Multi-objective optimization of hybrid microwave-fluidized bed drying conditions of rice using response surface methodology

The present study is aimed to examine the drying of paddy to a moisture level of 12% wet basis using a microwave-assisted fluidized bed dryer. The experiments were designed and analysed using response surface methodology through which, the drying conditions were optimized in terms of minimized energy consumption and maximized quality of the final dried rice. A central composite design was applied for the three featured parameters including air temperature (40–80 °C), air velocity (1–5 m/s) and microwave power level (100–900 W). The quality of dried rice was specified in terms of whiteness index, water uptake ratio, elongation ratio and head rice yield. A modified cubic model was observed in all responses with high R2 values (>0.9). The whiteness index of dried paddy increased as the microwave power and fluidization velocity was simultaneously raised. Moreover, higher microwave power and lower temperatures resulted in improved head rice yield of paddy samples. On the other hand, high microwave power accompanied by high temperatures reduced the drying time and correspondingly the specific energy consumption. It could be concluded that a microwave-fluidized bed dryer provides samples with better properties than the single fluidized bed dryer. The optimum conditions of microwave-fluidized bed drying of rice with maximum product quality and minimum time and energy consumption were: air velocity of 4.2 m/s, air temperature of 72 °C and microwave power of 652 W.