Mathematical modeling on thin-layer heat pump drying of yacon (Smallanthus sonchifolius) slices

Heat pump drying kinetics and quality characteristics of yacon at different drying temperatures (5, 15, 25, 35 and 45 degrees C) and air velocities (0.5, 1.0, 1.5 and 2.0 m/s) were investigated. Results indicated that drying took place in the falling rate period. The drying temperature and air velocity had a significant effect on drying behavior of yacon slices. Drying time decreased with increasing drying temperature and air velocity. Eight mathematical models were selected to describe and compare the drying kinetics of yacon. Comparisons were based on the coefficient of determination (R-2), reduced chi-square (chi(2)) and root mean square error (RMSE). Among the models tested, the Midilli et al. model achieved the best fit. Moisture transfer from yacon slice was described by applying the Fick's diffusion model. Effective diffusivity coefficients (D-eff) increased with increasing drying temperature and were found to range from 1.092 to 7.388 x 10(-7) m(2)/s. The temperature dependence of the effective diffusivity was described by the Arrhenius-type relationship and the activation energy for the diffusion of the moisture associated with the yacon was found to be 30.00 kJ/mol. Drying temperature and air velocity had little effects (p > 0.05) on the total color difference, shrinkage rate and rehydration rate of dried yacon slcies. (C) 2013 Elsevier Ltd. All rights reserved.