Effect of Drying Method and Hydrothermal Treatment on Physicomechanical Properties of Parboiled Rice

Abstract Rice grain was obtained from milled paddy, in which the husk and bran are removed. During the milling process, rice varieties having inherent poor milling quality or those have been processed under unfavorable conditions, break and so their head rice yield (HRY) decrease. Parboiling is one of the hydrothermal postharvest treatment before drying and milling processes. After parboiling, the grains become stronger and breakage of rice is very much reduced during milling, leading to often a remarkable increase in the HRY. In this study the paddy was soaked in hot water (70 °C) for 1 h and drained, then steaming was done at four periods (0, 5, 10 and 15 minutes) at atmospheric pressure. Drying process was carried out using two driers techniques, solar and continuous, at three inlet air temperature to the chamber dryer of 35, 40 and 45○C. Results revealed that the effects of drying air temperature and steaming time showed a significant effect on HRY and broken kernels for Hashemi rough rice cultivar (p<0.05) in continuous dryer. In both drying techniques the higher HRY performance was appeared at 35○C drying air temperature and 10 min steaming time. It was observed that dried kernels in continuous dryer were stronger (more bending strength) than those dried in the solar dryer and had a higher rate of HRY during subsequent processing operations. It was proved that increasing the steaming time to a safe level caused a reduction in broken kernels which was associated with higher HRY.