Journal of Food Engineering 48 (2001) 91±94
Temperature eect on the moisture sorption isotherms for methylcellulose and ethylcellulose ®lms G. Vel azquez de la Cruz a, J.A. Torres b,*, M.O. Martõn-Polo a a
Facultad de Quõmica, Departamento de Investigaci on y Posgrado en Alimentos, Universidad Aut onoma de Quer etaro, Cerro de las Campanas s/n, Quer etaro Qro. C.P.76010, Mexico b Department of Food Science and Technology, Oregon State University, 100 Wiegand Hall, Corvallis, OR 97331-6602, USA Received 27 September 1999; received in revised form 25 July 2000; accepted 25 July 2000
Abstract Moisture sorption isotherms were obtained at 9°C, 15°C, 20°C, 25°C and 35°C for methylcellulose (MC) and ethylcellulose (EC) ®lms following the normalized microclime method. Cast MC and EC ®lm samples were equilibrated to 0.22, 0.44, 0.57, 0.75, and 0.90 aw values. MC ®lms had an equilibrium moisture content about ®ve times higher than the EC ®lms. The Guggenheim±Anderson±deB oer (GAB) and Brunauer±Emmett±Teller (BET) equations were ®tted to the experimental data using nonlinear regression. The regression coecient (R2 ) used to evaluate model ®t showed a best ®t for the GAB model. The monolayer moisture content (xm ) decreased with temperature and correlated better with values calculated using the BET equation. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ethylcellulose; Methylcellulose; Moisture sorption; Isotherms
1. Introduction Moisture sorption isotherms are important to evaluate the eect of temperature (T) and relative humidity (RH) on ®lm properties (Cadden, 1988; Lai, 1998). Addition or removal of water may cause phase transitions in the macromolecular structure (Schwartzberg, 1986; Cadden, 1988; Gennadios & Weller, 1991; Torres, 1993; Hernandez, 1994) which can be studied by the Brunauer±Emmett±Teller (BET) and the Guggenheim± Anderson±deB oer (GAB) models. The BET model assumes a ®rst adsorbed water layer binding to speci®c substrate sites resulting in an increased heat of condensation. The GAB model adds the assumption that the heat of adsorption from the second to the ninth layer diers from the heat of pure water condensation by a constant amount (Labuza, Kaanane, & Chen, 1985; Saravacos, Tsiourvas, & Tsami, 1986; Tsami, MarinosKouris, & Maroulis, 1990; Dural & Hines, 1993; Mishra, Ooraikul, & Temelli, 1996; Diosady, Rizvi, Cai, & Jagdeo, 1996). This study evaluates the temperature * Corresponding author. Tel.: +1-541-737-4757; fax: +1-541-7376174. E-mail addresses: [email protected]
(G. VelaÂzquez de la Cruz), [email protected]
eect on the interaction between water and methylcellulose (MC) or ethylcellulose (EC) ®lms using moisture sorption isotherms determined at 9°C, 15°C, 20°C, 25°C and 35°C.
2. Materials and methods 2.1. Film preparation Ethylcellulose (EC) (Sigma Chemical, St. Louis, MO) or MC (Methocel A15-LV Premium, Dow Chemical, Midland, MI) (4.25 g) were dissolved in 75 ml of ethanol (J.T. Baker, Phillipsburg, NJ) for EC ®lms and ethanol:water 2:1 (v/v) for MC ®lms (Debeaufort, Martõn-Polo, & Voilley, 1993; Romero-Bastida, MartõnPolo, & Velazquez de la Cruz, 1995). The mixture was kept at 75°C while stirring for 20 min and then cast on glass plates (20 20 cm2 ) with a thin layer chromatography spreader (Kamper & Fennenma, 1985). Spreading thickness was ®xed to 1 mm and coated plates were dried in a 75°C oven for 45 and 30 min for MC and EC ®lms, respectively. After drying and cooling, ®lms were removed from the glass plates, placed in plastic bags and stored at room temperature in a desiccator over silica gel.
