Drying conditions and drying model of papaya slices during combined microwave-convective hot air dehydration

In order to study the drying characteristics and optimum conditions of combined microwave-convective hot air drying of papaya slices, several test index values, such as colour change, rehydration ratio, drying rate and unit energy consumption during drying process were investigated, and the technological parameters of hot air temperatures, microwave power densities and hot air velocities were optimized. The laboratory-made dryer, having the characteristics of microwave power continuously adjustable and the material heated evenly, was used to do single factor test and orthogonal test for combined microwave-convective hot air drying. Combined microwave-convective hot air drying curve and drying rate curve of papaya slices were obtained for different technological parameters.The drying kinetics model and effective moisture diffusion model of papaya slices during combined microwave-convective hot air drying were established. The results showed that combined microwave-convective hot air drying process of papaya slices consisted of a brief acceleration period and a prolonged deceleration period.Combination drying with hot air temperature of 60 ℃, microwave power density of 0.5 W/g and hot air velocity of 0.5 m/s was optimal in terms of comprehensive drying index. Microwave power density was the most significant experimental factor in orthogonal test of combination drying.Page model was proved to be more suitable in describing combined microwave-convective hot air drying characteristics of papaya slices, and the equation was: M_R= exp (-0.0011T-0.0069PD+ 0.073t ⁽0.0015T2-0.1993T+7.9642) ) .The effective moisture diffusivity was among 2.533 × 10^-9~ 6.0792 × 10^-9m²/s, and the effective moisture diffusion model of combination drying of papaya slices was: 10^-10Deff= 0.507 T + 6.72P_D+ 10.1v-32.