- Email: [email protected]
Moisture sorption isotherms of chickpea seeds at several temperatures
Journal of Food Engineering 45 (2000) 189±194
www.elsevier.com/locate/jfoodeng
Moisture sorption isotherms of chickpea seeds at several temperatures Nikolay D. Menkov * Department of Process Engineering, Higher Institute of Food and Flavour Industries, 26 Maritza Blvd., 4000 Plovdiv, Bulgaria Received 15 October 1999; accepted 28 February 2000
Abstract The equilibrium moisture contents of chickpea seeds were determined using the gravimetric static method at 5°C, 20°C, 40°C and 60°C over a range of water activities from 0.110 to 0.877. The sorption capacity of seeds decreased with the increase in temperature at constant water activity. Six models, modi®ed Chung±Pfost, modi®ed Halsey, modi®ed Oswin, modi®ed Henderson, Guggenheim±Anderson±de Boer (GAB), and a new Fraction±linear (FL), were applied to analyze the experimental data. The FL model was found to be the most suitable for describing the sorption data. For practical considerations a single monolayer value (in water activity (aW ) range 0.16±0.18) can be assumed. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Chickpea (Cicer arietinum L.) is a valuable foodstu due to its high content of vegetable protein with essential amino acids (Chavan, Kadam & Salunke, 1986; Clemente, Sanchez-Vioque, Vioque, Bautista & Millan, 1998; Jood, Bichnoi & Sharma, 1998; Rincon, Martinez & Iba nez, 1998; Sanchez-Vioque, Clemente, Vioque, Bautista & Millan, 1999). The legume seeds meant for food or sowing are normally subjected to long-term storage. During that time, important physico-chemical and biological changes take place with a strong impact on the nutrient properties. To a large extent, these changes depend on the moisture content of the seeds (Burr, Kon & Morris, 1968; Garcia-Vela & Stanley, 1989; Hentges, Weaver & Nielsen, 1991). The storage conditions of the legume seeds have a substantial in¯uence on their germination and longevity (Roberts, 1972; Vertucci, Roos & Crane, 1994). A number of models have been suggested in the literature to describe the relationship between the equilibrium moisture content (EMC) and the water activity (aW ) of food (Van den Berg & Bruin, 1981). Some of them take into account the eect of temperature. The modi®ed Chung±Pfost (Chung & Pfost, 1967; Pfost, Mourer, Chung & Milliken, 1976), modi*
Tel.: +359-32-449-393; fax: +359-32-440-102. E-mail address: nimenkov@hi-plovdiv.acad.bg (N.D. Menkov).
®ed Henderson (Henderson, 1952; Thompson, Peart & Foster, 1968), modi®ed Halsey (Halsey, 1948; Iglesias & Chirife, 1976a), modi®ed Oswin (Oswin, 1946; Chen & Morey, 1989) and GAB (Van den Berg, 1984; Labuza, Kaanane & Chen, 1985) equations have been adopted by the American Society of Agricultural Engineers as standard equations for describing sorption isotherms (ASAE, 1995). Veltchev and Menkov (2000) applied successfully a new four-parameter, Fraction±linear (FL) model, for describing the S-shape desorption isotherms of apples. The monolayer moisture content (MMC) which is of signi®cant importance to the physical and chemical stability of dehydrated materials with regard to lipid oxidation, enzyme activity, non-enzymatic browning, ¯avour components preservation, and structural characteristics (Labuza, Tannenbaum & Karel, 1970; Karel & Yong, 1981), can be determined from the equilibrium sorption isotherms by means of the two-parameter BET (Brunauer±Emmett±Teller) equation (Brunauer, Emmett & Teller, 1938). The knowledge of the equilibrium moisture content of chickpea seeds at various temperatures would allow to specify the storage conditions for the seeds. The object of this work is to: (1) determine experimentally the equilibrium sorption isotherms of chickpea seeds at 5°C, 20°C, 40°C and 60°C; (2) select the most suitable model describing the isotherms; (3) calculate the monolayer moisture content as a function of temperature.
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 5 2 - 2
190
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
Modi®ed Halsey ÿ exp A Bt aW exp : MC
2. Material and methods 2.1. Material Chickpea seeds of the Resurs 1 variety (provided by the Agricultural University, Plovdiv, Bulgaria, 1998) were tested. The seeds had been gathered 6 months in advance. After harvest they were sun-dried to approximately 12% dry basis (d.b.) and kept in a dry place (aW < 0.55) at room temperature. The mass of 1000 seeds was about 270 g. The seeds contained 58.4% d.b. carbohydrate, 23.1% d.b. protein, 4.0% d.b. fat and 2.8% d.b. ash.
