Starch gelatinization and drying of paddy using microwave rotary drum dryer: Optimization, kinetics, and cooking studies

The microwave-assisted starch gelatinization was carried out in a semi-pilot microwave rotary drum dryer. Gelatinization experiments were planned using a central composite design and the process was optimized. The effect of microwave power density, time of treatment, and water application rate on final moisture content, crystallinity percentage, and specific energy consumption was studied. The optimized conditions in the starch gelatinization process were found at 1 kW kg(-1) power density, 60 min treatment time, and 90 mL/10 min water application rate. Microwave power density and treatment time significantly (p < 0.01) affected the crystallinity percentage and specific energy consumption. Further, the gelatinized paddy was dried in the same dryer at 0.25, 0.50, 0.75, 1.00, and 1.25 kW kg(-1) microwave power density levels. The drying characteristics, milling yield, cooking characteristics and cooking kinetics of the milled rice were analyzed. Drying rates were increased with an increase in microwave power density. The head rice yield and specific energy consumption were lower in microwave drying compared with hot air drying. The crystallinity percentage of microwave dried rice was decreased with an increase in power density. At higher power density, the microstructure of starch granules showed formations of internal fissures as well as the effect on the color and cooking rate constants of the rice.