Effects of Hot-Air Coupled Microwave on Characteristics and Kinetics Drying of Lotus Root Slices

Abstract

Hot-air coupled microwave was employed to dry lotus root slices. The effects of lotus root slice thickness (5, 8, 11, 14, and 17 mm), hot-air velocity (1.5, 2.0, 2.5, 3.0, and 3.5 m/s), hot-air temperature (50, 55, 60, 65, and 70 °C), and microwave power density (2, 4, 6, 8, and 10 W/g) on drying characteristics and kinetics were studied. Results indicated that the drying process involved both the accelerating and decelerating periods but no constant rate period. The drying rate reached the maximum of 1.52 kg/kg when microwave power density was 8 W/g and reached the minimum of 0.02 kg/kg at the last stage of drying. In addition, the drying kinetics of lotus root slices were also investigated using eleven previously reported models. Among the models, the Verma et al. model was the most suitable for description of the drying behaviors of lotus root slices based on R2, root-mean-square error, and chi-square. The moisture transfer from lotus root slices can be effectively described by Fick’s diffusion model. Regardless of drying conditions, the effective diffusivity coefficients ranged from 8.23 × 10–7 to 7.08 × 10–6 m2/s, and their variations were mostly in agreement with those of moisture ratios. The activation energy of moisture diffusion related to lotus root slices was determined to be 13.754 kJ/mol.