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Effect of temperature on the moisture sorption isotherms of some cookies and corn snacks
Journalof FoodEngineering31(1997)85-93 Copyright 0 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 0260-8774197 $17.00 +O.OO
PII:SO260-8774(96)00019-2 ELSEVIER
Effect of Temperature on the Moisture Sorption Isotherms of some Cookies and Corn Snacks E. Palou, A. Lbpez-Malo”
& A. Argaiz
Departamento de Ingenieria Quimica y de Alimentos, Universidad de las Americas-Puebla, P.O. Box 100, Sta. Catarina Martir, Cholula (72820) Puebla, Mexico (Received
22 August 1994; revised 9 November
1995; accepted 9 February
1996)
ABSTRACT Three cookies and two corn snacks were analyzed for major components and their moisture adsorption characteristics were evaluated at 25, 35 and 45°C. The main composition differences were in fat and total carbohydrate content. The isotherms of each product were different (p <0*05) and significantly affected by temperature. The mathematical description of the adsorption data was obtained applying some of the most common sorption equations. Peleg’s model gave the best descrtption of the experimental data, followed by GAB equation. The mean relative deviations for Peleg’s equation were higher as the temperature increased and varied from 2.81 to 10*60%. Monolayer moisture content evaluated with BET and GAB models were in general lower for the cookies. The maximum net isosteric heats of adsorption were lower than 11 WI mol. 0 1997 Elsevier Science Limited. All rights reserved.
INTRODUCTION Food moisture isotherms and the equations that describe this relationship are important for the solution of engineering problems. Knowledge of the water adsorption characteristics is needed for shelf life predictions and determination of critical moisture and water activity (a,) for acceptability of products that deteriorate mainly by moisture gain, such as cookies and snacks (Katz & Labuza, 1981) and are important in drying, packaging and storage, as well as in problems that involve moisture transfer, like ingredient mixing. However, in most cases the sorption data are obtained at one temperature, usually the temperature of storage, but for the thermodynamic analysis of sorption and to model the drying and storage stability processes, it is necessary to know the isotherms in a range of temperatures. The *To whom correspondence
should be addressed: 85
Tel. (22) 29 24 09 Fax. (22) 29 20 32.
86
E. Palou et al.
knowledge of the dependency of the sorption phenomena on temperature provides valuable information about the changes related to the system’s energy. An important thermodynamic parameter is the net sorption isosteric heat which measures the binding energy or the forces between water vapor molecules and the adsorbent surfaces. The level of moisture content at which the heat of sorption approaches the heat of vaporization of water is often taken as indicative of the amount of ‘bound water’ existing in the food (Duckworth, 1972). Knowledge of the heat of sorption is very important in equipment and process design (Becker & Sallans, 1956; Rizvi, 1986), since with the integrated Clausius-Clapeyron equation the isotherms can be predicted at other temperatures. The objectives of this work were: (a) to determine the adsorption isotherms of three cookies and two corn snacks at 25, 35 and 45°C; (b) to evaluate several models and compare their goodness of fit in describing the isotherms; (c) to evaluate and compare the monolayer moisture content obtained with the BET and GAB equations; and (d) to evaluate the net isosteric heat of sorption of these products.
MATERIALS
AND METHODS
Sample preparation and proximate analysis Three commercial brands of cookies: Habaneras@, Ricanelas@ and Animalitos@ (Gamesa, Mexico) and two corn snacks: Doritos@ and Tostitos@ (Sabritas, Mexico) were purchased in a local supermarket and milled to pass mesh #40. The samples were dried in a vacuum oven (24 in Hg, 50°C) for 24 h and then placed for 7 days in vacuum desiccators (24 in Hg) over anhydrous calcium sulfate. A.O.A.C. (1984) standard procedures were used for triplicate determinations of the moisture content (14.062) ash (14.063) crude fiber (14*064), crude fat (14*066), protein (14.067) and sodium chloride (24.010). Total carbohydrates were calculated by difference. Determination of adsorption isotherms The water adsorption isotherms were determined gravimetrically by exposing the samples to atmospheres of known relative humidities, following the COST method to determine moisture sorption isotherms (Spiess & Wolf, 1986) and the modification reported by Argaiz & Lopez-Ma10 (1994). Triplicate samples of each product were placed inside the sorption jars. Saturated salt solutions of lithium chloride, potassium acetate, magnesium chloride, potassium carbonate, magnesium nitrate, sodium bromide, strontium chloride, sodium chloride, potassium chloride and barium chloride were used, their a, values used at different temperatures were those reported by Lopez-Ma10 et al. (1994). The jars were kept in temperaturecontrolled convection cabinets at 25 + 0.1, 35 f0.1 and 45 k O*l”C. Isotherm models and net isosteric heat of sorption The equilibrium moisture data were fitted using some traditional isotherm models such as: BET, Bradley, Halsey, Oswin, Smith and GAB (Chirife & Iglesias, 1978; Iglesias & Chirife, 1982; Lomauro et al., 1985a,b) and Peleg’s model (Peleg, 1993).
