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Journal of Food Engineering 77 (2006) 194–199

Research note

Water adsorption isotherms of texturized soy protein

A.S. Cassini a, L.D.F. Marczak a,*, C.P.Z. Norena b

a Chemical Engineering Department, Federal University of Rio Grande do Sul (UFRGS), Rua Luis Englert, s/n.

Campus Central, ZIP: 90040-000 Porto Alegre, RS, Brazilb Institute of Food Science and Technology, Federal University of Rio Grande do Sul (UFRGS), Av. Bento Goncalves,

9500. Campus do Vale, ZIP: 91540-000 Porto Alegre, RS, Brazil

Received 6 June 2004; received in revised form 12 May 2005; accepted 24 May 2005Available online 9 August 2005

Abstract

Texturized soy proteins (TSP) have been used for many years as a substitute of animal protein. In recent times, TSP was used as afunctional ingredient in several food applications; its process involves a drying step. The water adsorption isotherms of TSP weredetermined using the static method of saturated salt solutions at 10, 20, 30 and 40 �C. The experimental data were fitted to Oswin,Halsey, BET, GAB, Peleg and Darcy Watt models. The equilibrium moisture content at water activities up to 0.9 decreased as thetemperature was increased from 20 to 40 �C. At higher water activities, the moisture content showed an inverse behavior, resulting acrossover of the isotherms. The GAB and Peleg equation showed the best fit for the experimental curves. The total heat of sorptionof TSP increased with decreased moisture content.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Texturized soy protein; Sorption isotherms; Water activity

1. Introduction

Proteins are essential components of cells and biolog-ical processes. They are involved in regulatory functionsand controlling intra and extracellular conditions.

Around 1950, the nutritional importance of proteinand the high content of this nutrient in the soybean (soy-bean contains about 40% of vegetable protein) wereemphasized. On these grands, the food industry startedproduction of a defatted soybean flour designed for hu-man feed. This production has increased and, nowadays,millions of tons of defatted meal are produced world-wide (Bunge, 2001). Given their high applicability inthe food industry, this product was processed furtherinto texturized, concentrated and isolated soy protein.

0260-8774/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.jfoodeng.2005.05.059

* Corresponding author. Tel.: +55 51 33163304; fax: +55 5133163277.

E-mail address: ligia@enq.ufrgs.br (L.D.F. Marczak).

At the present time, the use of TSP as meat replacerin meat processed products is very common. Studiesproving the relationship of soy protein consumption tocholesterol reduction and prevention of heart diseaseshave raised the interest of the whole food industry to de-velop soy-based products. The range of new soy proteinapplications increased as well: nutritional bars, bever-ages, cereals, biscuits, sauces, chocolates and snacksare some examples.

The main functions of TSP in a food product may in-clude: increase water and protein content, reduce prod-uct cost, enhance product texture and hardness andreplace a portion of the meat to keep the original proteincontent.

In the production of TSP, one of the principal steps isthe drying process, which is necessary to decrease productmoisture content until the required level. The objective ofdehydration in foods is to inhibit degradation causedby growth of bacteria, yeasts and molds. Moreover,

Table 1Models for fitting the adsorption isotherms of TSP

Model

Oswin X eq ¼ Aaw

1� aw

� �B

ð1Þ

Halsey aw ¼ exp�AX B

eq

!ð2Þ

BET X eq ¼ðX mCawÞð1� ðN þ 1ÞaN

wNaNþ1w Þ

ð1� awÞð1� ðC � 1Þaw � CaNþ1w Þ

ð3Þ

GAB X eq ¼X mCKaw

ð1� K awÞð1� Kaw þ CKawÞð4Þ

Peleg X eq ¼ k1an1w þ k2an2

w ð5Þ

D�arcy Watt X eq ¼K1K2aw

1þ K1 aw

þ K5aw þK3K4aw

1� K3aw

ð6Þ

A.S. Cassini et al. / Journal of Food Engineering 77 (2006) 194–199 195

undesirable biochemical reactions—which are alsoresponsible for reduction of product shelf life—are mini-mized by drying. Inhibition of microbiological activity isachieved for water activity (aw) lower than 0.7. Depend-ing on the product, this value means moisture (wet basis)between 5 and 25% (Geankoplis, 1993).

The sorption isotherms describe the relationship be-tween water activity (aw) and the equilibrium moisturecontent of a given food at constant temperature. Indus-try has great interest in sorption isotherms determina-tion because they provide data about the shelf lifestability of a product. Beyond that, it is used in dryingto determine the final moisture. This moisture combinesmicrobiological and biochemical stability with the bestdrying cost (McLaughlin & Magee, 1998). In addition,isotherms give significant information to other processsteps, as packaging and storage.