0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 1 4 3 - 6
G.V. de la Cruz et al. / Journal of Food Engineering 48 (2001) 91±94 Notation x aw xm
moisture content (g water/g dry solids) water activity monolayer moisture content (g water/g dry solids)
2.2. Sorption isotherms Sorption studies followed the static microclime method (Wolf, Spiess, & Jung, 1985) with some equipment modi®cations. Air-tight glass jars (1 l) containing saturated KC2 H3 O2 ; K2 CO3 , NaBr, NaCl and BaCl2 solutions for 0.22, 0.44, 0.57, 0.75 and 0.90 aw at 25°C, respectively, were used to hold triplicate circular samples (u 3:2 cm) of MC and EC ®lms supported on micropipette tips. Jars were kept for 5 days in stirred and thermostatically controlled water baths (15°C, 30°C, 35°C) or in temperature controlled chambers (9°C, 25°C). Weight of moisture equilibrated samples, and
c k R2 T
constant related to thermal eects GAB constant regression coecient temperature (°C)
after drying in an oven at 105°C for 10 h, were determined with 0.0001 g precision. The experiment was repeated three times using dierent ®lm batches. 2.3. Data analysis 2.3.1. Moisture isotherms The GAB and BET equations were used to describe the dry basis moisture content (x) as a function of water activity (aw ): The BET model : x
xm caw xm a w ;
1 caw 1 ÿ aw
Fig. 1. Moisture sorption isotherms of methylcellulose (a) and ethylcellulose (b) cast ®lms at 9°C, 15°C, 20°C, 25°C and 35°C. Symbols show experimental data and lines connect points generated by the GAB model.
G.V. de la Cruz et al. / Journal of Food Engineering 48 (2001) 91±94
The GAB model : x
xm ckaw :
1 ÿ kaw
1 ÿ kaw ckaw
show a higher R2 for the GAB model. For MC ®lms, xm at 25°C was 3.7 (g water/g dry solids) and similar to the value reported by Debeaufort, Verschueren, MartõnPolo, and Voilley (1994) for ®lms made with 10% polyethylene glycol (3.1 g water/g dry solids). The expected decrease in monolayer value with temperature was estimated better by the BET equation (Table 1). The monolayer values calculated from the GAB equation were higher than those calculated from the BET. The same trend has been observed for starch (Van den Berg, 1985), ®sh ¯our and corn meal (Labuza et al., 1985), and ground sun¯ower (Mok & Hettiarachchy, 1990). The GAB parameters xm , c and k, are not independent, i.e., even a combination of values lacking physical meaning can give satisfactory estimations (Tsami, Vanegas, & Marinos-Kouris, 1992). Furthermore, the sensitivity of nonlinear regression to the number and scattering of experimental data makes it dicult to determine whether small changes in xm re¯ect real changes in the sorption process or only mathematical artifacts (Schuchmann et al., 1990). This may explain why the GAB model fails to describe accurately the interaction between water and substrate, particularly the eect of temperature on xm values.
In Eqs. (1) and (2), xm is the monolayer moisture content, c a constant related to thermal eects and k is the GAB constant related to the properties of multilayer water molecules with respect to bulk liquid. Equation parameters were estimated by nonlinear regression (SAS, 1985). Nonlinear regression does not ensure unique solutions, especially for three-parameter equations (Schuchmann, Roy, & Peleg, 1990). SAS minimizes this problem by allowing changes in starting values (0.5±5, increments of 0.5) and uses the combination with the least residual error as starting values for the ®nal optimization. The linear form of the BET equation was used to obtain initial xm and c values to solve its nonlinear form. For the GAB equation, the polynomial form was used to obtain starting values for estimation with the nonlinear form (Roos, 1992). Model ®t was estimated from regression coecient (R2 ) values.
3. Results and discussion Sorption isotherms of MC and EC ®lms at 9°C, 15°C, 20°C, 25°C and 35°C are shown in Fig. 1. Each point is the average and standard deviation for the three ®lm preparation batches with triplicate samples. An analysis of variance and Tukey test (P 0:05) showed no consistent dierence between ®lm preparation batches. Equilibrium moisture content decreased with temperature and was about ®ve times higher for MC ®lms than for EC ®lms. The model parameters calculated by the GAB and BET equations are presented in Table 1 and
4. Conclusions Moisture sorption isotherms for MC and EC ®lms, as a function of temperature showed the typical behavior of hydrophilic and moderately hydrophobic materials. Mathematical criteria used in ®tting procedures should be considered when relating numerical GAB parameters with their physical meaning in sorption process.