87
Effect of temperature on moisture sorption isotherms
The parameter values of the tested models were obtained by non-linear regression using the KaleidagraphO 3.0 package (Synergy Software, Reading, PA). To confirm that the regression parameters were indeed unique, the regression was repeated with various initial guessed values above and below those calculated. Monolayer moisture contents (m,,) were evaluated using the BET (a, up to O-45) and GAB (a, up to 0.90) models. The goodness of fit of the different models was evaluated with the mean relative deviation (%E) between the experimental (m,) and predicted (m,) moisture content, as defined by Iglesias & Chirife (1976):
where n is the number of observations. The net isosteric heat of sorption is a differential molar quantity derived from the temperature dependence of the sorption isotherms and it was calculated applying the Clausius-Clapeyron equation to the isosteres obtained at constant moisture contents up to 13% (dry basis) following the procedure reported by Iglesias & Chirife (1976).
RESULTS
AND DISCUSSION
Table 1 shows the proximate analysis of the products studied. The main composition differences in the analyzed products were in their fat content, ~23-24% for corn snacks and ~5-16% for cookies, and their total carbohydrate content, around 64% for the snacks and higher than 74% for cookies. Figures 1 and 2 show the isotherms at 25°C for cookies and corn snacks, respectively. All isotherms exhibit a sigmoid shape, which is described as a type I1 isotherm in the classification of Brunauer et al. (1938). The variability coefficients in the equilibrium moisture content ranged from 0.06 to 556% and in general, as temperature and moisture content increase the variability coefficient increased. The time to reach the equilibrium varied from 7 to 21 days and were shorter at lower a, and at higher temperatures.
TABLE 1
Major Composition Analysis (g component/l00
g) of the Products Studied
Hahaneras
Ricanelas
Animalitos
Moisture content
4.26+@18
3.05 + 0.24
7.10+0.31
2.34kO.15
1.37$0’11
Ash Fiber Fat Protein Total carbohydrate” Sodium chloride
1.06 * 0.03 055 + 0.04 9.92 _t 0.08 6.53 _+0.22 77.68 1.35kO.03
158+011 0.34 * 0.03 1559+0~09 508 + 0.38 74.26 0.94 t_ 0.3
1~13f0~10 1~00+0~09 4.87 + 0.03 6.10+0.08 79.80 1.02,0.02
2.93 + 0.29 I .96 + 0.08 22.77 + 0.32 6.07 + 0.77 63.93 1.42kO.18
1.18+0.09 3.37kO.15 24.42 + 0.04 5.40 * 0.86 65.26 2.16_+0.09
“Calculated
by difference
Doritos
-__ Tostitos
Component
88
E. Palou et al.
Figure 3 shows the temperature dependence of the moisture adsorption isotherms for one cookie (‘Ricanelas’) in the a, range 0.3-0.7, similar behavior was observed for the other products studied. The isotherms at room temperature of many foods are known (Iglesias & Chirife, 1982). However, few investigations have been con50
1
Animalitos
40 30 20 10 0 0.0
0.2
0.4
0.6
0.6
1 .o
0.6
1 .o
0.8
1 .o
water activity 40 Habaneras 30
20
10 I 0.0
0.2
0.4
0.6
water activity 50
,
-I
Ricanelas
0.2
0.4
0.6
water activity
Fig. 1. Moisture adsorption isotherms of three brands of cookies (‘Animalitos’, and ‘Ricanelas’)
at 25°C.
‘Habaneras’
89
Effect of temperature on moisture sorption isotherms
ducted to determine isotherms at temperatures higher than room temperature, and in most of these works the effect of temperature on the a, of the saturated salts solutions, used to control the relative humidity is not accounted, so errors in subsequent computations, like heat of sorption or isotherm prediction, are introduced. As can be seen (Fig. 3) the equilibrium moisture content increased at the same uH as temperature decreased, or at the same equilibrium moisture content, a, increased as temperature increased. The equilibrium moisture contents of the different products at the same temperature and for each product at different temperatures were found significatively different @
.z”
Tostitos
20
10
0 0.2
0.0
0.6
0.4
0.0
1 .o
water activity
40 Doritos 30 d s $ z 3 0,
20-
10 -
0
I 0.0
*
0.2
I 0.4
.
I 0.6
.
I 0.8
.
1
1 .o
water activity
Fig. 2. Moisture
adsorption
isotherms
of two brands of corn snacks (‘Tostitos’ and ‘Doritos’) at 25°C.
E. Palou
90
0.3
0.4
0.5
et al.
0.6
0.7
0.8
wateractivity Fig. 3. Moisture adsorption
isotherms of ‘Ricanelas’ at three temperatures
(25, 35 and 45°C).