The use of TSP as a functional ingredient is novel andvery few sorption data are available in literature. There-fore, the objective of this study was to determine thesorption isotherms of the texturized soy protein at thetemperatures of 10, 20, 30 and 40 �C. The total heat ofsorption and the water surface area of TSP were alsodetermined.

2. Theoretical considerations

Food products have a very complex composition andtheoretical predictions are not accurate. For this reason,sorption isotherms at different temperatures should bedetermined experimentally.

Several researches—including Ajibola, 1986;Alhamdan and Hassan, 1999; Basunia and Abe, 2001;Kaymak-Ertekin and Sultanoglu, 2001; Maskan andGogus, 1997; McLaughlin and Magee, 1998; Menkov,2000; Park, Vohnikova, and Brod, 2002; Sandoval andBarreiro, 2002; Saravacos, Tsiourvas, and Tsami,1986; Shivahare, Arora, Ahmed, and Raghavan, 2004;Tsami, Marinos-Kouris, and Maroulis, 1990—have re-ported sorption data for many different food products,but sorption isotherms of TSP are not available inliterature.

In the present study, the experimental data obtainedwere fitted by two two-parameter equations, Halsey(Park et al., 2002) and Oswin (Kaymak-Ertekin &Sultanoglu, 2001), two three-parameter equations,BET (Park et al., 2002) and GAB (Heldman & Hartel,2000), one four-parameter equation, Peleg, 1993, andone five-parameter equation, D�arcy Watt (Saravacoset al., 1986). Table 1 shows each model used for fittingexperimental curves, where Xeq and Xm are, respectively,the equilibrium and the monolayer moisture content, inkg water/kg db, aw is the water activity, in kg water/kgdry air and A, B, C, K, k1, k2, K1, K2, K3, K4, K5, N, n1

and n2 are constants of the models listed.

The sorption isotherms of a given food can also beused in the estimation of other important parametersof this food: the monolayer moisture content (Xm), thetotal heat of sorption (Qst) and the water surface area(S0).

The monolayer moisture content (Xm) is an impor-tant parameter to define physical and chemical stabilityof foods, since it has a direct influence on lipid oxida-tion, enzyme activity, non-enzymatic browning, flavorpreservation and product structure (Menkov, 2000).

The heat of sorption (Qs), estimated by the Clausius–Clapeyron equation, is used to estimate the total energydemand during drying. The heat of sorption is a mea-sure of the binding energy of absorbed water by the solidmaterials (McLaughlin & Magee, 1998). The moisturecontent at which the total heat of sorption approachesthe heat of vaporization of pure water is often takenas an indication of �bound� water in the foodstuff(Kaymak-Ertekin & Sultanoglu, 2001). At higher mois-ture contents, water is available for utilization by micro-organisms as it is mechanically free in the void spaces ofthe system (Fasina & Sokhansanj, 1993).

As mentioned earlier the ‘‘net’’ isosteric heat of sorp-tion is obtained using the equation of Clausius–Clapeyron,

dðln awÞdð1=T Þ ¼ �

QS

RG

ð7Þ

where T is the absolute temperature and RG is the uni-versal gas constant (RG = 8.319 kJ/mol K).

The total heat of sorption (Qst) is the sum of the‘‘net’’ isosteric heat of sorption and the heat of vapori-

196 A.S. Cassini et al. / Journal of Food Engineering 77 (2006) 194–199

zation of pure water (DH0, assumed constant with tem-perature and equal to 44.09 kJ/mol).

Qst ¼ Qs þ DH 0 ð8ÞThe water surface area of the product (S0), given in

m2/g of solid, can be determined from monolayer mois-ture content data and can be measured by assuming thearea of a water molecule to be 10.6 A2, using the follow-ing equation

S0 ¼ X m

1

PMH2O

N 0AH2O ¼ 3.5� 103X m; ð9Þ

where PMH2O is the molecular weight of water (18g/mol), N0 is the number of Avogadro (6 · 1023 mole-cules/mol) and AH2O is the area of a water molecule(10.6 · 10�20 m2) (Labuza, 1968).

S0 is associated with amount quantity of polymerspresents in the foodstuff (Labuza, 1968).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Xeq

(kg

wat

er/k

g db

) T = 10 ºC

T = 20 ºC

T = 30 ºC

T = 40 ºC

1aw

Fig. 1. Sorption data of TSP in the temperatures of 10, 20, 30 and40 �C.

3. Materials and methods

The TSP used in this study were obtained from com-mercial samples (The Solae Company Ind. e Com. deAlimentos LTDA – Esteio, RS, Brasil). The initial mois-ture content of the samples was about 6% (w.b.). Thesamples used have the following composition: 50%protein, 20% sugars, 0% fat, 20% fibers and 4% ashes(Central Lab, The Solae Company).

