MICROWAVE-VACUUM DRYING OF PARBOILED RICE

Abstract

ABSTRACT This study evaluated the use of microwave energy in a reduced pressure environment for drying parboiled rice using a single pass. The effects of parboil soaking conditions, steaming conditions, microwave power level, drying pressure, drying time, and final moisture content were evaluated. Parboiling conditions had no direct effect on drying rate and drying efficiency. A slight reduction in milling yields was associated with longer steaming time. Drying rates were directly proportional to microwave power levels and they remained essentially constant until rice moisture content fell below 18%. Lower operating pressures in the dryer increased drying rate, increased drying efficiency, and decreased rice temperature. These results indicate that diffusion of liquid water was not rate limiting and the rate of adsorption/desorption of water on the surface of the rice was a significant factor in determining overall drying rate. Rice kernel density measurements indicated that the kernels did not shrink during microwave drying as they normally do during conventional hot air drying. Milling yields were significantly related to the same variables that affected drying rate (power, pressure, and final moisture). At water removal rates of 7.4 g of water per min per kg dry rice or less, the parboiled rice could be dried in a single pass to safe storage levels without any significant decrease in head yield. Microwave-vacuum drying provides rice processors with an altemative method for rice drying.