(1983) and Lomauro et al. (1985a,b) stated that the GAB model is, in general, the best one to predict food isotherms. Lomauro et al. (1985b) reported that the GAB equation can predict, with mean relative deviations less than 5, the 80.8% of the starchy food isotherms studied followed by the Oswin equation, that can predict the 77%. Bouquet et al. (1978) found that the Oswin equation was the best one to describe the isotherms of this kind of foods, but in this study the GAB equation was not tested. Peleg’s (1993) model was proposed later than these works. The Peleg and GAB equations are adequate to predict in average the isotherms with mean relative deviation smaller than 7%, what is considered a good description (Lomauro et al., 1985a,b). The BET equation, in the a, range up to 0.45 gave in average a good prediction (%E, 3.7) Chirife & Iglesias (1978) stated that usually the BET equation can be applied in a a, range of 0.O.S0.45, what is considered a disadvantage when comparing it with the GAB equation, because with this equation the isotherms can be predicted up to a, values of 0.90. As Peleg (1993) stated, his model had the same or better fit than the GAB model, at the expense of an extra constant, but determination of the model constants by non linear regression does not require elaborated guessing. The monolayer moisture contents (mo) for each temperature calculated with BET and GAB equations are shown in Table 2. The GAB equation gave greater m. values for ‘Habaneras’, ‘Ricanelas’ and ‘Tostitos’ and in general the m,, values calculated by both models were significantly different (p
91
Effect of temperature on moisture sorption isotherms TABLE 2 Parameter Values for the BET, GAB and Peleg Models
BET’ “C
C
m,
Used to Describe the Isotherms
GAB %Eh
C
K
m,,
PELEG %Eh
kl
nl
3.69 2.89 4.17 7.84 6.32 7.33 7.23 8.56 6.59
10.16 10.05 10.47 9.32 10.16 8.81 9.86 10.31 10.09
0.49 0.51 0.60 0.82 0.91 0.87 0.77 0.91 0.96
25 10.79 3.32 2.14 11.18 1.01 3.26 8.45 35 8.97 3.40 2.47 11.01 1.06 3.06 10.07 45 7.81 3.13 0.86 8.01 1.04 3.00 10.26 25 8.13 3.42 4.75 7.23 0.95 3.71 6.96 35 6.56 3.41 4.63 5.84 0.94 3.77 6.53 45 7.50 3.23 4.43 6.33 0.97 3.45 6.41
7.94 9.25 7.45 7.46 7.02 7.75
kz
_~ n2
%Eh
COOKIES Habaneras
Ricanelas
Animalitos
CORN SNACKS Doritos
Tostitos
25 26.17 4.23 2.41 17.75 0.95 4.58 35 20.33 4.17 2.22 15.28 0.95 4.53 45 15.58 3.97 2.56 12.91 0.99 4.15 25 3.62 4.17 5.34 3.65 1.02 4.09 35 3.43 4.06 4.63 3.34 1.01 4.09 45 3.63 3.71 5.60 3.26 1.01 3.91 25 4.15 4.46 5.42 4.20 1.02 4.41 35 3.26 4.49 6.55 3.59 1.05 4.11 45 3.50 4.22 1.74 3.55 1.05 3.97
35.72 32.75 53.15 55.56 53.10 40.36 54.67 76.22 71.23
5.44 2.81 5.36 3.21 6.82 5.18 5.63 3.95 6.08 5.31 5.07 4.70 5.43 4.69 6.30 9.87 6.08 10.60
0.59 45.85 6.52 0.71 140.25 9.98 0.66 47.60 6.18 0.60 24.91 4.63 0.62 21.80 4.09 0.68 27.28 5.13
7.50 7.82 7.21 h.06 5.42 h.12
” Up to a, 0.45. h %E, mean relative deviation.
vary
with the composition and process. in general for starchy foods, values that vary from 3.2 to 16.0% had been reported (Lomauro et al., 1985b). The monolayer values found for the cookies agree with those reported at 20°C (3.78-5.53) by Tubert & Iglesias (1986) and by Iglesias & Chirife (1982) for the monolayer in wheat flour. The monolayer moisture contents for the cookies were higher than those found for corn snacks and in general decreased as temperature increased. Figure 4 shows the net isosteric sorption heat as a function of the moisture content for two of the studied products. The maximum heat of adsorption were obtained in the moisture content range 5-7% dry basis and was higher for ‘Tostitos’. Similar behavior was observed for the other products. The maximum net isosteric heat of sorption for the cookies were between 6.7 and 10~1 kJ/mol and for the corn snacks were around 7.5 kJ/mol. The maximum isosteric heats found are lower than those reported by Labuza et al. (1985) for corn flour (186 kJ/mol); Eglesias & Chirife (1976) for tapioca (12.5 kJ/mol); and Rizvi & Benado (1984) for sorghum (17.0 kJ/mol), this is probably due to the extensive heat treatment that the studied products received during processing, which could have damaged the sorption sites. The isosteric heat increased until a maximum and then decreased with the increase in moisture content (Fig. 4). The increase in isosteric heat at low moisture content can be explained considering that the sorption of water by the dry matrix lead to swelling of the food polymers resulting in the exposure of sorption sites of higher binding energies not previously available, After the maximum the decrease in the isosteric heat with the amount of water sorbed can be qualitatively explained considering that initially, sorption occurs on the most active available sites given rise to
92
E. Palou et al.
0
4
8
12
16
moisture content (g water1100 g ds) Fig. 4. Net isosteric
heat of sorption (Qs) as function of moisture (‘Ricanelas’) and a corn snack (‘Tostitos’).
