Influence of different tempering period and vacuum conditions on the rice grain breakage in a thin layer dryer

Abstract

Drying and milling of the paddy are known as the most important process influencing the rice grain breakage. Therefore in this study, influence of tempering period and the reduced atmospheric pressure (vacuum) on the crack formation of two rice varieties (Nemat and Pajouhesh) during the tempering stage is presented. Four different level of drying temperature (40, 50, 60 and 70°C) were used in this study. After drying, paddies have been transferred to a vacuum chamber ranging from 0.4 at to 1 atm (ambient pressure). Based on the results obtained in this study, crack formation in rice grains increases with increasing drying time and drying temperature. In contrast, a reduced pressure during the relaxation stage significantly reduced the rice grain breakage. During the relaxation period in the vacuum chamber, heat transfer within the rice grain occurs slowly, yields minimizing the thermal and moisture stresses. The optimized combination of temperature, time, and pressure for the Nemat and Pajouhesh rice varieties obtained 60°C, 5 hr, and 0.8 atm, respectively.