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Moisture sorption characteristics of chickpea flour
Journal of Food Engineering 68 (2005) 535–539 www.elsevier.com/locate/jfoodeng
Research note
Moisture sorption characteristics of chickpea flour Albena G. Durakova, Nikolay D. Menkov
*
Department of Process Engineering, University of Food Technologies, 26 Maritza Blvd., 4000 Plovdiv, Bulgaria Received 22 March 2004; accepted 28 June 2004
Abstract Moisture equilibrium data (adsorption and desorption) of chickpea flour were determined using the static gravimetric method of saturated salt solutions at four storage temperatures: 10, 20, 30 and 40 °C. The range of water activities for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with the increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Chung–Pfost, modified Oswin, modified Halsey and modified Henderson). The monolayer moisture content was estimated from sorption data using the Brunauer–Emmett–Teller (BET) equation. The isosteric heats of sorption were evaluated using Clausius–Clapeyron equation. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Chickpea flour; Heat of sorption; Moisture content; Water activity
1. Introduction With recent consumer interest in functional food, chickpea flour is increasingly being used as an ingredient and a functional modifier in many foods (CardosoSantiago & Areas, 2001; Goni & Valentin-Gamazo, 2003; Suhasini & Malleshi, 2003). The moisture content of the flour as a hygroscopic material exerts a strong influence on its quality and technological properties (Abdullah & Nawawi, 2000; Linfeng & Floters, 1999; Nasir, Butt, Anjum, Sharif, & Minhas, 2003; Teunou & Fitzpatrick, 1999). The sorption properties of foods (equilibrium moisture content, monolayer moisture, heat of sorption) are essential for the design and optimization of many processes such as drying, packaging and storage (Al-Muhtaseb, McMinn, & Magee, 2002). Several researchers have conducted equilibrium isotherm studies of chickpea seeds (Menkov, 2000; Moreira, Vazquez, & *
Corresponding author. Tel.: +359 32 603 689; fax: +359 32 644
102. E-mail addresses: [email protected] (A.G. Durakova), nimenkov@hiffi-plovdiv.acad.bg (N.D. Menkov). 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.06.019
Chenlo, 2002), but the sorption data on chickpea flour are limited. A number of models have been suggested in the literature for the dependence between the equilibrium moisture content (EMC) and the water activity (aw) (Van den Berg & Bruin, 1981). The modified Chung–Pfost, modified Henderson, modified Halsey and modified Oswin equations which incorporate the temperature effect have been proposed by Chen and Morey (1989). The models have been adopted as standard equations by the ASAE for the description of sorption isotherms (ASAE, 2001). The objectives of this work are: (1) to obtain experimental equilibrium data of chickpea flour at 10, 20, 30 and 40 °C; (2) to find out the suitable model describing the isotherms; (3) to calculate the monolayer moisture content; (4) to calculate the heat of sorption of the flour.
2. Material and methods 2.1. Material Commercial chickpea flour produced in Bulgaria was used in this study. The initial moisture content of the
536
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539
flour was 12.58% wet basis. AOAC (1990) standard procedures were used for the determination of crude fat–– 4.66%, protein––19.63%, carbohydrate––61.90% and ash––1.23%. 2.2. Procedure The EMC of the chickpea flour was determined at 10, 20, 30 and 40 °C. The static gravimetric method was applied (Wolf, Spiess, & Jung, 1985). For the adsorption process, flour was dehydrated in a desiccator over P2O5 at a room temperature for 20 days prior to the beginning of the experiment. The desorption isotherms were determined on samples hydrated in a glass jar over distilled water at a room temperature to approximately 30% dry basis (d.b.) moisture content. Samples of 1 ± 0.02 g were weighed in weighing bottles. The weighing bottles were then put in hygrostats with six saturated salt solutions (LiCl, MgCl2, K2CO3, NaBr, NaCl, KCl) used to obtain constant water activities environments (Bell & Labuza, 2000). All used salts were of reagent grade. At high water activities (aw > 0.70) crystalline thymol was placed in the hygrostats to prevent the microbial spoilage of the flour. The hygrostats were kept in thermostats at 10, 20, 30 and 40 ± 0.2 °C. Samples were weighed (balance, sensitivity ±0.0001 g) every three days. Equilibrium was acknowledged when three consecutive weight measurements showed a difference less than 0.001 g. The moisture content of each sample was determined by the oven method (105 °C for 24 h) by means of triplicate measurements. 2.3. Analysis of data The description of the sorption isotherms was verified according to the following four models: Modified Chung–Pfost A aw ¼ exp expðCMÞ tþB Modified Halsey expðA þ BtÞ aw ¼ exp MC Modified Oswin C aw M ¼ ðA þ BtÞ 1 aw
ð1Þ
ð2Þ
ð3Þ
Modified Henderson 1 aw ¼ exp½Aðt þ BÞM C
ð4Þ
where M is the moisture content, % d.b; aw is the water activity, decimal; A, B and C are coefficients; t is the temperature, °C.