content
for a cookie
greatest interaction energy. As these sites become occupied, sorption occurs on the less active sites given lower heats of sorption (Iglesias & Chirife, 1976). CONCLUSIONS Temperature has an effect on the adsorption isotherms of cookies and corn snacks; the relationship between a,,, and equilibrium moisture content of both products can be described by the GAB and Peleg equations in the a, range of O-11-0*90, and by the BET equation in the a, range of 0.11-0.45. The monolayer moisture content calculated with the GAB and BET equations were of similar magnitude but statistically different. The maximum net heats of adsorption were obtained in the moisture content range 5-7% dry basis and were lower than 11 kJ/mol. REFERENCES A. 0. A. C. (1984). Oj$cial Methods ofAnalysis, 14th edn. Association of Official Chemists, Washington, D.C. Argaiz, A. & Lopez-Malo, A. (1994). Implementation de una tecnica estandar de determinacion de Isotermas basada en el COST. Avarices en Ingenieria Quimica, 1, 10-16. Becker, H. A. & Sallans, H. R. (1956). A study of desorption isotherms of wheat at 25 and 50°C. Cereal Chem., 33, 79.
Effect of temperature on moisture sorption isotherms
93
Bizot, H. (1983). Using the ‘GAB’ model to construct sorption isotherms. In Physical Properties of Foods, ed. R. Jowitt, F. Escher, B. Hallstrom, H. Meffer, W. L. Spiess, & G. Vos. Applied Science Publishers, London. Bouquet, R., Chirife, J. & Iglesias, H. A. (1978). Equations for fitting water sorption isotherms of foods. II, Evaluation of various two-parameters models. 1. Food Technol., 13, 319-327.
Brunauer, S., Emmett, P. H. & Teller, E. (1938). Adsorption of gases in multimolecular layers. J. Am. Chem. Sot., 60, 309-319. Chirife, J. & Iglesias, H. A. (1978). Equations for fitting water sorption isotherms of foods. Part I. A Review. J. Food Technol., 13, 159. Duckworth, R. B. (1972). The properties of water around the surfaces of food colloids. Proc. Inst. Food Sci. Technol., 5, 50. Iglesias H. A. & Chirife J. (1976). Isosteric heat of water vapour sorption on dehydrated foods. Part I. Analysis of the differential heat curves. Lehensm.-wisxu-technol., 9. 116-122. Iglesias, H. A. & Chirife, J. (1982). Handbook of Food Isotherms. Academic Press, New York. Katz, E. E. & Labuza, T. P. (1981). Effect of water activity on sensory crispness and mechanical deformation of snack food products. J. Food Sci., 46, 403-409. Labuza, T. P., Khanawe, A. & Chen, J. Y. (1985). Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. J. Food Sci., 50, 385-391.
Lomauro, C. J., Bakshi, A. S. & Labuza, T. P. (1985a). Evaluation of food moisture sorption isotherms equations. Part I. Fruit, vegetable and meat products. Lehensm.-wiss.u-technol.. 18, 111-117.
Lomauro, C. J., Bakshi, A. S. & Labuza, T. P. (1985b). Evaluation of food moisture sorption isotherms equations. Part II, Milk, coffee, tea, nuts, oilseeds, spices and starchy Foods. Lebensm. -wiss.u-technol., 18, 118- 124. Lopez-Malo, A., Palou, E. & Argaiz, A. (1994). Measurement of water activity of saturated salt solution at various temperatures. Proc. Poster Session Int. Symp. on the Properties of Water; Practicum II, Universidad de las Americas-Puebla, Mexico, pp. 113-116. Peleg, M. (1993). Assessment of a semi-empirical four parameter general model for sigmoid moisture sorption isotherms. J. Food Process Engng., 16, 21-37. Rizvi. S. S. H. (1986). Thermodynamics properties of foods in dehydration. In Engineering Properties of Foods, ed. M. A. Rao, & S. S. H. Rizvi. Marcel Dekker, New York. Rizvi, S. S. H. & Benado, A. L. (1984). Thermodynamic analysis of drying foods. Drying Technol., 2(4), 1484- 1488.
Spiess, W. E. L. & Wolf, W. (1986). Critical evaluation of methods to determine moisture sorption isotherms. In Water Activity: Theory and Applications to Food, ed. L. B. Rockland & L. R. Beuchat. Marcel Dekker, New York. Tuber& A. H. & Iglesias, H. A. (1986). Water sorption isotherms and prediction of moisture gain during storage of packaged cereal crackers. Lebensm.-wiss.u-technol., 19, 365-368. Van den Berg, C. & Bruin, S. (1981). Water activity and its estimation in food systems: theoretical aspects. In Water Activity: Influences on Food Quality, ed. L. B. Rockland & G. F. Stewart. Academic Press, New York.
