Temperature effect on the moisture sorption isotherms for methylcellulose and ethylcellulose films

Temperature effect on the moisture sorption isotherms for methylcellulose and ethylcellulose films

Journal of Food Engineering 48 (2001) 91±94 www.elsevier.com/locate/jfoodeng Research Note Temperature e€ect on the moisture sorption isotherms for...

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Journal of Food Engineering 48 (2001) 91±94

www.elsevier.com/locate/jfoodeng

Research Note

Temperature e€ect 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 coecient (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 e€ect 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 di€ers 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] (J.A. Torres).

e€ect 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 ˆ 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

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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 ˆ 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 e€ects GAB constant regression coecient 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 di€erent ®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 ˆ

xm caw xm a w ‡ ; …1 ‡ caw † 1 ÿ aw

…1†

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 ˆ

xm ckaw : …1 ÿ kaw †…1 ÿ kaw ‡ ckaw †

93

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 dicult 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 e€ect of temperature on xm values.

…2†

In Eqs. (1) and (2), xm is the monolayer moisture content, c a constant related to thermal e€ects 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 coecient (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 ˆ 0:05) showed no consistent di€erence 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 coecient (R2 ) of GAB and BET models for moisture absorption isotherms of methylcellulose and ethylcellulose cast ®lms Model

Methylcellulose GAB

BET

Ethylcellulose GAB

BET

Parameter

Temperature (°C) 9

15

20

25

35

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

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