Modi®ed Henderson 1 ÿ aW exp ÿ A t BM C :
1 ÿ
B0 a
The water vapor sorption equilibrium of the chickpea seeds was determined at 5°C, 20°C, 40°C and 60°C, using the static gravimetric method (Wolf, Spiess & Jung, 1985). For the adsorption process, seeds were ®rst dehydrated in a desiccator over P2 O5 at room temperature for 20 days to an initial moisture content below 1% d.b. Samples for desorption measurements were ®rst hydrated in a glass jar over distilled water at 4°C for 20 days to approximately 32% d.b. For sorption measurements samples of 4 0:2 g were weighed in weighing bottles. The weighing bottles were then put in 10 hygrostats with dierent saturated salt solutions (LiCl, CH3 COOK, MgCl2 , K2 CO3 , NaBr, Mg(NO3 )2 , SrCl2 , NaCl, KBr, KCl), to obtain constant water activities (Greenspan, 1977; Weisser, 1986; Maroulis, Tsami, Marinos-Kouris & Saravacos, 1988). All used salts were of reagent grade. At high water activities aW > 0:70 crystalline thymol was placed in the hygrostats to prevent microbial spoilage of the seeds (Wolf et al., 1985). The hygrostats were kept in thermostats at 5°C, 20°C, 40°C and 60 0.2°C. Samples were weighed (balance, sensitivity 0.0001 g) every 3 days. Equilibrium was acknowledged when three consecutive weight measurements showed a dierence less than 0.001 g. The moisture content of each sample was determined by the vacuum oven method (AOAC, 1990). The time for the samples to reach equilibrium varied from 5 to 40 days, depending on the temperature and water activity. The equilibrium moisture contents were determined by means of triplicate measurements. 2.3. Analysis of data The description of the relationship between EMC, aW and temperature was veri®ed according to the following six models:
1
3
4
GAB M
2.2. Sorption isotherms
Modi®ed Chung±Pfost ÿA exp ÿ CM : aW exp tB
Modi®ed Oswin C aW : M A Bt 1 ÿ aW
2
B0 B exp
h1 RT
C 0 C exp
AB0 C 0 aW ; 0 0 0 W 1 ÿ B aW B C aW
h2 RT
5
;
6
:
7
FL M
A BT DaW ; 1 ÿ CaW
8
where M is the moisture content (% d.b.); aW the water activity; A, B, C, D, h1 , h2 the model coecients; t the temperature (°C); T the absolute temperature (K) and R 8:314 is the universal gas constant (J molÿ1 Kÿ1 ). A nonlinear, least-squares regression program was used to ®t the six models to the experimental data (all replications). The suitability of the equations was evaluated using the mean relative error (MRE, %), standard error of moisture (SEM) and randomness of residual (Chen & Morey, 1989): ^ i 100 X Mi ÿ M 9 MRE ; Mi N s P ^ i 2 Mi ÿ M SEM : df
10
Residual ^i; ei Mi ÿ M
11
^ i are experimental and predicted values, where Mi and M N is the number of data points and df is the degree of freedom (number of data points minus number of coecients in the model). The residuals obtained for each model with its respective coecients were plotted against measured EMC and assessed visually as random or patterned. In order to determine the monolayer moisture content, the two-parameter BET equation which is valid for aW < 0:45 (Labuza et al., 1985) was applied:
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
191
rithm described in the literature (Iglesias & Chirife, 1976b; Menkov, Paskalev, Galyazkov & KerezievaRakova, 1999).
BET
Mm CaW ; 12 M 1 ÿ aW 1 ÿ aW CaW where Mm is the MMC (% d.b.). The MMC values for each temperature were calculated following an algo-
3. Results and discussion In Fig. 1, the adsorption and desorption isotherms of chickpea seeds at 5°C and 60°C are presented. The sorption isotherms have an S-shape pro®le typical for legume seeds (Vertucci & Leopold, 1987; Pappas & Rao, 1987; Mazza & Jayas, 1991). The hysteresis eect was distinctly expressed at 5°C. At 60°C the dierences between the experimental data obtained after adsorption and desorption are not statistically signi®cant (level of signi®cance 0.05) for the most experimental points. The experimental data of all measurements are given in Table 1. The EMC values decreased with an increase in the temperature at constant aW . Similar trends for many seeds and food materials have been reported in the literature (Labuza, 1968; Menkov, 1997; Walters & Hill, 1998). The comparison between the results obtained and the data provided by Sethi, Soni and Chopra (1977) for chickpea ¯our shows that the seeds had a lower water sorption capacity. Similar observations were reported by
Fig. 1. Desorption and adsorption isotherms of chickpea seeds at 5°C and 60°C showing the hysteresis eect.
Table 1 Adsorption (ads) and desorption (des) data (M, % d.b.) of chickpea seeds at dierent temperatures 5°C aW
a b
20°C Ma
SDb
aW
40°C M
SD
aW
60°C M
SD
aW
M
SD
ads des
0.113 0.113
4.94 6.58
0.12 0.11
0.113 0.113
4.58 4.40
0.07 0.06
0.112 0.112
3.27 3.71
0.11 0.08
0.110 0.110
3.08 3.25
0.10 0.17
ads des
0.245 0.245
7.20 8.73
0.14 0.14
0.231 0.231
6.67 7.26
0.05 0.09
0.201 0.201
4.99 5.72
0.08 0.19
0.160 0.160
4.13 4.18
0.03 0.06
ads des
0.336 0.336
8.16 9.63
0.18 0.11
0.331 0.331
7.62 9.05
0.06 0.21
0.316 0.316
5.57 6.74
0.05 0.14
0.293 0.293
5.37 5.19
0.10 0.17
ads des
0.431 0.431
9.22 10.66
0.08 0.12
0.432 0.432
8.88 10.12
0.14 0.12
0.432 0.432
7.48 8.53
0.19 0.10
0.432 0.432
6.84 6.64
0.06 0.18
ads des
0.589 0.589
10.89 12.63
0.16 0.08
0.544 0.544
10.15 11.28
0.21 0.12
0.484 0.484
7.90 8.75
0.14 0.24
0.440 0.440
7.03 6.95
0.12 0.06
ads des
0.635 0.635
12.74 15.19
0.20 0.15
0.591 0.591
10.87 12.51
0.12 0.19
0.532 0.532
9.02 9.65
0.07 0.13
0.497 0.497
7.78 8.11
0.18 0.17
ads des
0.757 0.757
17.80 20.53
0.14 0.10
0.725 0.725
14.55 17.24
0.11 0.08
0.658 0.658
12.20 12.57
0.14 0.17
0.580 0.580
8.85 9.22
0.11 0.12
ads des
0.771 0.771
18.38 21.44
0.09 0.20
0.755 0.755
17.39 18.16
0.16 0.22
0.747 0.747
15.33 16.09
0.23 0.07
0.745 0.745
13.50 12.44
0.08 0.20
ads des
0.851 0.851
24.72 25.44
0.20 0.07
0.817 0.817
21.83 23.52
0.15 0.14
0.794 0.794
17.11 16.60
0.23 0.22
0.789 0.789
14.34 14.09
0.22 0.17
ads des
0.877 0.877
28.70 29.15
0.22 0.25
0.851 0.851
24.65 24.51
0.21 0.21
0.823 0.823
19.34 19.12
0.06 0.17
0.803 0.803
14.95 16.29
0.28 0.09
Mean of n 3 replications. Standard deviations.
