Milling behavior and microstructure of rice dried using microwave set at 915 MHz frequency

In industrial milling of rice, the practice is to target certain degree of milling (DOM) which primarily depend on end-use requirement. Unlike convective heated air (CHA) drying, volumetric heating phenomenon which is associated with industrial microwave (MW) drying of rice at 915 MHz frequency could induce unique changes in rice microstructure; this is likely to impact the rice milling characteristics, especially the DOM. Medium grain rough rice at 24% moisture content (wet basis) at bed thicknesses of 0.01 and 0.05 m was dried in a one-pass, continuous drying operation for 8 min using pilot scale MW set at specific energy of 450, 600 and 750 kJ/kg. Samples from each treatment were milled for durations of 0, 15, 30, 45 and 60 s. The result shows that, more than 80% of the SLC were removed by 30 s of milling duration for both MW and CHA dried samples. Irregular and hexagonal structure typically alluded to starch granules reduced as the specific energy increased. At specific energy of 750 kJ/kg, the core of the MW dried rice kernel indicated features related to thermal decomposition. The study findings are crucial to determining MW drying condition that maintain the rice milling characteristics.