PII:SO260-8774(96)00019-2 ELSEVIER
Effect of Temperature on the Moisture Sorption Isotherms of some Cookies and Corn Snacks E. Palou, A. Lbpez-Malo”
& A. Argaiz
Departamento de Ingenieria Quimica y de Alimentos, Universidad de las Americas-Puebla, P.O. Box 100, Sta. Catarina Martir, Cholula (72820) Puebla, Mexico (Received
22 August 1994; revised 9 November
1995; accepted 9 February
1996)
ABSTRACT Three cookies and two corn snacks were analyzed for major components and their moisture adsorption characteristics were evaluated at 25, 35 and 45°C. The main composition differences were in fat and total carbohydrate content. The isotherms of each product were different (p <0*05) and significantly affected by temperature. The mathematical description of the adsorption data was obtained applying some of the most common sorption equations. Peleg’s model gave the best descrtption of the experimental data, followed by GAB equation. The mean relative deviations for Peleg’s equation were higher as the temperature increased and varied from 2.81 to 10*60%. Monolayer moisture content evaluated with BET and GAB models were in general lower for the cookies. The maximum net isosteric heats of adsorption were lower than 11 WI mol. 0 1997 Elsevier Science Limited. All rights reserved.
INTRODUCTION Food moisture isotherms and the equations that describe this relationship are important for the solution of engineering problems. Knowledge of the water adsorption characteristics is needed for shelf life predictions and determination of critical moisture and water activity (a,) for acceptability of products that deteriorate mainly by moisture gain, such as cookies and snacks (Katz & Labuza, 1981) and are important in drying, packaging and storage, as well as in problems that involve moisture transfer, like ingredient mixing. However, in most cases the sorption data are obtained at one temperature, usually the temperature of storage, but for the thermodynamic analysis of sorption and to model the drying and storage stability processes, it is necessary to know the isotherms in a range of temperatures. The *To whom correspondence
should be addressed: 85
Tel. (22) 29 24 09 Fax. (22) 29 20 32.
86
E. Palou et al.
knowledge of the dependency of the sorption phenomena on temperature provides valuable information about the changes related to the system’s energy. An important thermodynamic parameter is the net sorption isosteric heat which measures the binding energy or the forces between water vapor molecules and the adsorbent surfaces. The level of moisture content at which the heat of sorption approaches the heat of vaporization of water is often taken as indicative of the amount of ‘bound water’ existing in the food (Duckworth, 1972). Knowledge of the heat of sorption is very important in equipment and process design (Becker & Sallans, 1956; Rizvi, 1986), since with the integrated Clausius-Clapeyron equation the isotherms can be predicted at other temperatures. The objectives of this work were: (a) to determine the adsorption isotherms of three cookies and two corn snacks at 25, 35 and 45°C; (b) to evaluate several models and compare their goodness of fit in describing the isotherms; (c) to evaluate and compare the monolayer moisture content obtained with the BET and GAB equations; and (d) to evaluate the net isosteric heat of sorption of these products.
MATERIALS
AND METHODS
Sample preparation and proximate analysis Three commercial brands of cookies: Habaneras@, Ricanelas@ and Animalitos@ (Gamesa, Mexico) and two corn snacks: Doritos@ and Tostitos@ (Sabritas, Mexico) were purchased in a local supermarket and milled to pass mesh #40. The samples were dried in a vacuum oven (24 in Hg, 50°C) for 24 h and then placed for 7 days in vacuum desiccators (24 in Hg) over anhydrous calcium sulfate. A.O.A.C. (1984) standard procedures were used for triplicate determinations of the moisture content (14.062) ash (14.063) crude fiber (14*064), crude fat (14*066), protein (14.067) and sodium chloride (24.010). Total carbohydrates were calculated by difference. Determination of adsorption isotherms The water adsorption isotherms were determined gravimetrically by exposing the samples to atmospheres of known relative humidities, following the COST method to determine moisture sorption isotherms (Spiess & Wolf, 1986) and the modification reported by Argaiz & Lopez-Ma10 (1994). Triplicate samples of each product were placed inside the sorption jars. Saturated salt solutions of lithium chloride, potassium acetate, magnesium chloride, potassium carbonate, magnesium nitrate, sodium bromide, strontium chloride, sodium chloride, potassium chloride and barium chloride were used, their a, values used at different temperatures were those reported by Lopez-Ma10 et al. (1994). The jars were kept in temperaturecontrolled convection cabinets at 25 + 0.1, 35 f0.1 and 45 k O*l”C. Isotherm models and net isosteric heat of sorption The equilibrium moisture data were fitted using some traditional isotherm models such as: BET, Bradley, Halsey, Oswin, Smith and GAB (Chirife & Iglesias, 1978; Iglesias & Chirife, 1982; Lomauro et al., 1985a,b) and Peleg’s model (Peleg, 1993).