192
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
Chhinnan and Beuchat (1985) with whole cowpeas and cowpea ¯ours. The coecients for the three- and four-parameter models together with the corresponding MRE and SEM values are presented in Table 2. The parameters of the GAB models are presented in Table 3. Example of
the analysis of the residuals are presented in Fig. 2. For the modi®ed Chung±Pfost (for adsorption and desorption as well) and modi®ed Henderson equations (for adsorption), the residual plots were patterned, which makes them unsuitable. The residuals obtained by all the other models were randomly distributed. For these
Table 2 Estimated values of model coecients (A, B, C, D); mean relative error (MRE, %) and standard error of moisture (SEM) of three- and fourparameter models for adsorption (ads) and desorption (des) Parameter
Model Modi®ed Chung±Pfost
Modi®ed Halsey
Modi®ed Oswin
Modi®ed Henderson
FL
A
ads des
455.2110 308.1214
3.337690 4.098313
10.79086 12.71118
0.000138 0.000328
10.35033 15.70636
B
ads des
101.6365 52.78201
)0.007900 )0.012647
)0.05010 )0.07701
12.82720 48.96227
)0.022551 )0.038772
C
ads des
0.166102 0.159547
1.582279 1.777402
0.49602 0.43913
2.138215 1.415975
0.954224 0.910345
D
ads des
± ±
± ±
± ±
± ±
4.469358 5.778511
MRE
ads des
12.13 10.76
4.63 6.20
6.71 5.01
15.31 10.40
4.88 5.24
SEM
ads des
2.16 1.81
0.59 0.78
0.77 0.75
3.01 1.30
0.58 0.70
Residual
ads des
Patterned Patterned
Random Random
Random Random
Patterned Random
Random Random
Table 3 Estimated values of model coecients (A, B, C, h1 , h2 ); mean relative error (MRE, %) and standard error of moisture (SEM) of the GAB model
Adsorption Desorption
A
B
C
h1
h2
MRE
SEM
Residual
5.372119 6.976963
0.547510 0.356268
0.000561 0.000490
1225.049 2071.703
26279.24 25363.67
5.06 6.19
0.65 0.80
Random Random
Fig. 2. Plot of residuals ®t of Chung±Pfost and FL models to adsorption data of chickpea seeds.
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
models the respective MRE and SEM values were similar. For the desorption data at 60°C the MRE values obtained by the modi®ed Halsey and the GAB models were high: 8.3% and 9.8%, respectively. For adsorption data at 20°C the MRE value obtained by the modi®ed Oswin model was 9.1%. The MRE values obtained by the FL model at the studied temperatures varied small: from 4.2% to 5.3% for adsorption and from 4.8 to 6.3 for desorption. This gives us grounds to recommend the FL model for description of the equilibrium isotherms of chickpea seeds. The temperature in¯uence on the monolayer moisture content values is shown in Fig. 3. There is also a hysteresis eect with respect to the monolayer moisture content. The values diminish with the increase in temperature. Menkov et al. (1999) have shown for a large number of biological products that the dependence between MMC and temperature can be described by a linear equation. A modi®cation of the BET model yields: Modi®ed BET A BtCaW ; M 1 ÿ aW 1 ÿ aW CaW
13
Mm A Bt;
14
where A, B and C are coecients independent of temperature. Applying the chickpea data the coecients of Eq. (13) were determined direct, using a nonlinear, leastsquares regression program on the basis of experimental data at aW < 0:45 (all replications): for adsorption A 5:9589; B ÿ0:0322; C 23:13263; for desorption A 7:10650; B ÿ0:0514; C 25:66617. Fig. 3 shows a comparison of the MMC values obtained by the stan-
193
dard procedure with Eq. (12) and Eqs. (13) and (14). It can be seen from the ®gures that in this case the linear equation provides a very good description of the data obtained. The corresponding water activity was determined by Eq. (13) after putting M Mm A Bt p Cÿ1 : 15 aW m Cÿ1 Despite the hysteresis eect, the (aW )m values obtained are close: for adsorption aW m 0:165; for desorption aW m 0:172. The (aW )m value is independent of temperature (the coecient C in Eq. (13) is independent of temperature). For practical considerations a single monolayer value (in the aW range 0.16±0.18) can be assumed, with respective EMC values from 4.1% to 5.7% for adsorption and from 4.2% to 6.7% for desorption.
4. Conclusions The sorption capacity of chickpea seeds decreased with an increase in temperature at constant water activity. The FL model is the most suitable for describing the relationship between the equilibrium moisture content and the water activity for the seeds under study. For practical considerations a single monolayer value (in the aW range 0.16±0.18) can be assumed.
Acknowledgements This study was conducted with the kind support of the National Research Fund, Bulgaria. References
Fig. 3. Eect of temperature on the monolayer moisture content.
AOAC (1990). Ocial methods of analysis of the Association of Ocial Analytical Chemists (15th ed.). Washington, DC: AOAC. ASAE (1995). Moisture relationship of plant-based agricultural products. ASAE Standard D245.5. St. Joseph, Michigan. Brunauer, S., Emmett, P. H., & Teller, E. (1938). Adsorption of gases in multimolecular layers. Journal of the American Chemical Society, 60, 309±319. Burr, K. H., Kon, S., & Morris, H. J. (1968). Cooking rates of dry beans as in¯uenced by moisture content and temperature and time of storage. Food Technology, 22, 336±338. Chavan, J., Kadam, S., & Salunke, D. (1986). Biochemistry and technology of chickpea (Cicer arietinum, L.) seeds. Critical Reviews of Food Science and Nutrition, 25, 107±158. Chen, C. C., & Morey, R. V. (1989). Comparison of four EMC/ERH equations. Transactions of the American Society of Agricultural Engineers, 32, 983±989. Chhinnan, M. S., & Beuchat, L. R. (1985). Sorption isotherms of whole cowpeas and ¯ours. Lebensmittel-Wissenschaft und-Technologie, 18, 83±88.
