable network is saidto be formed that consists of armorhpous regions (containing plasticising water) and hydratedmicroscrystalline regions which did not dissolve during the partial gelatinisation and serveas junction zones.[31] Chuzel et al.[6] suggest that increase in both temperature and wateractivity initiates a collapse process, which makes the soluble starch (armorphous fractionsand branched segments) to leach out thereby increasing the number of available adsorp-tion sites (glucose residue). The drying of the ungarified products is much slower (dayscompare to less than 1 hour for the garified products) and also at a much lower tempera-ture (less than 40°C compared to above 100°C for the garified products). Consequently,there is neither partial gelatinization, nor swelling, and the products do not possess thecrisp and hard grains like the garified products. The net effect of these is that the ungari-fied products’ capacity for moisture adsorption is lower than that for the garified products.

Moisture-Binding Energy

From Eq. (4), plots of ln(aw) vs (1/T) at different moisture contents can be consid-ered as straight lines, whose slope yield ΔHS/ R, which can be calculated by regressionanalysis.[32] Table 5 shows the values of excess heat of sorption for both materialsobtained by such method of analysis. The values obtained varied from 4.92 kJ/mol at 20%moisture content to 39.47 kJ/mol at 1% moisture content, dry basis for the garified prod-uct. The values obtained for the ungarified product varied from 4.79 kJ/mol at 20% mois-ture content to 26.98 kJ/mol at 1% moisture content, dry basis. Fasina et al.[33] observedthat the heat of vaporization of gari reduces with increase in moisture content. The resultsalso show that the process of water sorption by both materials is endothermic. This agreeswith Chuzel and Zakhia[6] who observed that when the soluble fraction of gari starchundergoes collapse and leach out, the sorption phenomenon becomes endothermic rather

Table 5 Excess heat of sorption, kJ/mol as a function of moisture content for both materials.

Moisture Contentg H2O/g solids Garified material Ungarifeid material

1 39.47 26.983 38.60 25.255 34.23 21.176 30.26 18.468 21.23 13.4710 14.57 10.1111 12.34 8.9613 9.30 7.3315 7.41 6.2716 6.72 5.8718 5.78 5.2420 4.92 4.79

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174 IKHU-OMOREGBE

than the usual exothermic behavior found in sorption theory. The monolayer concept isalso very relevant to physical and chemical deterioration of dehydrated foods, such aslipid, enzyme activity, non-enzymic browning reactions, aroma retention and texturalcharacteristics.[12,34] Furthermore, Iglesias and Chirife[35] explained that as more water isadsorbed there is a decrease in sorption energy due to reduced activity at the sorption sites.The results also suggest that the garified product has greater interaction energy at the sorp-tion sites compared to the ungarified product. This is supported by the observation that thegarified product is more crisp and harder when felt with the fingers.

Application of aw to Shelf-Life Estimation

The establishment of the sorption isotherm of a packaged food can aid in the estima-tion of its shelf-life in given storage conditions. The model of Heiss and Eichner[36] can beused to estimate the potential storage time based on a critical aw for a particular systemunder given storage conditions. This model was based on the assumption that water sorp-tion vapor is the determining factor, amongst others such as the presence of spoilage bac-teria, oxygen, and light that could limit shelf-life. The equation is given as:

KS is permeability of the package of moisture vapor, kg . m−2 . Pa.−1. day−1, ts is potentialshelf life of product (time in days for the packaged product to spoil by microbial and bio-chemical deterioration with loss of sensory quality). A is surface area of package, m2; WSis weight of the product (dry matter), kg; P0, is the vapor pressure at storage temperature,Pa; S is slope of the products isotherm, assumed linear over the range Xe, and Xc. Xe isequilibrium moisture content, Xc is safe storage moisture content and Xi is initial moisturecontent of material when packaged, kg/kg dry basis.

The shelf-life of packaged cassava products in 5-kg polyethylene bags (A = 0.266 m−2;KS = 2.28 × 10−6 kg H2O . m−2 . Pa−1 . day−1) was estimated at three temperatures (20, 30, 40°C).For the purpose of estimation, we will assume ambient storage conditions for which therelative humidity is 0.90 (which is reasonable for the tropical conditions that prevail in thecountries where these products are a part of the staple food), and a water activity of 0.7.[6]

The theoretical shelf-life estimated for different initial moisture contents are shown inTables 6 and 7. The results indicate that the garified products will store for longer timecompared to the ungarified products as well as having a higher capacity to absorb mois-ture. This is due to the fact that the garified product was obtained by a more severe heat

tX X X X

K A W P SSe i e C

S S

=− −ln ( ) /( )

( / )( / )

[ ]

0

(5)

Table 6 Estimated shelf-life (days) for safe storage at aw 0.7 at different initial moisture content, (dry basis) forthe garified product.

Initial moisture content, % (d.b) 20°C 30°C 40°C

5 209 148 868 163 65 5310 85 58 3712 54 38 20

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COMPARISON OF THE SORPTION ISOTHERM 175

treatment process resulting in a harder and more crisp substance, and consequently made itless susceptible to mould attack.

It is possible to suggest that at these conditions for a low cost storage for at leastthree months shelf-life, one would recommend an initial moisture content of less than 8%for both products at a temperature of less than 30°C using polyethylene materials. Thisvalue must be taken with caution, as it is very difficult to dry the products to this lowmoisture level. Polyethylene material was chosen because of its low cost compared to themore expensive polypropylene, which has a lower permeability to water vapor and oxy-gen. In this article, attention was directed only to permeability of water vapor to polyethyl-ene, however, it must be mentioned that it is also permeable to oxygen and carbon dioxide,which can cause oxidation and hence deterioration.

CONCLUSION

The sorption isotherms at temperatures of 20, 30 and 40°C for the two cassava-based products were measured using the conventional gravimetric method. The resultsshow that the sigmoid characteristic curves of sorption isotherms were obtained asexpected. The data obtained were well described by the three models tested. However, theBET gave a poorer fit as it gave a rather high %RMS values for both products. The mono-layer water content values were found not to be a particular function of temperature butwere dependent on the particular model used for its estimation. Excess heat of sorptionvalues varied from 4.92 kJ/mol at 20% to 39.47 kJ/mol at 1% moisture content, dry basisfor the garified product. For the ungarified products, the values ranged from 4.97 kJ/mol at20% to 26.58 kJ/mol at 1% moisture content, dry basis. A theoretical estimation of theshelf-life for both materials shows that a low cost storage of at least three months can beobtained using 5 kg polyethylene bags for both products at less than 8% moisture content.It was observed that though the garified product has a higher moisture adsorbing capacity,it also has a longer safe storage period. This was explained to be due to the more severeheat treatment process for the garified product which resulted in a harder, more crisp andhygroscopic material.

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Table 7 Estimated shelf-life (days) for safe storage at aw 0.7 at different initial moisture content, (dry basis) forthe ungarified product.

Initial moisture content, % (d.b) 20°C 30°C 40°C

5 169 121 738 123 53 3810 52 37 2212 27 18 8

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