Effect of high-power microwave drying on moisture removal, fissuring and milling yield of rough rice

Abstract

<b><sc>Abstract. </sc></b>Drying rough rice (RR) by employing a microwave (MW) dryer is one of the promising technologies for high throughput drying of high moisture rice. The milling quality of RR in MW drying is important for stakeholders to adopt the drying method. Hence, drying experiments were conducted with an industrial MW dryer operating at 915 MHz. The power levels and heating durations employed were 16, 18, and 20 kW and 1, 2, and 3 min, respectively for a single pass drying. The RR's moisture content, surface temperature, fissuring and head rice yield (HRY) were determined. The maximum measured surface temperature of RR at severe (20 kW for 3 min) and least severe (16 kW and 1 min) treatment conditions were 91.86°C and 62.56°C, respectively. The initial moisture content of RR was 21%. The severe MW drying conditions of 20 kW for 3 min, 18 kW for 3 min and 16 kW for 3 min decreased the moisture content by 9.69, 8.91, and 7.80 percentage points, respectively, from their respective initial moisture content. Maximum fissuring percentages of 90.70% and 93.67% were observed at 18 kW for 3 min drying after one day and after seven days, respectively. Interestingly, the HRY at short MW drying conditions i.e. for one minute at all the MW power levels were higher than that of the gently dried condition (25°C, 56% RH). In conclusion, the high power and short duration MW drying of RR showed huge potential for drying of RR. MW drying has the promise of reducing drying duration and hence resolve issues related to bottlenecks of low drying throughputs that are associated with conventional methods.