Table 1 Parameters and regression coecient (R2 ) of GAB and BET models for moisture absorption isotherms of methylcellulose and ethylcellulose cast ®lms Model
Temperature (°C) 9
xm c k R2 xm c R2
3.9284 11.0592 0.9743 0.9980 4.1068 4.3930 0.9821
4.0573 8.6868 0.9564 0.9944 3.8900 5.6681 0.9216
3.5236 11.0973 0.9642 0.9957 3.3327 7.8642 0.9458
3.6727 5.0327 0.9259 0.9972 3.2134 4.0878 0.9809
4.8565 4.3591 0.8773 0.9926 3.3309 5.0241 0.9217
xm c k R2 xm c R2
1.1593 5.3597 0.9082 0.9871 1.7993 2.5817 0.9177
1.0867 4.6522 0.9035 0.9767 1.6025 2.1289 0.7840
0.9068 4.4003 0.9109 0.9811 1.3665 2.7378 0.8189
0.7885 3.7338 0.9067 0.9808 1.0805 4.1888 0.6634
0.7505 3.0674 0.9224 0.9660 0.706 2.0159 0.6580
G.V. de la Cruz et al. / Journal of Food Engineering 48 (2001) 91±94
Acknowledgements We acknowledge the ®nancial assistance from the Consejo Nacional de Ciencia y Tecnologõa (CONACyT) and the Consejo de Ciencia y Tecnologõa del Estado de Queretaro (CONCyTEQ) in Mexico. References Cadden, A. N. (1988). Moisture sorption characteristics of several food ®bers. Journal of Food Science, 53, 1150±1155. Debeaufort, F., Martõn-Polo, M., & Voilley, A. (1993). Polarity homogeneity and structure aect water vapor permeability of model edible ®lm. Journal of Food Science, 58, 426±429. Debeaufort, F., Verschueren, K., Martõn-Polo, M. O., & Voilley, A. (1994). Water vapor permeability of edible barriers. Presented at the International Symposium on the Properties of Water, Practicum II, Food Preservation by Moisture Control, Puebla, Mexico, June 19±24. Diosady, L. L., Rizvi, S. S. H., Cai, W., & Jagdeo, D. J. (1996). Moisture sorption isotherms of canola meals, and applications to packaging. Journal of Food Science, 61, 204±208. Dural, N. H., & Hines, A. L. (1993). A new theoretical isotherm equation for water vapor-food system: multilayer adsorption on heterogeneous surfaces. Journal of Food Engineering, 20, 75±96. Gennadios, A. and Weller, C. (1991). Moisture sorption isotherms of edible ®lms. Presented at the International Winter Meeting of the American Society of Agricultural Engineers. ASAE Paper No. 916521, Chicago, IL. Hernandez, R. J. (1994). Eect of water vapor on the transport properties of oxygen through polyamide packaging materials. Journal of Food Engineering, 22, 495±507. Kamper, S. L., & Fennema, O. (1985). Use of an edible ®lm to maintain water vapor gradients in foods. Journal of Food Science, 50, 382±384. 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. Lai, H. M. (1998). Water vapor barrier properties of zein ®lms plasticized with oleic acid. Cereal Chemistry, 75, 194±199.
Mishra, V. K., Ooraikul, B., & Temelli, F. (1996). Physical characterization and water sorption on freeze dried dulse Palmaria palmata powder. Journal of Food Processing and Preservation, 20, 25±39. Mok, C., & Hettiarachchy, N. S. (1990). Moisture sorption characteristics of ground sun¯ower nutmeat and its products. Journal of Food Science, 55, 786±789. Romero-Bastida, C., Martõn-Polo, M. O., & Velazquez de la Cruz, G. (1995). Permeation and moisture sorption behavior of zeineethylcellulose based ®lms. Paper # 12e-8 presented at the IFT Annual Meeting, Anaheim, CA. Roos, H. Y. (1992). Water activity and physical state eects on amorphous food stability. Journal of Food Processing and Preservation, 16, 433±437. Saravacos, G. D., Tsiourvas, D. A., & Tsami, E. (1986). Eect of temperature on the water adsorption isotherms of sultana raisins. Journal of Food Science, 51, 381±383. A. SAS. 1985. SAS/STAT, User's Guide (Release 5). SAS Inst. Inc., Cary, NC. Schuchmann, H., Roy, I., & Peleg, M. (1990). Empirical models for moisture sorption isotherms at very high water activities. Journal of Food Science, 55, 759±762. Schwartzberg, H. G. (1986). Modeling of gas and vapour transport through hydrophilic ®lms. In M. Mathlouthi (Ed.), Food packaging and preservation: theory and practice (pp. 115). London: Elsevier. Torres, J. A. (1993). Edible coatings and ®lms from proteins. In: A. Hettiarachchy, G. Ziegler (Eds.), Protein functionality in food systems (pp. 467). New York: IFT/Marcel Dekker. Tsami, E., Marinos-Kouris, D., & Maroulis, Z. B. (1990). Water sorption isotherms of raisins, currants, ®gs, prunes and apricots. Journal of Food Science, 55, 1594±1597. Tsami, E., Vanegas, G. K., & Marinos-Kouris, D. (1992). Moisture sorption isotherms of pectins. Journal of Food Processing and Preservation, 16, 151±161. Van den Berg, C. (1985). Development of B.E.T.-like models for sorption of water on foods, theory and relevance. In D. Simatos, J. L. Multon (Eds.), Properties of water in foods (pp. 119). Dordrecht: Martinum Nijho. 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 (Eds.), Properties of water in foods (pp. 661). Dordrecht: Martinus Nijho.