Abstract: The effects of material thickness, air temperature, air velocity and microwave power on the drying characteristics and effective moisture diffusion coefficient were investigated by using microwave coupled with hot air. Meanwhile, a model for describing the drying kinetics was established through nonlinear fitting. The experimental results showed that the drying process was mainly divided into two periods, namely accelerated and decelerated change of dry basis moisture content, without obvious constant-speed period. The effective moisture diffusion coefficients, ranging from 0.879 1 × 10-6 to 8.245 8 × 10-6 m 2/s, were directly proportional to material thickness, air temperature and microwave power density and initially decreased and then increased with increasing air velocity. Furthermore, the effects of material thickness and microwave power density on effective moisture diffusion coefficient were greater than those of air temperature and air velocity. Among 9 common drying kinetic models, the mean values of R2, χ2 and root-mean-square error for the Two-term exponential model were minimum, which were 0.998 0, 0.000 2 and 0.014 7, respectively. The predicted values from this model were nearly consistent with the experimental values under identical experimental conditions. Therefore, the model could be used to predict the moisture change of Chinese yam during the drying process. These research findings can provide a technical foundation for the application of microwave coupled with hot air in the drying of Chinese yam and other agricultural products.

Key words: Chinese yam, microwave coupled with hot air, drying characteristics