A nonlinear, least squares regression program was used to fit the four models to the experimental data (all replications). The suitability of the equations was evaluated and compared using the mean relative error P, %; standard error of moisture (SEM) and randomness of residuals ei (Chen & Morey, 1989): ^ i 100 X M i M ð5Þ P¼ M N i sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ^ i Þ2 ðM i M SEM ¼ df ^i ei ¼ M i M
ð6Þ ð7Þ
^ i are experimentally observed and prewhere Mi and M dicted by the model value of the equilibrium moisture content, respectively, N is the number of data points, and df is the number of degree of freedom (number of data points minus number of constants in the model). The monolayer moisture contents (MMC) for each temperature were calculated using the Brunauer– Emmett–Teller (BET) equation (Brunauer, Emmett, & Teller, 1938) and the experimental data for water activities up to 0.45 (Bell & Labuza, 2000): M¼
M e Caw ð1 aw Þð1 aw þ Caw Þ
ð8Þ
where Me is the MMC, % d.b.; C is the coefficient. The values of heat of sorption were calculated using the Clausius–Clapeyron equation from the slope of the plot between the values of ln aw and 1/T at constant moisture: Q 1 þ constant ð9Þ ln aw ¼ st R T where Qst is isosteric heat of sorption, kJ mol1 ; T is the absolute temperature, K; R is the universal gas constant, kJ mol1 K1. Using Eq. (2), the values of aw were determined at the four temperatures and constant moisture contents 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22% and 24% d.b.
3. Results and discussion The EMC mean values obtained and the standard deviations based on the triplicate measurements for the respective water activity and temperature are presented in Table 1 for adsorption and in Table 2 for desorption. The EMC values decreased with an increase in the temperature at constant aw. Similar trends for many foods have been reported in the literature (AlMuhtaseb et al., 2002). The comparison with the published sorption data for chickpea seeds (Menkov, 2000; Moreira et al., 2002) showed that the sorption capacity
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539
537
Table 1 Equilibrium moisture content M, % d.b. of chickpea flour obtained by adsorption at different water activities and temperatures 10 °C
Salt
LiCl MgCl2 KCO3 NaBr NaCl KCl a b
20 °C a
aw
M
0.113 0.335 0.431 0.622 0.757 0.868
7.66 10.48 12.71 15.38 19.07 25.99
sd
b
0.16 0.07 0.24 0.09 0.13 0.22
30 °C
40 °C
aw
M
sd
aw
M
sd
aw
M
sd
0.113 0.331 0.432 0.591 0.755 0.851
6.89 9.31 11.59 12.57 18.20 23.85
0.08 0.03 0.23 0.13 0.10 0.18
0.113 0.324 0.432 0.560 0.751 0.836
5.71 8.61 9.58 11.78 17.70 20.22
0.18 0.21 0.18 0.11 0.17 0.11
0.112 0.316 0.432 0.532 0.747 0.823
4.95 7.35 8.96 11.26 16.09 19.40
0.14 0.13 0.12 0.16 0.18 0.20
Mean of three replications. Standard deviation based on three replications.
