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to make it more stable through reducing its water activity (Aw). Materials and Methods Raw materials. Corn (Zea mays), cocoa (Theobroma cocoa), mamee sapote seeds (Calocarpum mammosum), Rosita de Cacao (Quararibea funebris), corozo, and lime were bought in the local market of Oaxaca. These products were well inspected to avoid insects and microbial damage. Tejate preparation. 1kg of white corn, 100g of cocoa, 20g of Rosita de cacao, 20g of mamme sapote seeds, 3L of water and 250g of lime were used for tejate preparation (Cortes, 1999). The corn was cooked with lime for 2h. Then, it was washed and mixed with the other ingredients previously roasted. The mixture was grounded until an homogeneous dough was obtained. Tejate drying. The tejate was dried in a cabinet drier at 60, 70 and 80 °C in different layer thicknesses (0.25, 0.5 and 0.75 cm) and 1-1.5 m/s of air velocity. During drying, samples were taken to obtain the drying kinetics. The dried product was grounded and sieved to obtain particle sizes smaller than 500 mm (Mendoza, 2004). Sorption Isotherms. The gravimetric method, with sulfuric acid solutions, was used to obtain the sorption isotherm at 15, 25 and 35°C. The samples were put in glass containers with the solutions of known Aw. The containers were closed and vacuum was made into the containers to avoid variations in the inside relative humidity. The samples remained in the glasses until equilibrium was reached. Equations of Chung-Pfost ⎡ C ⎤ (1967): Aw = exp ⎢− 1 exp( − C2H ) ⎥⎦ ⎣ RT and Henderson (1952):. C ln( 1− Aw ) = −C TH 2 1 Were used, where Aw is water activity, T is temperature (K), M is water content (g H2O/g dm), R is constant of gases and C1, C2 are constants. Water Content. Water content of samples taken during drying and sorption isotherms were determined by the AOAC recommended method (1991). Results and Discussion Preliminary drying experiments showed that dried tejate was very unstable. Besides, it developed a rancid flavor attributed to corozo, which is rich in polyunsaturated fatty acids. Drying temperatures catalyzed oxidation reactions provoking the off flavors. Instead of using antioxidants to avoid this problem, corozo was eliminated from the original formulation. The new formulation was tasted by 100 tejate producers and consumers. No difference was found between the original and new formulations. Drying Kinetics. Kinetics of tejate drying are shown in Figure 1 at the three tested temperatures. The effect of layer thickness is clearly notorious and it was increased with temperature. At 60 C, this effect is more important between 0.25 and 0.50 cm than 0.50 and 0.75 cm thickness. Nevertheless, when the temperature increases the thicknesses effect is less important.
The tejate dough is mainly constituted by starch. During processing, this starch is partially gelatinized and water is caught into its structure. This effect is seen at 0.75 cm thickness (Figure 1C). Water content (X/Xo) Water content (X/Xo) Water content (X/Xo) 1 0.8 0.6 0.4 0.2 60 C 0.25 cm 60 C 0.50 cm 60 C 0.75 cm 0 0 50 100 150 200 250 300 350 Time (min) 1 0.8 0.6 0.4 0.2 a 70 C 0.25 cm 70 C 0.50 cm 70 C 0.75 cm 0 0 50 100 150 200 Time (min) 250 300 350 400 1 0.8 0.6 0.4 0.2 b 80 C 0.25 cm 80 C 0.50 cm 80 C 0.75 cm 0 0 50 100 150 200 Time (min) 250 300 350 400 c Figure 1 Thickness effect in Tejate drying kinetics at 60, 70 and 80 C Water content (X/Xo) Water content (X/Xo) Water content (X/Xo) 1 0.8 0.6 0.4 0.2 60 C 0.25 cm 70 C 0.25 cm 80 C 0.25 cm 0 0 20 40 60 80 100 Time (min) 120 140 160 180 200 1 0.8 0.6 0.4 0.2 a 0 0 50 100 150 200 250 300 350 1 0.8 0.6 0.4 0.2 Time (min) b 60 C 0.50 cm 70 C 0.50 cm 80 C 0.50 cm 60 C 0.75 cm 70 C 0.75 cm 80 C 0.75 cm 0 0 50 100 150 200 Time (min) 250 300 350 400 c Figure 1 Temperature effect in Tejate drying kinetics at 60, 70 and 80 C When drying rate is high, the water transports solids to the surface and a barrier is formed due to water evaporation (Figure 2C). This phenomenon diminishes the temperature effect in drying. A compromise between drying time and
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Determination of the effect of the
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Graphic 3 presents the initial and
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