194
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
Chung, D. S., & Pfost, H. B. (1967). Adsorption and desorption of water vapor by cereal grains and their products. Part II: Development of the general isotherm equation. Transactions of the American Society of Agricultural Engineers, 10, 552±555. Clemente, A., Sanchez-Vioque, R., Vioque, J., Bautista, J., & Millan, F. (1998). Eect of cooking on protein quality of chickpea (Cicer arietinum) seeds. Food Chemistry, 62, 1±6. Garcia-Vela, L. A., & Stanley, D. W. (1989). Protein denaturation and starch gelatinization in hard-to-cook beans. Journal of Food Science, 54, 1284±1292. Greenspan, L. (1977). Humidity ®xed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards ± A. Physics and Chemistry, 81A, 89±96. Halsey, G. (1948). Physical adsorption on non-uniform surfaces. Journal of Chemical Physics, 16, 931±937. Henderson, S. M. (1952). A basic concept of equilibrium moisture. Agricultural Engineering, 33, 9±32. Hentges, D. L., Weaver, C. M., & Nielsen, S. S. (1991). Changes of selected physical and chemical components in development of the hard-to-cook bean defect. Journal of Food Science, 56, 436±442. Iglesias, H. A., & Chirife, J. (1976a). Prediction of eect of temperature on water sorption isotherms of food materials. Journal of Food Technology, 11, 109±116. Iglesias, H. A., & Chirife, J. (1976b). B.E.T. monolayer values in dehydrated foods and food components. Lebensmittel-Wissenschaft und-Technologie, 9, 107±113. Jood, S., Bichnoi, S., & Sharma, A. (1998). Chemical analysis and physico-chemical properties of chickpea and lentil cultivars. Nahrung, 42, 71±74. Karel, M., & Yong, S. (1981). Autoxidation-initiated reactions in foods. In L. B. Rockland, & G. F. Stewart, Water Activity: In¯uences on Food Quality (pp. 511±529). New York: Academic Press. Labuza, T. P. (1968). Sorption phenomena in foods. Food Technology, 22, 263±272. Labuza, T. P., Kaanane, A., & Chen, J. Y. (1985). Eect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. Journal of Food Science, 50, 385± 391. Labuza, T. P., Tannenbaum, S. R., & Karel, M. (1970). Water content and stability of low-moisture and intermediate-moisture foods. Food Technology, 24, 543±549. Maroulis, Z. B., Tsami, E., Marinos-Kouris, D., & Saravacos, G. D. (1988). Application of the GAB model to the moisture sorption isotherms for dried fruits. Journal of Food Engineering, 7, 63±80. Mazza, G., & Jayas, D. S. (1991). Evaluation of four three-parameter equations for the description of the moisture sorption data of Lathyrus pea seeds. Lebensmittel-Wissenschaft und-Technologie, 24, 562±565. Menkov, N. D. (1997). Ein¯uss der Temperatur auf die Gleichgewichtsfeuchte bei konstanter relativer Luftfeuchte. Nahrung, 41, 349±351. Menkov, N. D., Paskalev, H. M., Galyazkov, D. I., & KerezievaRakova, M. (1999). Applying the linear equation of correlation of
Brunauer±Emmet±Teller (BET) ± monolayer moisture content with temperature. Nahrung, 43, 118±121. Oswin, C. R. (1946). The kinetics of package life. III. Isotherm. Journal of the Society of Chemical Industry, London, 65, 419±421. Pappas, G., & Rao, V. N. M. (1987). Sorption isotherms of cowpeas from 25°C to 70°C. Transactions of the American Society of Agricultural Engineers, 30, 1478±1483. Pfost, H. B., Mourer, S. G., Chung, D. S., & Milliken, G. A. (1976). Summarizing and reporting equilibrium moisture data for grains. American Society Agricultural Engineers, Paper No. 76-3520. St. Joseph, Michigan. Rincon, F., Martinez, B., & Iba nez, M. V. (1998). Proximate composition and antinutritive substances in chickpea (Cicer arietinum L.) as aected by the biotype factor. Journal of the Science of Food and Agriculture, 78, 382±388. Roberts, E. H. (1972). Storage environment and the control of viability. In E. H. Roberts, Viability of Seeds (pp 14±58). London: Chapman and Hall. Sanchez-Vioque, R., Clemente, A., Vioque, J., Bautista, J., & Millan, F. (1999). Protein isolates from chickpea (Cicer arietinum L.): chemical composition, functional properties and protein characterization. Food Chemistry, 64, 237±243. Sethi, R. K., Soni, G. L., & Chopra, S. I. (1977). Varietal dierences in chickpea ¯our (Cicer Arietinum) in relation to water vapor sorption. Cereal Chemistry, 54, 79±92. Thompson, T. L., Peart, R. M., & Foster, G. H. (1968). Mathematical simulation of corn drying ± a new model. Transactions of the American Society of Agricultural Engineers, 11, 582±586. Van den Berg, C. (1984). Description of water activity of foods for engineering purposes by means of the GAB model of sorption. In B. M. McKenna, Engineering and Foods (pp. 119±131). New York: Elsevier. Van den Berg, C., & Bruin, S. (1981). Water activity and its estimation in food systems: theoretical aspects. In L. B. Rockland, & G. F. Stewart, Water Activity: In¯uences on Food Quality (pp. 1±61). New York: Academic Press. Veltchev, Z. N., & Menkov, N. D. (2000). Desorption isotherms of apples at several temperatures, Drying Technology, 18, 1127±1138. Vertucci, C. W., & Leopold, A. C. (1987). Water binding in legume seeds. Plant Physiology, 85, 224±231. Vertucci, C. W., Roos, E. E., & Crane, J. (1994). Theoretical basis of protocols for seed storage III. Optimum moisture contents for pea seeds stored at dierent temperatures. Annals of Botany, 74, 531± 540. Walters, C., & Hill, L. M. (1998). Water sorption isotherms of seeds from ultra-dry experiments. Seed Science Research, 8, 69±73. Weisser, H. (1986). In¯uence of temperature on sorption isotherms. In M. Le Maguer, & P. Jelen, Food Engineering and Process Applications, vol. 1 (pp. 189±200). New York: Elsevier. Wolf, W., Spiess, W. E. L., & Jung, G. (1985). Standardization of isotherm measurements (COST-project 90 and 90 bis). In D. Simatos, & J. L. Multon, Properties of Water in Foods in Relation to Quality and Stability (pp. 661±679). Dordrecht: Martinus Nijho.
