Drying kinetics and heat mass transfer characteristics of thin layer pineapple during microwave vacuum drying

Abstract

Microwave vacuum drying of pineapple slices was carried out to investigate the microwave power degree on the drying kinetics and heat mass transfer characteristics. Numerical simulation and bench test were used to study the variation characteristic of microwave volumetric heating and temperature and moisture during the drying process, and the best drying model was selected. The result shows that the drying process of the pineapple slices belonged to the falling rate period without constant drying stage; Among 6 common thin layer drying kinetic models, the mean values of $R^2$ were maximum, ${\chi }^2$ and RMSE for the Two-term exponential model were minimum, which were 0.9993, 0.000107 and 0.00839, respectively. As the temperature rose to 40 $\sim$ 45 ${}^{\circ }$ C at the late stage of drying, the thermal runaway phenomenon appeared, and the internal temperature of pineapple slices rose sharply. Hot spots appeared at the center and edge of pineapple at the late stage of drying, and it generated by microwave volumetric heating focusing. The predicted values of moisture ratio (MR) obtained from COMSOL simulation showed good consistency with the Two-term exponential model and the experimental. In order to improve the drying quality of the pineapple, the microwave power should be varied and reasonable interval to make the drying temperature lower than the thermal runaway temperature range.