Assessment of One-Pass Drying of Rough Rice with an Industrial Microwave System on Milling Quality

Abstract

Abstract. The volumetric heating phenomenon of microwaves has the potential to dry rough rice rapidly with reduced inter-kernel rice temperature and moisture content (MC) gradients thereby minimizing rice fissuring and maintaining milled rice quality. The objective of this study was to determine the feasibility of using an industrial-type microwave heating system to achieve one-pass rice drying with minimum implications on rice milling quality, especially the head rice yield. Freshly-harvested, medium-grain rough rice samples (cv. Jupiter) at initial moisture content (IMC) of 23% to 24% (wet basis) were heated using an industrial microwave system with a frequency of 915 MHz and set to transmit energy to rice at power levels of 2, 5, 10, and 15 kW for durations of 1, 2, 3, and 4 min. The effects of natural air and forced air cooling and tempering of the rice after microwave treatments on moisture removal and head rice yield reduction were determined. The goodness of fit of linear, quadratic, and cubic models to describe the kinetics of the head rice yield reduction due to the treatments were determined. Results showed that microwave treatments at power levels of 5 and 15 kW for 4 and 1 min, respectively, bore much promise in decreasing the rice MC to 13.0% (wet basis) for a rice bed thickness at 0.03 m. Supplying microwave energy of up to 600 kJ/kg grain followed by 4 h of tempering at 60°C dried the rice to final MCs of 14% to 16%, and the resulting head rice yield was not significantly (p >0.001) different from that of rice dried with natural air at 25°C and relative humidity of 65%. Without a tempering step, microwave heating with energy input exceeding 300 kJ/kg-grain resulted in head rice yield that was 20 percentage points lower than that of the control samples. The cubic model best fitted the correlations of specific energy input and the head rice yield with Root Mean Square Errors (RMSE) of 1.19%, 4.70%, and 5.56% and coefficients of determination (R 2 ) of 0.597, 0.911, and 0.889 for treatment with microwave heating followed by tempering and natural air cooling, microwave heating followed by forced air cooling, and microwave heating followed by natural air cooling, respectively. The marginal reduction in head rice yield, especially resulting from microwave heating followed by tempering treatment, provided a strong justification to optimize the treatments to achieve commercially viable rough rice drying throughput; it was concluded that such endeavors may have immediate applications, especially in rice parboiling operations which typically require rapid drying of rice at high initial moisture contents – often greater than 24% wet basis.