Table 2 Equilibrium moisture content M, % d.b. of chickpea flour obtained by desorption at different water activities and temperatures 10 °C
Salt
LiCl MgCl2 KCO3 NaBr NaCl KCl a b
20 °C
30 °C
Ma
sdb
aw
M
sd
aw
M
sd
aw
M
sd
0.113 0.335 0.431 0.622 0.757 0.868
8.56 12.21 13.40 17.72 20.78 26.74
0.19 0.25 0.11 0.23 0.24 0.21
0.113 0.331 0.432 0.591 0.755 0.851
8.01 10.62 12.43 14.40 19.08 24.96
0.08 0.16 0.17 0.20 0.10 0.11
0.113 0.324 0.432 0.560 0.751 0.836
7.43 9.02 11.99 12.65 18.23 24.05
0.08 0.12 0.17 0.27 0.20 0.14
0.112 0.316 0.432 0.532 0.747 0.823
6.44 8.46 11.29 12.04 16.98 23.60
0.13 0.12 0.17 0.18 0.13 0.08
Mean of three replications. Standard deviation based on three replications.
of the flour was higher. A similar effect has been reported for cowpea flour (Chhinnan & Beuchat, 1985) and rice flour (Durakova & Menkov, 2004). Fig. 1 gives the experimental data obtained after adsorption and desorption at 20 °C. The sorption isotherms have an S-shape profile. The hysteresis effect was not distinctly expressed. The coefficients for the three-parameter models, P and SEM values are presented in Table 3 for adsorption and Table 4 for desorption. For adsorption and desorption the P and SEM
30 Equilibrium moisture content. %d.b.
40 °C
aw
values obtained by the modified Halsey and modified Oswin equations were lower. The analysis of the residuals only for both of the models is presented in Fig. 2. The residuals obtained by both models denoted uniformly scattered points in the residual plots. Therefore we recommend the modified Halsey and modified Oswin models for description of the chickpea flour equilibrium isotherms. The calculated MMC values at the four temperatures are presented in Table 5. The MMC decreased with the increase in temperature. Menkov, Paskalev, and Galyazkov (1999) and Lahsasni, Kouhila, and Mahrouz (2004) employed a modification of the BET model which incorporated the temperature effect:
25
M¼
20
ðA þ BtÞCaw ð1 aw Þð1 aw þ Caw Þ
ð10Þ
15
M m ¼ A þ Bt
10
The coefficients values of the modified BET model, P and SEM are presented in Table 3 for adsorption and Table 4 for desorption. The low values of P and SEM show that the modified BET model can be used to calculate the MMC. The net isosteric heats of sorption as a function of the moisture content are presented in Fig. 3. The figure demonstrates that the heats of sorption decrease continually with the increasing moisture content. The heat of
5 0 0
0.2
0.4
0.6
0.8
1
Water activity
Fig. 1. Comparison of isotherms at 20 °C: (d) desorption; (s) adsorption; (—) modified Halsey model.
ð11Þ
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A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539
Table 3 Model coefficients (A, B, C), mean relative error (P, %) and standard error of moisture (SEM) for adsorption Model Modified Modified Modified Modified Modified
Chung–Pfost Oswin Halsey Henderson BET
A
B
C
P
SEM
314.2460 13.92884 4.91108 0.000184 8.043770
35.46810 0.08215 0.018856 2.239437 0.074712
0.169053 0.361772 1.978204 1.943038 77.94349
6.95 5.30 4.91 15.63 2.78
1.18 0.74 1.04 3.48 0.28
Table 4 Model coefficients (A, B, C), mean relative error (P, %) and standard error of moisture (SEM) for desorption Model Modified Modified Modified Modified Modified
Chung–Pfost Oswin Halsey Henderson BET
A
B
C
P
SEM
390.1014 14.90687 5.531604 0.000161 8.381076
38.32762 0.061772 0.017202 3.137333 0.057495
0.164011 0.341096 2.145665 1.98053 256.9351
7.06 6.53 4.73 15.81 3.06
1.45 1.08 1.28 3.23 0.44
Modified Oswin, desorption 4
2
2
Residual, %d.b
Residual, % d.b.
