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product quality needs to be taken into account during food processing. Sorption isotherms. Figure 3 shows the sorption isotherms at 15, 25 and 35C. The figure indicates that there is a great influence of temperature on the isotherm sorption. Data are exponentially distributed as shown in other high starch products (Chen, 2002). Water content (g H2O/g d.m) 0.8 0.6 0.4 0.2 15°C 25°C 35°C 0 0.2 0.4 0.6 Aw 0.8 1 Figure 3 Sorption isotherm of Tejate at 15, 25 and 35 C The Chung-Pfost equation was linerized to calculate the constant values ln ⎛ ( − ln Aw ) = ⎜ln + ( − b) M ⎟ ⎝ T ⎠ a where: a=C1/R and b=C2 and also Henderson equation [ − ln( 1− A ) ] = ln aT b ln M ln w + a=C1 and b=C2. Table 1 shows the constant values and the correlation coefficient (r 2 ) for Chung-Pfost and Henderson models. The accuracy of models at the studied temperatures is the same for both equations. They represent ⎞ isotherm behavior but they were more accurate at low temperatures than at high temperatures. Chung-Pfost and Henderson models have been used to predict sorption isotherms for cereals or starchy products (Sun D.W., 1998, Silakul and Jindal, 2002, Chen, 2002). Both equations have been modified to increase their accuracy. Chung-Pfost equation has been recommended to use in rice products and mushroom (Durakova and Mendkov, 2004, Shivhare et al., 2004). Figure 4 fits both equations and experimental results at 15 C. There is a region between 0.7 and 0.95 of Aw where the isotherm was not well described by the models. Table 1. Values of constants and r 2 of Chung-Pfost and Henderson equations for Tejate. Chung-Pfost a= 668.84 b= 8.5224 Henderson a= 0.0243 b= 1.1473 r 2 15C 25C 35C 0.9841 0.9851 Water content (g water/g d.m.) 0.6 0.5 0.4 0.3 0.2 0.1 Experimental Chung-Pfost Henderson 0.9650 0.9718 0 0 0.2 0.4 0.6 0.8 1 0.9343 0.9469 Aw Figure 4. Experimental and predicted absorption isotherms by Chung-Pfost and Henderson models at 15C
CONCLUSION A tejate formulation was developed. The best operational and sensory drying conditions were 70C and 0.05 cm thickness under the conditions used in this study. Adsortion isotherms showed an important temperature effect to take in account during storage of dried tejate. The models evaluated described the sorption isotherm behavior of tejate powder. REFERENCES Chen C.(2002). Sorption isotherms of sweet potato slices. Biosystems Engineering. 83(1), 85-95. Cheng D.S., Pfost H.B. 1967. Adsortion and desorption of water vapor by Cereal grains and their products. Transactions of ASAE, 10(4), 552-555. Cortez-Noh , M. M; 1999. Estudio Nutricional y Bebidas a base de Vegetales en la Alimentación, Tesis Inst. Tec. Veracruz. México. Durakova A.G., Menkov, N.D, 2004. Moisture sorption characteristics. Nahrung/Food, 48(2), 137-140. Henderson S.M. 1952. A basic concept of equilibrium moisture. Agricultural Engineering, 33(1), 29- 32. Silakul T.,and Jindal V.K. 2002. Equilibrium moisture content isotherms of mungbean. International Journal of food properties, 5(1), 25- 35. Shuvhare U.S., Arora S., Ahamed J., Raghavan G.S.V. 2004. Moisture adsorption isotherms for mushroom. Lebensmittel-Wissenschsft und Technologie. 37(1), 133-137. Sun D-W. 1999. Comparison and selection of EMC/ERH isotherm equations for rice. Journal of Stored Products Research, 35, 249-264.
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