www.elsevier.com/locate/jfoodeng
Moisture sorption isotherms of chickpea seeds at several temperatures Nikolay D. Menkov * Department of Process Engineering, Higher Institute of Food and Flavour Industries, 26 Maritza Blvd., 4000 Plovdiv, Bulgaria Received 15 October 1999; accepted 28 February 2000
Abstract The equilibrium moisture contents of chickpea seeds were determined using the gravimetric static method at 5°C, 20°C, 40°C and 60°C over a range of water activities from 0.110 to 0.877. The sorption capacity of seeds decreased with the increase in temperature at constant water activity. Six models, modi®ed Chung±Pfost, modi®ed Halsey, modi®ed Oswin, modi®ed Henderson, Guggenheim±Anderson±de Boer (GAB), and a new Fraction±linear (FL), were applied to analyze the experimental data. The FL model was found to be the most suitable for describing the sorption data. For practical considerations a single monolayer value (in water activity (aW ) range 0.16±0.18) can be assumed. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Chickpea (Cicer arietinum L.) is a valuable foodstu due to its high content of vegetable protein with essential amino acids (Chavan, Kadam & Salunke, 1986; Clemente, Sanchez-Vioque, Vioque, Bautista & Millan, 1998; Jood, Bichnoi & Sharma, 1998; Rincon, Martinez & Iba nez, 1998; Sanchez-Vioque, Clemente, Vioque, Bautista & Millan, 1999). The legume seeds meant for food or sowing are normally subjected to long-term storage. During that time, important physico-chemical and biological changes take place with a strong impact on the nutrient properties. To a large extent, these changes depend on the moisture content of the seeds (Burr, Kon & Morris, 1968; Garcia-Vela & Stanley, 1989; Hentges, Weaver & Nielsen, 1991). The storage conditions of the legume seeds have a substantial in¯uence on their germination and longevity (Roberts, 1972; Vertucci, Roos & Crane, 1994). A number of models have been suggested in the literature to describe the relationship between the equilibrium moisture content (EMC) and the water activity (aW ) of food (Van den Berg & Bruin, 1981). Some of them take into account the eect of temperature. The modi®ed Chung±Pfost (Chung & Pfost, 1967; Pfost, Mourer, Chung & Milliken, 1976), modi*
Tel.: +359-32-449-393; fax: +359-32-440-102. E-mail address: nimenkov@hi-plovdiv.acad.bg (N.D. Menkov).
®ed Henderson (Henderson, 1952; Thompson, Peart & Foster, 1968), modi®ed Halsey (Halsey, 1948; Iglesias & Chirife, 1976a), modi®ed Oswin (Oswin, 1946; Chen & Morey, 1989) and GAB (Van den Berg, 1984; Labuza, Kaanane & Chen, 1985) equations have been adopted by the American Society of Agricultural Engineers as standard equations for describing sorption isotherms (ASAE, 1995). Veltchev and Menkov (2000) applied successfully a new four-parameter, Fraction±linear (FL) model, for describing the S-shape desorption isotherms of apples. The monolayer moisture content (MMC) which is of signi®cant importance to the physical and chemical stability of dehydrated materials with regard to lipid oxidation, enzyme activity, non-enzymatic browning, ¯avour components preservation, and structural characteristics (Labuza, Tannenbaum & Karel, 1970; Karel & Yong, 1981), can be determined from the equilibrium sorption isotherms by means of the two-parameter BET (Brunauer±Emmett±Teller) equation (Brunauer, Emmett & Teller, 1938). The knowledge of the equilibrium moisture content of chickpea seeds at various temperatures would allow to specify the storage conditions for the seeds. The object of this work is to: (1) determine experimentally the equilibrium sorption isotherms of chickpea seeds at 5°C, 20°C, 40°C and 60°C; (2) select the most suitable model describing the isotherms; (3) calculate the monolayer moisture content as a function of temperature.
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 5 2 - 2
190
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
Modi®ed Halsey ÿ exp A Bt aW exp : MC
2. Material and methods 2.1. Material Chickpea seeds of the Resurs 1 variety (provided by the Agricultural University, Plovdiv, Bulgaria, 1998) were tested. The seeds had been gathered 6 months in advance. After harvest they were sun-dried to approximately 12% dry basis (d.b.) and kept in a dry place (aW < 0.55) at room temperature. The mass of 1000 seeds was about 270 g. The seeds contained 58.4% d.b. carbohydrate, 23.1% d.b. protein, 4.0% d.b. fat and 2.8% d.b. ash.