Modified Oswin, adsorption 4
0
-2
0
-2
-4
-4 0
5
10
15
20
25
30
0
Equilibrium moisture content, % d.b. Modified Halsey, adsorption
10
15
20
25
30
Modified Halsey, desorption
4
4
2
2
Residual, %d.b.
Residual, %d.b.
5
Equilibrium moisture content, %d.b.
0
-2
-4
0
-2
-4 0
5
10
15
20
25
30
Equilibrium moisture content, %d.b.
0
10
20
30
40
Equilibrium moisture content, %d.b.
Fig. 2. Plot of residuals fit of modified Oswin and modified Halsey models to adsorption and desorption data.
Table 5 BET monolayer moisture content (% d.b.) of chickpea flour at several temperatures t (°C)
Adsorption
Desorption
10 20 30 40
7.26 6.59 5.63 5.28
7.78 7.06 6.66 6.46
desorption values are higher than those of adsorption only at lower moisture contents. A power function was used to describe the relationship between isosteric heat of sorption and moisture content (Hossain, Bala, Hossain, & Mondol, 2001): Adsorption: Qst ¼ 1194:8M 1:9781 ;
r2 ¼ 1
ð12Þ
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539 30
Heat of sorption, kJ/mol
25
20
15
10
5
0 5
10
15
20
25
Equilibrium moisture content, % d.b.
Fig. 3. Net isosteric heat of sorption of chickpea flour: (d) desorption; () adsorption; (—) power model.
Desorption: Qst ¼ 2070:0M 2:1388 ;
r2 ¼ 0:9999
ð13Þ
The high coefficient of determination values show that the power function can be used to calculate the heat of sorption of chickpea flour for varying moisture content.
4. Conclusions The sorption capacity of chickpea flour decreased with an increase in temperature at constant water activity. The modified Halsey and modified Oswin models are suitable for describing the relationship between the equilibrium moisture content, the water activity, and the temperature for chickpea flour. The modified BET equation can be used to calculate the monolayer moisture content. The power function describes the relationship between the heat of sorption and the moisture content of the flour.
Acknowledgement This study was conducted with the kind support of the National Research Fund, Ministry of Education and Science, Bulgaria.
References Abdullah, N., & Nawawi, A. (2000). Fungal spoilage of starch based foods in relation to its water activity. Journal of Stored Products Research, 36, 47–54.
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Al-Muhtaseb, A. H., McMinn, W. A. M., & Magee, T. R. A. (2002). Moisture sorption isotherm characteristics of food products: A review. Food and Bioproducts Processing, 80, 118–129. AOAC. (1990). Official Methods of Analysis (15th ed.). Association of Official Analytical Chemists, Washington, DC. ASAE. (2001). Moisture relationship of plant-based agricultural products. ASAE Standard D245.5, St. Joseph, Michigan. Bell, L., & Labuza, T. (2000). Moisture sorption: Practical aspects of isotherm measurement and use. St. Paul: American Association of Cereal Chemists, Inc. (pp. 33–36). Brunauer, S., Emmett, P. H., & Teller, E. (1938). Adsorption of gases in multi-molecular layers. Journal of American Chemical Society, 60, 309–316. Cardoso-Santiago, R. A., & Areas, J. A. G. (2001). Nutritional evaluation of snacks obtained from chickpea and bovine lung blends. Food Chemistry, 74, 35–40. Chen, C., & Morey, R. V. (1989). Comparison of four EMC/ERH equations. Transactions of ASAE, 32, 983–990. Chhinnan, M. S., & Beuchat, L. R. (1985). Sorption isotherms of whole cowpeas and flours. Lebensmittel- Wissenschaft und Technologie, 18, 83–88. Durakova, A., & Menkov, N. (2004). Moisture sorption characteristics of chickpea flour. Nahrung, 48, 137–140. Goni, I., & Valentin-Gamaza, C. (2003). Chickpea flour ingredient slows glycemic response to pasta in healthy volunteers. Food Chemistry, 81, 511–515. Hossain, M. D., Bala, B. K., Hossain, M. A., & Mondol, M. R. A. (2001). Sorption isotherms and heat of sorption of pineapple. Journal of Food Engineering, 48, 103–107. Lahsasni, S., Kouhila, M., & Mahrouz, M. (2004). Adsorption– desorption isotherms and heat of sorption of prickly pear fruit (Opuntia ficus indica). Energy Conversion and Management, 45, 249–261. Linfeng, W., & Floters, R. A. (1999). The effects of storage on flour quality and baking performance. Food Research International, 15, 215–234. Menkov, N. D. (2000). Moisture sorption isotherms of chickpea seeds at several temperatures. Journal of Food Engineering, 45, 189– 