87
Effect of temperature on moisture sorption isotherms
The parameter values of the tested models were obtained by non-linear regression using the KaleidagraphO 3.0 package (Synergy Software, Reading, PA). To confirm that the regression parameters were indeed unique, the regression was repeated with various initial guessed values above and below those calculated. Monolayer moisture contents (m,,) were evaluated using the BET (a, up to O-45) and GAB (a, up to 0.90) models. The goodness of fit of the different models was evaluated with the mean relative deviation (%E) between the experimental (m,) and predicted (m,) moisture content, as defined by Iglesias & Chirife (1976):
where n is the number of observations. The net isosteric heat of sorption is a differential molar quantity derived from the temperature dependence of the sorption isotherms and it was calculated applying the Clausius-Clapeyron equation to the isosteres obtained at constant moisture contents up to 13% (dry basis) following the procedure reported by Iglesias & Chirife (1976).
RESULTS
AND DISCUSSION
Table 1 shows the proximate analysis of the products studied. The main composition differences in the analyzed products were in their fat content, ~23-24% for corn snacks and ~5-16% for cookies, and their total carbohydrate content, around 64% for the snacks and higher than 74% for cookies. Figures 1 and 2 show the isotherms at 25°C for cookies and corn snacks, respectively. All isotherms exhibit a sigmoid shape, which is described as a type I1 isotherm in the classification of Brunauer et al. (1938). The variability coefficients in the equilibrium moisture content ranged from 0.06 to 556% and in general, as temperature and moisture content increase the variability coefficient increased. The time to reach the equilibrium varied from 7 to 21 days and were shorter at lower a, and at higher temperatures.
TABLE 1
Major Composition Analysis (g component/l00
g) of the Products Studied
Hahaneras
Ricanelas
Animalitos
Moisture content
4.26+@18
3.05 + 0.24
7.10+0.31
2.34kO.15
1.37$0’11
Ash Fiber Fat Protein Total carbohydrate” Sodium chloride
1.06 * 0.03 055 + 0.04 9.92 _t 0.08 6.53 _+0.22 77.68 1.35kO.03
158+011 0.34 * 0.03 1559+0~09 508 + 0.38 74.26 0.94 t_ 0.3
1~13f0~10 1~00+0~09 4.87 + 0.03 6.10+0.08 79.80 1.02,0.02
2.93 + 0.29 I .96 + 0.08 22.77 + 0.32 6.07 + 0.77 63.93 1.42kO.18
1.18+0.09 3.37kO.15 24.42 + 0.04 5.40 * 0.86 65.26 2.16_+0.09
“Calculated
by difference
Doritos
-__ Tostitos
Component
88
E. Palou et al.
Figure 3 shows the temperature dependence of the moisture adsorption isotherms for one cookie (‘Ricanelas’) in the a, range 0.3-0.7, similar behavior was observed for the other products studied. The isotherms at room temperature of many foods are known (Iglesias & Chirife, 1982). However, few investigations have been con50
1
Animalitos
40 30 20 10 0 0.0
0.2
0.4
0.6
0.6
1 .o
0.6
1 .o
0.8
1 .o
water activity 40 Habaneras 30
20
10 I 0.0
0.2
0.4
0.6
water activity 50
,
-I
Ricanelas
0.2
0.4
0.6
water activity
Fig. 1. Moisture adsorption isotherms of three brands of cookies (‘Animalitos’, and ‘Ricanelas’)
at 25°C.
‘Habaneras’
89
Effect of temperature on moisture sorption isotherms
ducted to determine isotherms at temperatures higher than room temperature, and in most of these works the effect of temperature on the a, of the saturated salts solutions, used to control the relative humidity is not accounted, so errors in subsequent computations, like heat of sorption or isotherm prediction, are introduced. As can be seen (Fig. 3) the equilibrium moisture content increased at the same uH as temperature decreased, or at the same equilibrium moisture content, a, increased as temperature increased. The equilibrium moisture contents of the different products at the same temperature and for each product at different temperatures were found significatively different @
.z”
Tostitos
20
10
0 0.2
0.0
0.6
0.4
0.0
1 .o
water activity
40 Doritos 30 d s $ z 3 0,
20-
10 -
0
I 0.0
*
0.2
I 0.4
.
I 0.6
.
I 0.8
.
1
1 .o
water activity
Fig. 2. Moisture
adsorption
isotherms
of two brands of corn snacks (‘Tostitos’ and ‘Doritos’) at 25°C.
E. Palou
90
0.3
0.4
0.5
et al.
0.6
0.7
0.8
wateractivity Fig. 3. Moisture adsorption
isotherms of ‘Ricanelas’ at three temperatures
(25, 35 and 45°C).