Modi®ed Henderson 1 ÿ aW exp ÿ A t BM C :
1 ÿ
B0 a
The water vapor sorption equilibrium of the chickpea seeds was determined at 5°C, 20°C, 40°C and 60°C, using the static gravimetric method (Wolf, Spiess & Jung, 1985). For the adsorption process, seeds were ®rst dehydrated in a desiccator over P2 O5 at room temperature for 20 days to an initial moisture content below 1% d.b. Samples for desorption measurements were ®rst hydrated in a glass jar over distilled water at 4°C for 20 days to approximately 32% d.b. For sorption measurements samples of 4 0:2 g were weighed in weighing bottles. The weighing bottles were then put in 10 hygrostats with dierent saturated salt solutions (LiCl, CH3 COOK, MgCl2 , K2 CO3 , NaBr, Mg(NO3 )2 , SrCl2 , NaCl, KBr, KCl), to obtain constant water activities (Greenspan, 1977; Weisser, 1986; Maroulis, Tsami, Marinos-Kouris & Saravacos, 1988). All used salts were of reagent grade. At high water activities aW > 0:70 crystalline thymol was placed in the hygrostats to prevent microbial spoilage of the seeds (Wolf et al., 1985). The hygrostats were kept in thermostats at 5°C, 20°C, 40°C and 60 0.2°C. Samples were weighed (balance, sensitivity 0.0001 g) every 3 days. Equilibrium was acknowledged when three consecutive weight measurements showed a dierence less than 0.001 g. The moisture content of each sample was determined by the vacuum oven method (AOAC, 1990). The time for the samples to reach equilibrium varied from 5 to 40 days, depending on the temperature and water activity. The equilibrium moisture contents were determined by means of triplicate measurements. 2.3. Analysis of data The description of the relationship between EMC, aW and temperature was veri®ed according to the following six models:
1
3
4
GAB M
2.2. Sorption isotherms
Modi®ed Chung±Pfost ÿA exp ÿ CM : aW exp tB
Modi®ed Oswin C aW : M A Bt 1 ÿ aW
2
B0 B exp
h1 RT
C 0 C exp
AB0 C 0 aW ; 0 0 0 W 1 ÿ B aW B C aW
h2 RT
5
;
6
:
7
FL M
A BT DaW ; 1 ÿ CaW
8
where M is the moisture content (% d.b.); aW the water activity; A, B, C, D, h1 , h2 the model coecients; t the temperature (°C); T the absolute temperature (K) and R 8:314 is the universal gas constant (J molÿ1 Kÿ1 ). A nonlinear, least-squares regression program was used to ®t the six models to the experimental data (all replications). The suitability of the equations was evaluated using the mean relative error (MRE, %), standard error of moisture (SEM) and randomness of residual (Chen & Morey, 1989): ^ i 100 X Mi ÿ M 9 MRE ; Mi N s P ^ i 2 Mi ÿ M SEM : df
10
Residual ^i; ei Mi ÿ M
11
^ i are experimental and predicted values, where Mi and M N is the number of data points and df is the degree of freedom (number of data points minus number of coecients in the model). The residuals obtained for each model with its respective coecients were plotted against measured EMC and assessed visually as random or patterned. In order to determine the monolayer moisture content, the two-parameter BET equation which is valid for aW < 0:45 (Labuza et al., 1985) was applied:
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
191
rithm described in the literature (Iglesias & Chirife, 1976b; Menkov, Paskalev, Galyazkov & KerezievaRakova, 1999).
BET
Mm CaW ; 12 M 1 ÿ aW 1 ÿ aW CaW where Mm is the MMC (% d.b.). The MMC values for each temperature were calculated following an algo-
3. Results and discussion In Fig. 1, the adsorption and desorption isotherms of chickpea seeds at 5°C and 60°C are presented. The sorption isotherms have an S-shape pro®le typical for legume seeds (Vertucci & Leopold, 1987; Pappas & Rao, 1987; Mazza & Jayas, 1991). The hysteresis eect was distinctly expressed at 5°C. At 60°C the dierences between the experimental data obtained after adsorption and desorption are not statistically signi®cant (level of signi®cance 0.05) for the most experimental points. The experimental data of all measurements are given in Table 1. The EMC values decreased with an increase in the temperature at constant aW . Similar trends for many seeds and food materials have been reported in the literature (Labuza, 1968; Menkov, 1997; Walters & Hill, 1998). The comparison between the results obtained and the data provided by Sethi, Soni and Chopra (1977) for chickpea ¯our shows that the seeds had a lower water sorption capacity. Similar observations were reported by
Fig. 1. Desorption and adsorption isotherms of chickpea seeds at 5°C and 60°C showing the hysteresis eect.
Table 1 Adsorption (ads) and desorption (des) data (M, % d.b.) of chickpea seeds at dierent temperatures 5°C aW
a b
20°C Ma
SDb
aW
40°C M
SD
aW
60°C M
SD
aW
M
SD
ads des
0.113 0.113
4.94 6.58
0.12 0.11
0.113 0.113
4.58 4.40
0.07 0.06
0.112 0.112
3.27 3.71
0.11 0.08
0.110 0.110
3.08 3.25
0.10 0.17
ads des
0.245 0.245
7.20 8.73
0.14 0.14
0.231 0.231
6.67 7.26
0.05 0.09
0.201 0.201
4.99 5.72
0.08 0.19
0.160 0.160
4.13 4.18
0.03 0.06
ads des
0.336 0.336
8.16 9.63
0.18 0.11
0.331 0.331
7.62 9.05
0.06 0.21
0.316 0.316
5.57 6.74
0.05 0.14
0.293 0.293
5.37 5.19
0.10 0.17
ads des
0.431 0.431
9.22 10.66
0.08 0.12
0.432 0.432
8.88 10.12
0.14 0.12
0.432 0.432
7.48 8.53
0.19 0.10
0.432 0.432
6.84 6.64
0.06 0.18
ads des
0.589 0.589
10.89 12.63
0.16 0.08
0.544 0.544
10.15 11.28
0.21 0.12
0.484 0.484
7.90 8.75
0.14 0.24
0.440 0.440
7.03 6.95
0.12 0.06
ads des
0.635 0.635
12.74 15.19
0.20 0.15
0.591 0.591
10.87 12.51
0.12 0.19
0.532 0.532
9.02 9.65
0.07 0.13
0.497 0.497
7.78 8.11
0.18 0.17
ads des
0.757 0.757
17.80 20.53
0.14 0.10
0.725 0.725
14.55 17.24
0.11 0.08
0.658 0.658
12.20 12.57
0.14 0.17
0.580 0.580
8.85 9.22
0.11 0.12
ads des
0.771 0.771
18.38 21.44
0.09 0.20
0.755 0.755
17.39 18.16
0.16 0.22
0.747 0.747
15.33 16.09
0.23 0.07
0.745 0.745
13.50 12.44
0.08 0.20
ads des
0.851 0.851
24.72 25.44
0.20 0.07
0.817 0.817
21.83 23.52
0.15 0.14
0.794 0.794
17.11 16.60
0.23 0.22
0.789 0.789
14.34 14.09
0.22 0.17
ads des
0.877 0.877
28.70 29.15
0.22 0.25
0.851 0.851
24.65 24.51
0.21 0.21
0.823 0.823
19.34 19.12
0.06 0.17
0.803 0.803
14.95 16.29
0.28 0.09
Mean of n 3 replications. Standard deviations.