194. Menkov, N. D., Paskalev, H., & Galyazkov, D. (1999). Applying the linear equation of correlation of Brunauer–Emmett–Teller (BET)monolayer moisture content with temperature. Nahrung, 43, 118–121. Moreira, R., Vazquez, G., & Chenlo, F. (2002). Influence of temperature on sorption isotherms of chickpea: evaluation of isosteric heat of sorption. Electronic Journal of Environmental, Agricultural and Food Chemistry, 1. Nasir, M., Butt, M. S., Anjum, F. M., Sharif, K., & Minhas, R. (2003). Effect of moisture on the shelf life of wheat flour. International Journal of Agricultural and Biology, 5, 458–459. Suhasini, A. W., & Malleshi, N. G. (2003). Nutritional and carbohydrate characteristics of wheat and chickpea based weaning foods. International Journal of Food Science and Nutrition, 54, 181–187. Teunou, E., & Fitzpatrick, J. J. (1999). Effect of relative humidity and temperature on food powder flowability. Journal of Food Engineering, 42, 109–116. Van den Berg, C., & Bruin, S. (1981). Water activity and its estimation in food systems: Theoretical aspects. In L. B. Rockland & G. F. Stewart (Eds.), Water activity: Influences on food quality (pp. 1–61). London: Academic Press. Wolf, W., Spiess, W. E. L., & Jung, G. (1985). Standardization of isotherm measurements (COST-Project 90 and 90 bis). In D. Stimatos & J. L. Multon (Eds.), Properties of water in foods in relation to quality and stability (pp. 661–679). Dordrecht: Martinus Nijhoff.
Research note
Moisture sorption characteristics of chickpea flour Albena G. Durakova, Nikolay D. Menkov
*
Department of Process Engineering, University of Food Technologies, 26 Maritza Blvd., 4000 Plovdiv, Bulgaria Received 22 March 2004; accepted 28 June 2004
Abstract Moisture equilibrium data (adsorption and desorption) of chickpea flour were determined using the static gravimetric method of saturated salt solutions at four storage temperatures: 10, 20, 30 and 40 °C. The range of water activities for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with the increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Chung–Pfost, modified Oswin, modified Halsey and modified Henderson). The monolayer moisture content was estimated from sorption data using the Brunauer–Emmett–Teller (BET) equation. The isosteric heats of sorption were evaluated using Clausius–Clapeyron equation. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Chickpea flour; Heat of sorption; Moisture content; Water activity
1. Introduction With recent consumer interest in functional food, chickpea flour is increasingly being used as an ingredient and a functional modifier in many foods (CardosoSantiago & Areas, 2001; Goni & Valentin-Gamazo, 2003; Suhasini & Malleshi, 2003). The moisture content of the flour as a hygroscopic material exerts a strong influence on its quality and technological properties (Abdullah & Nawawi, 2000; Linfeng & Floters, 1999; Nasir, Butt, Anjum, Sharif, & Minhas, 2003; Teunou & Fitzpatrick, 1999). The sorption properties of foods (equilibrium moisture content, monolayer moisture, heat of sorption) are essential for the design and optimization of many processes such as drying, packaging and storage (Al-Muhtaseb, McMinn, & Magee, 2002). Several researchers have conducted equilibrium isotherm studies of chickpea seeds (Menkov, 2000; Moreira, Vazquez, & *
Corresponding author. Tel.: +359 32 603 689; fax: +359 32 644
102. E-mail addresses: [email protected] (A.G. Durakova), nimenkov@hiffi-plovdiv.acad.bg (N.D. Menkov). 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.06.019
Chenlo, 2002), but the sorption data on chickpea flour are limited. A number of models have been suggested in the literature for the dependence between the equilibrium moisture content (EMC) and the water activity (aw) (Van den Berg & Bruin, 1981). The modified Chung–Pfost, modified Henderson, modified Halsey and modified Oswin equations which incorporate the temperature effect have been proposed by Chen and Morey (1989). The models have been adopted as standard equations by the ASAE for the description of sorption isotherms (ASAE, 2001). The objectives of this work are: (1) to obtain experimental equilibrium data of chickpea flour at 10, 20, 30 and 40 °C; (2) to find out the suitable model describing the isotherms; (3) to calculate the monolayer moisture content; (4) to calculate the heat of sorption of the flour.