(1983) and Lomauro et al. (1985a,b) stated that the GAB model is, in general, the best one to predict food isotherms. Lomauro et al. (1985b) reported that the GAB equation can predict, with mean relative deviations less than 5, the 80.8% of the starchy food isotherms studied followed by the Oswin equation, that can predict the 77%. Bouquet et al. (1978) found that the Oswin equation was the best one to describe the isotherms of this kind of foods, but in this study the GAB equation was not tested. Peleg’s (1993) model was proposed later than these works. The Peleg and GAB equations are adequate to predict in average the isotherms with mean relative deviation smaller than 7%, what is considered a good description (Lomauro et al., 1985a,b). The BET equation, in the a, range up to 0.45 gave in average a good prediction (%E, 3.7) Chirife & Iglesias (1978) stated that usually the BET equation can be applied in a a, range of 0.O.S0.45, what is considered a disadvantage when comparing it with the GAB equation, because with this equation the isotherms can be predicted up to a, values of 0.90. As Peleg (1993) stated, his model had the same or better fit than the GAB model, at the expense of an extra constant, but determination of the model constants by non linear regression does not require elaborated guessing. The monolayer moisture contents (mo) for each temperature calculated with BET and GAB equations are shown in Table 2. The GAB equation gave greater m. values for ‘Habaneras’, ‘Ricanelas’ and ‘Tostitos’ and in general the m,, values calculated by both models were significantly different (p
91
Effect of temperature on moisture sorption isotherms TABLE 2 Parameter Values for the BET, GAB and Peleg Models
BET’ “C
C
m,
Used to Describe the Isotherms
GAB %Eh
C
K
m,,
PELEG %Eh
kl
nl
3.69 2.89 4.17 7.84 6.32 7.33 7.23 8.56 6.59
10.16 10.05 10.47 9.32 10.16 8.81 9.86 10.31 10.09
0.49 0.51 0.60 0.82 0.91 0.87 0.77 0.91 0.96
25 10.79 3.32 2.14 11.18 1.01 3.26 8.45 35 8.97 3.40 2.47 11.01 1.06 3.06 10.07 45 7.81 3.13 0.86 8.01 1.04 3.00 10.26 25 8.13 3.42 4.75 7.23 0.95 3.71 6.96 35 6.56 3.41 4.63 5.84 0.94 3.77 6.53 45 7.50 3.23 4.43 6.33 0.97 3.45 6.41
7.94 9.25 7.45 7.46 7.02 7.75
kz
_~ n2
%Eh
COOKIES Habaneras
Ricanelas
Animalitos
CORN SNACKS Doritos
Tostitos
25 26.17 4.23 2.41 17.75 0.95 4.58 35 20.33 4.17 2.22 15.28 0.95 4.53 45 15.58 3.97 2.56 12.91 0.99 4.15 25 3.62 4.17 5.34 3.65 1.02 4.09 35 3.43 4.06 4.63 3.34 1.01 4.09 45 3.63 3.71 5.60 3.26 1.01 3.91 25 4.15 4.46 5.42 4.20 1.02 4.41 35 3.26 4.49 6.55 3.59 1.05 4.11 45 3.50 4.22 1.74 3.55 1.05 3.97
35.72 32.75 53.15 55.56 53.10 40.36 54.67 76.22 71.23
5.44 2.81 5.36 3.21 6.82 5.18 5.63 3.95 6.08 5.31 5.07 4.70 5.43 4.69 6.30 9.87 6.08 10.60
0.59 45.85 6.52 0.71 140.25 9.98 0.66 47.60 6.18 0.60 24.91 4.63 0.62 21.80 4.09 0.68 27.28 5.13
7.50 7.82 7.21 h.06 5.42 h.12
” Up to a, 0.45. h %E, mean relative deviation.
vary
with the composition and process. in general for starchy foods, values that vary from 3.2 to 16.0% had been reported (Lomauro et al., 1985b). The monolayer values found for the cookies agree with those reported at 20°C (3.78-5.53) by Tubert & Iglesias (1986) and by Iglesias & Chirife (1982) for the monolayer in wheat flour. The monolayer moisture contents for the cookies were higher than those found for corn snacks and in general decreased as temperature increased. Figure 4 shows the net isosteric sorption heat as a function of the moisture content for two of the studied products. The maximum heat of adsorption were obtained in the moisture content range 5-7% dry basis and was higher for ‘Tostitos’. Similar behavior was observed for the other products. The maximum net isosteric heat of sorption for the cookies were between 6.7 and 10~1 kJ/mol and for the corn snacks were around 7.5 kJ/mol. The maximum isosteric heats found are lower than those reported by Labuza et al. (1985) for corn flour (186 kJ/mol); Eglesias & Chirife (1976) for tapioca (12.5 kJ/mol); and Rizvi & Benado (1984) for sorghum (17.0 kJ/mol), this is probably due to the extensive heat treatment that the studied products received during processing, which could have damaged the sorption sites. The isosteric heat increased until a maximum and then decreased with the increase in moisture content (Fig. 4). The increase in isosteric heat at low moisture content can be explained considering that the sorption of water by the dry matrix lead to swelling of the food polymers resulting in the exposure of sorption sites of higher binding energies not previously available, After the maximum the decrease in the isosteric heat with the amount of water sorbed can be qualitatively explained considering that initially, sorption occurs on the most active available sites given rise to
92
E. Palou et al.
0
4
8
12
16
moisture content (g water1100 g ds) Fig. 4. Net isosteric
heat of sorption (Qs) as function of moisture (‘Ricanelas’) and a corn snack (‘Tostitos’).