192
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
Chhinnan and Beuchat (1985) with whole cowpeas and cowpea ¯ours. The coecients for the three- and four-parameter models together with the corresponding MRE and SEM values are presented in Table 2. The parameters of the GAB models are presented in Table 3. Example of
the analysis of the residuals are presented in Fig. 2. For the modi®ed Chung±Pfost (for adsorption and desorption as well) and modi®ed Henderson equations (for adsorption), the residual plots were patterned, which makes them unsuitable. The residuals obtained by all the other models were randomly distributed. For these
Table 2 Estimated values of model coecients (A, B, C, D); mean relative error (MRE, %) and standard error of moisture (SEM) of three- and fourparameter models for adsorption (ads) and desorption (des) Parameter
Model Modi®ed Chung±Pfost
Modi®ed Halsey
Modi®ed Oswin
Modi®ed Henderson
FL
A
ads des
455.2110 308.1214
3.337690 4.098313
10.79086 12.71118
0.000138 0.000328
10.35033 15.70636
B
ads des
101.6365 52.78201
)0.007900 )0.012647
)0.05010 )0.07701
12.82720 48.96227
)0.022551 )0.038772
C
ads des
0.166102 0.159547
1.582279 1.777402
0.49602 0.43913
2.138215 1.415975
0.954224 0.910345
D
ads des
± ±
± ±
± ±
± ±
4.469358 5.778511
MRE
ads des
12.13 10.76
4.63 6.20
6.71 5.01
15.31 10.40
4.88 5.24
SEM
ads des
2.16 1.81
0.59 0.78
0.77 0.75
3.01 1.30
0.58 0.70
Residual
ads des
Patterned Patterned
Random Random
Random Random
Patterned Random
Random Random
Table 3 Estimated values of model coecients (A, B, C, h1 , h2 ); mean relative error (MRE, %) and standard error of moisture (SEM) of the GAB model
Adsorption Desorption
A
B
C
h1
h2
MRE
SEM
Residual
5.372119 6.976963
0.547510 0.356268
0.000561 0.000490
1225.049 2071.703
26279.24 25363.67
5.06 6.19
0.65 0.80
Random Random
Fig. 2. Plot of residuals ®t of Chung±Pfost and FL models to adsorption data of chickpea seeds.
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
models the respective MRE and SEM values were similar. For the desorption data at 60°C the MRE values obtained by the modi®ed Halsey and the GAB models were high: 8.3% and 9.8%, respectively. For adsorption data at 20°C the MRE value obtained by the modi®ed Oswin model was 9.1%. The MRE values obtained by the FL model at the studied temperatures varied small: from 4.2% to 5.3% for adsorption and from 4.8 to 6.3 for desorption. This gives us grounds to recommend the FL model for description of the equilibrium isotherms of chickpea seeds. The temperature in¯uence on the monolayer moisture content values is shown in Fig. 3. There is also a hysteresis eect with respect to the monolayer moisture content. The values diminish with the increase in temperature. Menkov et al. (1999) have shown for a large number of biological products that the dependence between MMC and temperature can be described by a linear equation. A modi®cation of the BET model yields: Modi®ed BET A BtCaW ; M 1 ÿ aW 1 ÿ aW CaW
13
Mm A Bt;
14
where A, B and C are coecients independent of temperature. Applying the chickpea data the coecients of Eq. (13) were determined direct, using a nonlinear, leastsquares regression program on the basis of experimental data at aW < 0:45 (all replications): for adsorption A 5:9589; B ÿ0:0322; C 23:13263; for desorption A 7:10650; B ÿ0:0514; C 25:66617. Fig. 3 shows a comparison of the MMC values obtained by the stan-
193
dard procedure with Eq. (12) and Eqs. (13) and (14). It can be seen from the ®gures that in this case the linear equation provides a very good description of the data obtained. The corresponding water activity was determined by Eq. (13) after putting M Mm A Bt p Cÿ1 : 15 aW m Cÿ1 Despite the hysteresis eect, the (aW )m values obtained are close: for adsorption aW m 0:165; for desorption aW m 0:172. The (aW )m value is independent of temperature (the coecient C in Eq. (13) is independent of temperature). For practical considerations a single monolayer value (in the aW range 0.16±0.18) can be assumed, with respective EMC values from 4.1% to 5.7% for adsorption and from 4.2% to 6.7% for desorption.
4. Conclusions The sorption capacity of chickpea seeds decreased with an increase in temperature at constant water activity. The FL model is the most suitable for describing the relationship between the equilibrium moisture content and the water activity for the seeds under study. For practical considerations a single monolayer value (in the aW range 0.16±0.18) can be assumed.
Acknowledgements This study was conducted with the kind support of the National Research Fund, Bulgaria. References
Fig. 3. Eect of temperature on the monolayer moisture content.
AOAC (1990). Ocial methods of analysis of the Association of Ocial Analytical Chemists (15th ed.). Washington, DC: AOAC. ASAE (1995). Moisture relationship of plant-based agricultural products. ASAE Standard D245.5. St. Joseph, Michigan. Brunauer, S., Emmett, P. H., & Teller, E. (1938). Adsorption of gases in multimolecular layers. Journal of the American Chemical Society, 60, 309±319. Burr, K. H., Kon, S., & Morris, H. J. (1968). Cooking rates of dry beans as in¯uenced by moisture content and temperature and time of storage. Food Technology, 22, 336±338. Chavan, J., Kadam, S., & Salunke, D. (1986). Biochemistry and technology of chickpea (Cicer arietinum, L.) seeds. Critical Reviews of Food Science and Nutrition, 25, 107±158. Chen, C. C., & Morey, R. V. (1989). Comparison of four EMC/ERH equations. Transactions of the American Society of Agricultural Engineers, 32, 983±989. Chhinnan, M. S., & Beuchat, L. R. (1985). Sorption isotherms of whole cowpeas and ¯ours. Lebensmittel-Wissenschaft und-Technologie, 18, 83±88.