2. Material and methods 2.1. Material Commercial chickpea flour produced in Bulgaria was used in this study. The initial moisture content of the
536
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539
flour was 12.58% wet basis. AOAC (1990) standard procedures were used for the determination of crude fat–– 4.66%, protein––19.63%, carbohydrate––61.90% and ash––1.23%. 2.2. Procedure The EMC of the chickpea flour was determined at 10, 20, 30 and 40 °C. The static gravimetric method was applied (Wolf, Spiess, & Jung, 1985). For the adsorption process, flour was dehydrated in a desiccator over P2O5 at a room temperature for 20 days prior to the beginning of the experiment. The desorption isotherms were determined on samples hydrated in a glass jar over distilled water at a room temperature to approximately 30% dry basis (d.b.) moisture content. Samples of 1 ± 0.02 g were weighed in weighing bottles. The weighing bottles were then put in hygrostats with six saturated salt solutions (LiCl, MgCl2, K2CO3, NaBr, NaCl, KCl) used to obtain constant water activities environments (Bell & Labuza, 2000). All used salts were of reagent grade. At high water activities (aw > 0.70) crystalline thymol was placed in the hygrostats to prevent the microbial spoilage of the flour. The hygrostats were kept in thermostats at 10, 20, 30 and 40 ± 0.2 °C. Samples were weighed (balance, sensitivity ±0.0001 g) every three days. Equilibrium was acknowledged when three consecutive weight measurements showed a difference less than 0.001 g. The moisture content of each sample was determined by the oven method (105 °C for 24 h) by means of triplicate measurements. 2.3. Analysis of data The description of the sorption isotherms was verified according to the following four models: Modified Chung–Pfost A aw ¼ exp expðCMÞ tþB Modified Halsey expðA þ BtÞ aw ¼ exp MC Modified Oswin C aw M ¼ ðA þ BtÞ 1 aw
ð1Þ
ð2Þ
ð3Þ
Modified Henderson 1 aw ¼ exp½Aðt þ BÞM C
ð4Þ
where M is the moisture content, % d.b; aw is the water activity, decimal; A, B and C are coefficients; t is the temperature, °C.
A nonlinear, least squares regression program was used to fit the four models to the experimental data (all replications). The suitability of the equations was evaluated and compared using the mean relative error P, %; standard error of moisture (SEM) and randomness of residuals ei (Chen & Morey, 1989): ^ i 100 X M i M ð5Þ P¼ M N i sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ^ i Þ2 ðM i M SEM ¼ df ^i ei ¼ M i M
ð6Þ ð7Þ
^ i are experimentally observed and prewhere Mi and M dicted by the model value of the equilibrium moisture content, respectively, N is the number of data points, and df is the number of degree of freedom (number of data points minus number of constants in the model). The monolayer moisture contents (MMC) for each temperature were calculated using the Brunauer– Emmett–Teller (BET) equation (Brunauer, Emmett, & Teller, 1938) and the experimental data for water activities up to 0.45 (Bell & Labuza, 2000): M¼
M e Caw ð1 aw Þð1 aw þ Caw Þ
ð8Þ
where Me is the MMC, % d.b.; C is the coefficient. The values of heat of sorption were calculated using the Clausius–Clapeyron equation from the slope of the plot between the values of ln aw and 1/T at constant moisture: Q 1 þ constant ð9Þ ln aw ¼ st R T where Qst is isosteric heat of sorption, kJ mol1 ; T is the absolute temperature, K; R is the universal gas constant, kJ mol1 K1. Using Eq. (2), the values of aw were determined at the four temperatures and constant moisture contents 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22% and 24% d.b.