Equilibrium moisture content curves versus wateractivity at 10, 20, 30 and 40 �C were obtained throughthe standard gravimetric method recommended by theCOST 90 Project (Spiess & Wolf, 1983) using 10 satu-rated salt solutions–sodium hydroxide, lithium chloride,potassium acetate, magnesium chloride, potassium car-bonate, potassium nitrite, sodium chloride, potassiumchloride, barium chloride and copper sulfate–with rela-tive humidity ranging from 7 to 97%.

Two samples of about 5 g of TSP were placed on atripod above the saturated salt solution in each of 10hygrostats (glass jars, 400 mL). These jars were placedinside an air-circulating thermally-insulated tempera-ture-controlled equipment maintained at the specifiedtemperature within ±0.1 �C, until equilibrium had beenreached (about 20 days). To prevent microbial spoilageof samples, crystalline tymol was placed in the hygro-stats where high water activities occurred (aw > 0.7)(Wolf, Spiess, & Jung, 1985).

After the equilibrium had been reached, the sampleswere dried using the oven method at 105 �C for 24 h(AOAC, 1990).

The fitting of selected models to experimental data wascarried out with a non-linear estimation software (Statis-tica �98 Edition). Regressions were repeated with variousinitial estimated values above and below those calculatedto confirm that convergence was reliable (Peleg, 1993).

The coefficient determination (R2) and the mean rela-tive deviation modulus (MRD) were used to evaluatethe goodness of fit. The MRD value is given in percent-age and may be estimated as follow:

MRDð%Þ ¼ 100

N

XN

i¼1

V EXP;i � V CALC;ij jV EXP;i

; ð10Þ

where N is the number of experimental points andVEXP,i and VCALC,i are the experimental and calculatedmoisture contents.

A good fit must show MRD values below 10% (Parket al., 2002).

4. Results and discussion

Fig. 1 shows the experimental curves obtained relat-ing the equilibrium moisture content of TSP with itswater activities for all the studied temperatures. Theequilibrium moisture content at each aw represents themean value of tree replications.

This figure shows that, for a constant temperature,the equilibrium moisture content of the product in-creases with aw.

In addition, the curves of Fig. 1 were very close (prac-tically coincident), mainly for low values of aw. Thisindicates a low influence of temperature (between 10and 40 �C) in the isotherms of TSP. Sandoval andBarreiro (2002) found a similar behavior for non-fermented cocoa beans at 25, 30 and 35 �C.

Table 2 shows the fitting constants of the models(presented in Table 1), the correlation coefficient andthe mean relative deviation modulus.

All models presented correlation coefficients veryclose to unity indicating good fit to experimental data.However, considering the MRD index, fewer modelswere fit satisfactorily.

The best fit was obtained by the models of GAB andPeleg. The first presented the lowest MRD value for the

Table 2Results obtained from the fit of the experimental curves to the selectedmodels

Model Cts Temperatures

10 �C 20 �C 30 �C 40 �C

Oswin A 0.125 0.122 0.102 0.087B 0.378 0.461 0.558 0.666R2 0.971 0.990 0.998 0.999MRD 23.63 15.14 11.09 16.18

Halsey A 0.020 0.024 0.031 0.024B 1.490 1.437 1.271 1.386R2 0.991 0.997 0.998 0.998MRD 15.46 11.62 6.24 10.19

BET Xm 0.075 0.072 0.062 0.062C 1.020 1.055 1.094 1.123N 11.382 12.008 11.967 9.899R2 0.987 0.990 0.995 0.994MRD 29.73 31.93 30.60 32.44

GAB Xm 0.074 0.064 0.055 0.047C 5.839 10.588 8.955 30.828K 0.874 0.922 0.950 0.976R2 0.995 0.998 >0.999 >0.999MRD 14.70 10.61 7.99 4.02

Peleg k1 0.454 0.590 0.710 0.980k2 0.075 0.128 0.175 0.180n1 4.660 7.264 10.698 13.689n2 0.191 0.490 0.861 0.830R2 >0.999 0.999 0.999 0.998MRD 2.18 7.58 13.45 13.27

D�arcy Watt K1 �0.127 �1.004 �0.046 �0.045K2 385.601 �0.121 127.452 135.931K3 0.263 1.008 0.917 0.960K4 76.463 �0.088 0.101 0.068K5 29.133 0.141 6.079 6.256R2 0.999 0.997 0.999 0.999MRD 8.33 17.93 12.46 11.56

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9aw

Xeq

(kg

wat

er/k

gdb)

T = 20 ºC (exp)

T = 20 ºC (GAB)

T = 40 ºC (exp)

T = 40 ºC (GAB)

1

Fig. 2. Fitting of the experimental data to the GAB model.

