One-pass drying of rough rice with an industrial 915Â MHz microwave dryer: Quality and energy use consideration

Microwave (MW) at 915 MHz has potential to achieve one pass rough rice drying. However, optimising processing parameters to maintain the rice quality is crucial. Effects of MW treatment on rice moisture removal, milled rice characteristics, and energy requirements for continuous one pass drying operation were quantified. Freshly harvested rough rice with initial moisture content of 25% wet basis was dried in a pilot-scale 915 MHz microwave dryer. The dryer was set to transmit MW power ranging between 3 and 24 kW during 8 min of drying. During treatments, rough rice was conveyed at bed thicknesses, 0.01, 0.03, and 0.05 m; supplied specific energy was maintained at 450, 600 and 750 kJ kg−1 of rough rice. Moisture removed varied between 6% and 15% points, depending on rice bed thickness (0.01-0.05 m) and applied specific energy (450-750 kJ kg−1). Increasing rice bed thickness and specific energy reduced milling and head rice yields, increased final viscosity of milled rice, but marginally affected rice peak viscosity and surface lipid and protein contents (p < 0.05). To achieve the desired percentage point moisture content reduction (∼12% points) at specific energy of 600 kJ kg−1 and 750 kJ kg−1 of rough rice, 4574 kJ and 5986 kJ were required per kg of water removed, respectively; this translated to 13 and 16 USD per metric ton of dried rice, respectively. The study demonstrated feasibility of one pass MW drying of rough rice; 450-600 kJ kg−1 of rough rice was recommended to preserve rice quality and achieved better energy use efficiency.