Quality Aspects of Microwave-Vacuum-Dried Parboiled Rice

Abstract

One of the consequences of the rice parboiling process is an increase of the moisture content of the rice to approximately 35% (w.b.). It is essential to reduce the moisture of the parboiled rice before it can be stored or milled. The proper drying of parboiled rice thus becomes an important step. This study evaluates the combined use of microwave energy and an evacuated atmosphere to dry parboiled rice on its quality. The effect of two soaking times and two steaming times combined with two vacuum levels and three microwave power levels were tested. Drying times were adjusted to achieve target final moisture levels of 14%, 16%, and 18% (w.b.), respectively. The results showed that it was possible to dry parboiled rice using the microwave-vacuum method and the quality of the finished product was comparable to that produced by conventional methods. It was concluded that the whole kernel yield was reduced with increased microwave power, the effect being more marked at higher vacuum levels. A drop of 5% in whole kernel yield was seen at the lower vacuum level (absolute pressure ==53.3 kPa), this increased to 10% when the vacuum level was higher (absolute pressure = 6.67 kPa). The major factor in the reduction of yields appeared to be the increase of the drying rate as the microwave power and vacuum were increased. Results pertaining to color indicated that the independent variables did not affect it. The percentage of completely gelatinized kernels was found to decrease for increase of microwave power. At the 53.3 kPa level of absolute pressure, the percent gelatinized kernels dropped approximately 8% when the microwave power was increased from 600 W to 1800 W. When the pressure level was decreased to 6.67 kPa (vacuum increased), the corresponding drop in percent gelatinized kernels increased to nearly 14%. The steaming treatment tended to increase the deformity of the parboiled rice kernels.