A.S. Cassini et al. / Journal of Food Engineering 77 (2006) 194–199 197

higher temperatures (30 and 40 �C) and the second, forthe lower (10 and 20 �C).

Furthermore, the Halsey model was also accurate athigh temperatures. Iglesias and Chirife (1976) indicatedthe good fit of this model to the isotherms of high pro-tein foods.

The fitting of BET model to experimental data gener-ated the most elevated MRD values (above 30%) andthe minor correlation coefficients; this occurs, probably,because the use of BET model is indicated only in the fit-ting of water activities data until 0.5.

The fitting of GAB model to experimental data at 20and 40 �C are presented in Fig. 2. A little difference be-tween the two curves, mainly for water activities above0.5, was observed. Furthermore, a crossover of thesecurves can be seen at an aw value around 0.9.

This phenomenon is characteristic of sugar richfoods. Labuza (1984) found that isotherm inversion iscaused by microbial growth and/or the sugar dissolution

(the higher the temperature, the higher the dissolutionand the higher the equilibrium moisture content)(Maskan & Gogus, 1997).

Different studies reported isotherm crossover for var-ious types of sugars-rich foods. Tsami et al. (1990) deter-mined the sorption isotherms of some dried fruitsbetween 15 and 60 �C. The authors found that the high-er the sugars content, the lower the crossover aw. Thus,since the sugars content of TSP is about 20% (with amoisture content of 0.07 kg water/kg db), the crossingof the isotherms in a high value of aw (around 0.9) wereexpected.

As stated earlier, GAB and Peleg models presentedthe best fit to experimental curves of TSP between 10and 40 �C. The GAB model, however, is the only onethat predicts the isotherm crossover and, therefore, itis concluded that GAB is the best model to predict thesorption isotherms for this type of TSP in this tempera-ture range.

The values of monolayer moisture content of TSPestimated with GAB model at 10, 20, 30 and 40 �Ccan also be observed in Table 2. TSP presented a mono-layer moisture content of 7.4% (db) at the temperatureof 10 �C and this value decreased with increasing tem-perature. Similar results were found by McLaughlinand Magee (1998) for potatoes. Monolayer values ob-tained here are in the acceptable range for food prod-ucts. Labuza (1984) indicated 10% (db) as themaximum monolayer moisture content for foods.

Fig. 3 shows that total heat of sorption decreases forincreasing moisture content of TSP; this fact agrees withobservations by many researchers (Fasina &Sokhansanj, 1993), since the lower the moisture content,the higher the energy required to remove water from theproduct.

The analysis of Fig. 3 also demonstrates that totalheat of sorption of TSP reaches the heat of vaporizationof pure water at a moisture content about 28% (db).

Table 3 presents the water surface area of TSP be-tween 10 and 40 �C; it was estimated using Eq. (9) and

42

43

44

45

46

47

48

49

50

51

0 0.1 0.2 0.3 0.4 0.5 0.6

Qst

(kJ

/mol

)

Xeq (kg water/kg db)

Fig. 3. Total heat of sorption (kJ/mol) versus the equilibrium moisturecontent of TSP.

Table 3Water surface area (S0) of TSP (m2/g solid)

T (�C) S0 (m2/g solid)

10 260.0520 225.430 192.1540 162.75

198 A.S. Cassini et al. / Journal of Food Engineering 77 (2006) 194–199

the monolayer moisture contents of TSP obtained byGAB model.

These values are within the range commonly obtainedfor food products (100–250 m2/g solid) (Labuza, 1968).

5. Conclusions

This study presented adsorption isotherms of textur-ized soy protein at 10, 20, 30 and 40 �C. Several modelswere fitted to experimental data and the monolayermoisture content, the total heat of sorption and thewater surface area of the product were calculated.

The adsorption isotherms obtained in this range oftemperature were very close, indicating week effect oftemperature. Isotherms at 20 and at 40 �C crossed overat a water activity value around 0.9. This fact is charac-teristic of sugars-rich foods.

The GAB and Peleg models presented the best fit toexperimental data. As the GAB model predicted thecrossover, it was considered as the most suitable to pre-dict the adsorption isotherms of TSP in the temperaturerange covered.

The total heat of sorption obtained for TSP de-creased as the moisture content increased.

The monolayer moisture content of TSP varied be-tween 4.6% and 7.4% (db), within the studied range oftemperature, and decreased with increasing tempera-ture. The water surface area showed a similar behavior.

Acknowledgements

The authors gratefully acknowledge Solae do BrasilInd. e Com. de Alimentos LTDA. (Esteio, RS, Brasil)for providing the products and equipment necessaryfor this study.

The authors also acknowledge Capes for providingfinancial support.

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