3. Results and discussion The EMC mean values obtained and the standard deviations based on the triplicate measurements for the respective water activity and temperature are presented in Table 1 for adsorption and in Table 2 for desorption. The EMC values decreased with an increase in the temperature at constant aw. Similar trends for many foods have been reported in the literature (AlMuhtaseb et al., 2002). The comparison with the published sorption data for chickpea seeds (Menkov, 2000; Moreira et al., 2002) showed that the sorption capacity
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539
537
Table 1 Equilibrium moisture content M, % d.b. of chickpea flour obtained by adsorption at different water activities and temperatures 10 °C
Salt
LiCl MgCl2 KCO3 NaBr NaCl KCl a b
20 °C a
aw
M
0.113 0.335 0.431 0.622 0.757 0.868
7.66 10.48 12.71 15.38 19.07 25.99
sd
b
0.16 0.07 0.24 0.09 0.13 0.22
30 °C
40 °C
aw
M
sd
aw
M
sd
aw
M
sd
0.113 0.331 0.432 0.591 0.755 0.851
6.89 9.31 11.59 12.57 18.20 23.85
0.08 0.03 0.23 0.13 0.10 0.18
0.113 0.324 0.432 0.560 0.751 0.836
5.71 8.61 9.58 11.78 17.70 20.22
0.18 0.21 0.18 0.11 0.17 0.11
0.112 0.316 0.432 0.532 0.747 0.823
4.95 7.35 8.96 11.26 16.09 19.40
0.14 0.13 0.12 0.16 0.18 0.20
Mean of three replications. Standard deviation based on three replications.
Table 2 Equilibrium moisture content M, % d.b. of chickpea flour obtained by desorption at different water activities and temperatures 10 °C
Salt
LiCl MgCl2 KCO3 NaBr NaCl KCl a b
20 °C
30 °C
Ma
sdb
aw
M
sd
aw
M
sd
aw
M
sd
0.113 0.335 0.431 0.622 0.757 0.868
8.56 12.21 13.40 17.72 20.78 26.74
0.19 0.25 0.11 0.23 0.24 0.21
0.113 0.331 0.432 0.591 0.755 0.851
8.01 10.62 12.43 14.40 19.08 24.96
0.08 0.16 0.17 0.20 0.10 0.11
0.113 0.324 0.432 0.560 0.751 0.836
7.43 9.02 11.99 12.65 18.23 24.05
0.08 0.12 0.17 0.27 0.20 0.14
0.112 0.316 0.432 0.532 0.747 0.823
6.44 8.46 11.29 12.04 16.98 23.60
0.13 0.12 0.17 0.18 0.13 0.08
Mean of three replications. Standard deviation based on three replications.
of the flour was higher. A similar effect has been reported for cowpea flour (Chhinnan & Beuchat, 1985) and rice flour (Durakova & Menkov, 2004). Fig. 1 gives the experimental data obtained after adsorption and desorption at 20 °C. The sorption isotherms have an S-shape profile. The hysteresis effect was not distinctly expressed. The coefficients for the three-parameter models, P and SEM values are presented in Table 3 for adsorption and Table 4 for desorption. For adsorption and desorption the P and SEM
30 Equilibrium moisture content. %d.b.
40 °C
aw
values obtained by the modified Halsey and modified Oswin equations were lower. The analysis of the residuals only for both of the models is presented in Fig. 2. The residuals obtained by both models denoted uniformly scattered points in the residual plots. Therefore we recommend the modified Halsey and modified Oswin models for description of the chickpea flour equilibrium isotherms. The calculated MMC values at the four temperatures are presented in Table 5. The MMC decreased with the increase in temperature. Menkov, Paskalev, and Galyazkov (1999) and Lahsasni, Kouhila, and Mahrouz (2004) employed a modification of the BET model which incorporated the temperature effect:
25
M¼
20
ðA þ BtÞCaw ð1 aw Þð1 aw þ Caw Þ
ð10Þ
15
M m ¼ A þ Bt
10
The coefficients values of the modified BET model, P and SEM are presented in Table 3 for adsorption and Table 4 for desorption. The low values of P and SEM show that the modified BET model can be used to calculate the MMC. The net isosteric heats of sorption as a function of the moisture content are presented in Fig. 3. The figure demonstrates that the heats of sorption decrease continually with the increasing moisture content. The heat of
5 0 0
0.2
0.4
0.6
0.8
1
Water activity
Fig. 1. Comparison of isotherms at 20 °C: (d) desorption; (s) adsorption; (—) modified Halsey model.