194
N.D. Menkov / Journal of Food Engineering 45 (2000) 189±194
Chung, D. S., & Pfost, H. B. (1967). Adsorption and desorption of water vapor by cereal grains and their products. Part II: Development of the general isotherm equation. Transactions of the American Society of Agricultural Engineers, 10, 552±555. Clemente, A., Sanchez-Vioque, R., Vioque, J., Bautista, J., & Millan, F. (1998). Eect of cooking on protein quality of chickpea (Cicer arietinum) seeds. Food Chemistry, 62, 1±6. Garcia-Vela, L. A., & Stanley, D. W. (1989). Protein denaturation and starch gelatinization in hard-to-cook beans. Journal of Food Science, 54, 1284±1292. Greenspan, L. (1977). Humidity ®xed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards ± A. Physics and Chemistry, 81A, 89±96. Halsey, G. (1948). Physical adsorption on non-uniform surfaces. Journal of Chemical Physics, 16, 931±937. Henderson, S. M. (1952). A basic concept of equilibrium moisture. Agricultural Engineering, 33, 9±32. Hentges, D. L., Weaver, C. M., & Nielsen, S. S. (1991). Changes of selected physical and chemical components in development of the hard-to-cook bean defect. Journal of Food Science, 56, 436±442. Iglesias, H. A., & Chirife, J. (1976a). Prediction of eect of temperature on water sorption isotherms of food materials. Journal of Food Technology, 11, 109±116. Iglesias, H. A., & Chirife, J. (1976b). B.E.T. monolayer values in dehydrated foods and food components. Lebensmittel-Wissenschaft und-Technologie, 9, 107±113. Jood, S., Bichnoi, S., & Sharma, A. (1998). Chemical analysis and physico-chemical properties of chickpea and lentil cultivars. Nahrung, 42, 71±74. Karel, M., & Yong, S. (1981). Autoxidation-initiated reactions in foods. In L. B. Rockland, & G. F. Stewart, Water Activity: In¯uences on Food Quality (pp. 511±529). New York: Academic Press. Labuza, T. P. (1968). Sorption phenomena in foods. Food Technology, 22, 263±272. Labuza, T. P., Kaanane, A., & Chen, J. Y. (1985). Eect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. Journal of Food Science, 50, 385± 391. Labuza, T. P., Tannenbaum, S. R., & Karel, M. (1970). Water content and stability of low-moisture and intermediate-moisture foods. Food Technology, 24, 543±549. Maroulis, Z. B., Tsami, E., Marinos-Kouris, D., & Saravacos, G. D. (1988). Application of the GAB model to the moisture sorption isotherms for dried fruits. Journal of Food Engineering, 7, 63±80. Mazza, G., & Jayas, D. S. (1991). Evaluation of four three-parameter equations for the description of the moisture sorption data of Lathyrus pea seeds. Lebensmittel-Wissenschaft und-Technologie, 24, 562±565. Menkov, N. D. (1997). Ein¯uss der Temperatur auf die Gleichgewichtsfeuchte bei konstanter relativer Luftfeuchte. Nahrung, 41, 349±351. Menkov, N. D., Paskalev, H. M., Galyazkov, D. I., & KerezievaRakova, M. (1999). Applying the linear equation of correlation of
Brunauer±Emmet±Teller (BET) ± monolayer moisture content with temperature. Nahrung, 43, 118±121. Oswin, C. R. (1946). The kinetics of package life. III. Isotherm. Journal of the Society of Chemical Industry, London, 65, 419±421. Pappas, G., & Rao, V. N. M. (1987). Sorption isotherms of cowpeas from 25°C to 70°C. Transactions of the American Society of Agricultural Engineers, 30, 1478±1483. Pfost, H. B., Mourer, S. G., Chung, D. S., & Milliken, G. A. (1976). Summarizing and reporting equilibrium moisture data for grains. American Society Agricultural Engineers, Paper No. 76-3520. St. Joseph, Michigan. Rincon, F., Martinez, B., & Iba nez, M. V. (1998). Proximate composition and antinutritive substances in chickpea (Cicer arietinum L.) as aected by the biotype factor. Journal of the Science of Food and Agriculture, 78, 382±388. Roberts, E. H. (1972). Storage environment and the control of viability. In E. H. Roberts, Viability of Seeds (pp 14±58). London: Chapman and Hall. Sanchez-Vioque, R., Clemente, A., Vioque, J., Bautista, J., & Millan, F. (1999). Protein isolates from chickpea (Cicer arietinum L.): chemical composition, functional properties and protein characterization. Food Chemistry, 64, 237±243. Sethi, R. K., Soni, G. L., & Chopra, S. I. (1977). Varietal dierences in chickpea ¯our (Cicer Arietinum) in relation to water vapor sorption. Cereal Chemistry, 54, 79±92. Thompson, T. L., Peart, R. M., & Foster, G. H. (1968). Mathematical simulation of corn drying ± a new model. Transactions of the American Society of Agricultural Engineers, 11, 582±586. Van den Berg, C. (1984). Description of water activity of foods for engineering purposes by means of the GAB model of sorption. In B. M. McKenna, Engineering and Foods (pp. 119±131). New York: Elsevier. Van den Berg, C., & Bruin, S. (1981). Water activity and its estimation in food systems: theoretical aspects. In L. B. Rockland, & G. F. Stewart, Water Activity: In¯uences on Food Quality (pp. 1±61). New York: Academic Press. Veltchev, Z. N., & Menkov, N. D. (2000). Desorption isotherms of apples at several temperatures, Drying Technology, 18, 1127±1138. Vertucci, C. W., & Leopold, A. C. (1987). Water binding in legume seeds. Plant Physiology, 85, 224±231. Vertucci, C. W., Roos, E. E., & Crane, J. (1994). Theoretical basis of protocols for seed storage III. Optimum moisture contents for pea seeds stored at dierent temperatures. Annals of Botany, 74, 531± 540. Walters, C., & Hill, L. M. (1998). Water sorption isotherms of seeds from ultra-dry experiments. Seed Science Research, 8, 69±73. Weisser, H. (1986). In¯uence of temperature on sorption isotherms. In M. Le Maguer, & P. Jelen, Food Engineering and Process Applications, vol. 1 (pp. 189±200). New York: Elsevier. Wolf, W., Spiess, W. E. L., & Jung, G. (1985). Standardization of isotherm measurements (COST-project 90 and 90 bis). In D. Simatos, & J. L. Multon, Properties of Water in Foods in Relation to Quality and Stability (pp. 661±679). Dordrecht: Martinus Nijho.