Mathematical modelling on thin layer microwave drying of apple pomace with and without hot air pre-drying

Characteristics of thin layer microwave drying of apple pomace with and without hot air pre-drying were evaluated in a laboratory scale microwave dryer. The drying experiments were carried out at 150, 300, 450 and 600 W, and the hot air pre-drying experiment was performed at 105 °C. Ten commonly used mathematical models were evaluated with the experimental data. The results indicated that the Page model was most adequate in predicting moisture transfer for fresh and pre-dried apple pomace; just a falling rate period was observed in the microwave drying processes; in comparison with the fresh apple pomace, the drying times, or the energy consumptions in the drying of pre-dried apple pomace to remove the same moisture (40%, wb) decreased by 25% on the average at the experimental power levels, and their values of effective diffusivity were higher, which indicates that the pre-treatment with hot air drying can improve the microwave drying rate of apple pomace; four regression equations of drying rate against drying duration or moisture were found to describe very well the drying characteristics for fresh and pre-treated apple pomace respectively; it took nearly 70% of total drying time to remove the latter half of moisture (wb) in the microwave drying with or without pre-drying. The above findings can facilitate the design and operation of microwave drying of apple pomace.