ð11Þ
538
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539
Table 3 Model coefficients (A, B, C), mean relative error (P, %) and standard error of moisture (SEM) for adsorption Model Modified Modified Modified Modified Modified
Chung–Pfost Oswin Halsey Henderson BET
A
B
C
P
SEM
314.2460 13.92884 4.91108 0.000184 8.043770
35.46810 0.08215 0.018856 2.239437 0.074712
0.169053 0.361772 1.978204 1.943038 77.94349
6.95 5.30 4.91 15.63 2.78
1.18 0.74 1.04 3.48 0.28
Table 4 Model coefficients (A, B, C), mean relative error (P, %) and standard error of moisture (SEM) for desorption Model Modified Modified Modified Modified Modified
Chung–Pfost Oswin Halsey Henderson BET
A
B
C
P
SEM
390.1014 14.90687 5.531604 0.000161 8.381076
38.32762 0.061772 0.017202 3.137333 0.057495
0.164011 0.341096 2.145665 1.98053 256.9351
7.06 6.53 4.73 15.81 3.06
1.45 1.08 1.28 3.23 0.44
Modified Oswin, desorption 4
2
2
Residual, %d.b
Residual, % d.b.
Modified Oswin, adsorption 4
0
-2
0
-2
-4
-4 0
5
10
15
20
25
30
0
Equilibrium moisture content, % d.b. Modified Halsey, adsorption
10
15
20
25
30
Modified Halsey, desorption
4
4
2
2
Residual, %d.b.
Residual, %d.b.
5
Equilibrium moisture content, %d.b.
0
-2
-4
0
-2
-4 0
5
10
15
20
25
30
Equilibrium moisture content, %d.b.
0
10
20
30
40
Equilibrium moisture content, %d.b.
Fig. 2. Plot of residuals fit of modified Oswin and modified Halsey models to adsorption and desorption data.
Table 5 BET monolayer moisture content (% d.b.) of chickpea flour at several temperatures t (°C)
Adsorption
Desorption
10 20 30 40
7.26 6.59 5.63 5.28
7.78 7.06 6.66 6.46
desorption values are higher than those of adsorption only at lower moisture contents. A power function was used to describe the relationship between isosteric heat of sorption and moisture content (Hossain, Bala, Hossain, & Mondol, 2001): Adsorption: Qst ¼ 1194:8M 1:9781 ;
r2 ¼ 1
ð12Þ
A.G. Durakova, N.D. Menkov / Journal of Food Engineering 68 (2005) 535–539 30
Heat of sorption, kJ/mol
25
20
15
10
5
0 5
10
15
20
25
Equilibrium moisture content, % d.b.
Fig. 3. Net isosteric heat of sorption of chickpea flour: (d) desorption; () adsorption; (—) power model.
Desorption: Qst ¼ 2070:0M 2:1388 ;
r2 ¼ 0:9999
ð13Þ
The high coefficient of determination values show that the power function can be used to calculate the heat of sorption of chickpea flour for varying moisture content.
4. Conclusions The sorption capacity of chickpea flour decreased with an increase in temperature at constant water activity. The modified Halsey and modified Oswin models are suitable for describing the relationship between the equilibrium moisture content, the water activity, and the temperature for chickpea flour. The modified BET equation can be used to calculate the monolayer moisture content. The power function describes the relationship between the heat of sorption and the moisture content of the flour.
Acknowledgement This study was conducted with the kind support of the National Research Fund, Ministry of Education and Science, Bulgaria.
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