Effects of Stepwise Microwave Heating and Expanded Bed Height Control on the Performance of Combined Fluidized Bed/Microwave Drying for Preparing Instant Brown Rice

Drying is the most energy-intensive operation in instant brown rice production. The effects of stepwise microwave heating on energy consumption and product qualities were investigated for combined fluidized bed microwave (FDBMW) drying. Red jasmine rice (RJR) was used as the test sample and was dried to the desired level with a variation of microwave power ranging in 0-450 W, while fluidizing air temperature was kept constant at 98 degrees C. Two drying periods in stepwise modes were predicted using the Page drying equation. An image processing technique was applied to capture the bed kinetics during convective fluidized bed drying and further used to control bed stability in the FDBMW drying process. Energy consumption was reduced by 7 to 23% when the expanded bed height was controlled as a dense fluidization regime in the heat transfer zone. Based on energy saving concerns, FDBMW drying operating with a step-down mode from 300 to 150 W and control of bed stability was the optimal condition to produce instant good-quality brown rice.