content
for a cookie
greatest interaction energy. As these sites become occupied, sorption occurs on the less active sites given lower heats of sorption (Iglesias & Chirife, 1976). CONCLUSIONS Temperature has an effect on the adsorption isotherms of cookies and corn snacks; the relationship between a,,, and equilibrium moisture content of both products can be described by the GAB and Peleg equations in the a, range of O-11-0*90, and by the BET equation in the a, range of 0.11-0.45. The monolayer moisture content calculated with the GAB and BET equations were of similar magnitude but statistically different. The maximum net heats of adsorption were obtained in the moisture content range 5-7% dry basis and were lower than 11 kJ/mol. REFERENCES A. 0. A. C. (1984). Oj$cial Methods ofAnalysis, 14th edn. Association of Official Chemists, Washington, D.C. Argaiz, A. & Lopez-Malo, A. (1994). Implementation de una tecnica estandar de determinacion de Isotermas basada en el COST. Avarices en Ingenieria Quimica, 1, 10-16. Becker, H. A. & Sallans, H. R. (1956). A study of desorption isotherms of wheat at 25 and 50°C. Cereal Chem., 33, 79.
Effect of temperature on moisture sorption isotherms
93
Bizot, H. (1983). Using the ‘GAB’ model to construct sorption isotherms. In Physical Properties of Foods, ed. R. Jowitt, F. Escher, B. Hallstrom, H. Meffer, W. L. Spiess, & G. Vos. Applied Science Publishers, London. Bouquet, R., Chirife, J. & Iglesias, H. A. (1978). Equations for fitting water sorption isotherms of foods. II, Evaluation of various two-parameters models. 1. Food Technol., 13, 319-327.
Brunauer, S., Emmett, P. H. & Teller, E. (1938). Adsorption of gases in multimolecular layers. J. Am. Chem. Sot., 60, 309-319. Chirife, J. & Iglesias, H. A. (1978). Equations for fitting water sorption isotherms of foods. Part I. A Review. J. Food Technol., 13, 159. Duckworth, R. B. (1972). The properties of water around the surfaces of food colloids. Proc. Inst. Food Sci. Technol., 5, 50. Iglesias H. A. & Chirife J. (1976). Isosteric heat of water vapour sorption on dehydrated foods. Part I. Analysis of the differential heat curves. Lehensm.-wisxu-technol., 9. 116-122. Iglesias, H. A. & Chirife, J. (1982). Handbook of Food Isotherms. Academic Press, New York. Katz, E. E. & Labuza, T. P. (1981). Effect of water activity on sensory crispness and mechanical deformation of snack food products. J. Food Sci., 46, 403-409. Labuza, T. P., Khanawe, A. & Chen, J. Y. (1985). Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. J. Food Sci., 50, 385-391.
Lomauro, C. J., Bakshi, A. S. & Labuza, T. P. (1985a). Evaluation of food moisture sorption isotherms equations. Part I. Fruit, vegetable and meat products. Lehensm.-wiss.u-technol.. 18, 111-117.
Lomauro, C. J., Bakshi, A. S. & Labuza, T. P. (1985b). Evaluation of food moisture sorption isotherms equations. Part II, Milk, coffee, tea, nuts, oilseeds, spices and starchy Foods. Lebensm. -wiss.u-technol., 18, 118- 124. Lopez-Malo, A., Palou, E. & Argaiz, A. (1994). Measurement of water activity of saturated salt solution at various temperatures. Proc. Poster Session Int. Symp. on the Properties of Water; Practicum II, Universidad de las Americas-Puebla, Mexico, pp. 113-116. Peleg, M. (1993). Assessment of a semi-empirical four parameter general model for sigmoid moisture sorption isotherms. J. Food Process Engng., 16, 21-37. Rizvi. S. S. H. (1986). Thermodynamics properties of foods in dehydration. In Engineering Properties of Foods, ed. M. A. Rao, & S. S. H. Rizvi. Marcel Dekker, New York. Rizvi, S. S. H. & Benado, A. L. (1984). Thermodynamic analysis of drying foods. Drying Technol., 2(4), 1484- 1488.
Spiess, W. E. L. & Wolf, W. (1986). Critical evaluation of methods to determine moisture sorption isotherms. In Water Activity: Theory and Applications to Food, ed. L. B. Rockland & L. R. Beuchat. Marcel Dekker, New York. Tuber& A. H. & Iglesias, H. A. (1986). Water sorption isotherms and prediction of moisture gain during storage of packaged cereal crackers. Lebensm.-wiss.u-technol., 19, 365-368. Van den Berg, C. & Bruin, S. (1981). Water activity and its estimation in food systems: theoretical aspects. In Water Activity: Influences on Food Quality, ed. L. B. Rockland & G. F. Stewart. Academic Press, New York.