Drying characteristics and quality of rough rice under infrared radiation heating

Abstract

Infrared (IR) radiation heating could provide high heating rate and rapid moisture removal for rough rice drying. The objective of this research was to investigate the effect of drying bed thickness on drying characteristics and quality of rough rice under IR heating. The samples of freshly harvested medium grain rice (M202 variety) with 20.5 % and 23.8% (wb) moisture contents were used for this study. They were dried with two different radiation intensities, 4685 and 5348 W/m2, for six exposure times, 15, 30, 40, 60, 90 and 120 s, for each drying bed thickness. The tested three drying bed thicknesses were single layer, 5 mm and 10 mm. After IR drying, the samples were tempered for 4 hours followed by slow cooling. The drying rate, moisture removal and temperature of rice during drying were measured and calculated. The rice temperatures after the IR heating were in the range of 35.9 to 71.4 oC. The heating and drying rates decreased with the increase of bed thickness. A significant amount of moisture was removed during slow cooling after tempering, without additional energy input. The quality of milled rice, including total rice yield, head rice yield, and degree of milling of the dried rice was evaluated. It was concluded that a high heating rate, fast drying and good rice quality could be achieved by heating rough rice to about 60 oC followed by tempering and slow cooling with a bed thickness up to 10 mm.