Drying Characteristics and Quality of Rough Rice under Infrared Radiation Heating

Abstract

Infrared (IR) radiation heating could provide a high heating rate and rapid moisture removal for rough rice drying. The objective of this research was to investigate the effect of the drying bed thickness on drying characteristics and quality of rough rice subjected to IR heating. Samples of freshly harvested medium grain rice (M202 variety) with 20.5% and 23.8% (w.b.) moisture contents were used for this study. They were dried with two different radiation intensities (4685 and 5348 W m-2) and exposure times of 15, 30, 40, 60, 90, and 120 s for each drying bed thickness. The three tested drying bed thicknesses were single layer, 5 mm, and 10 mm. After IR drying, the samples were tempered for 4 h followed by natural cooling. The drying rate, moisture removal, and temperature of the rice were determined. The rice temperatures after the IR heating were in the range of 35.9°C to 71.4°C. The heating and drying rates decreased with the increase of bed thickness. Up to 2.2% of moisture was removed during natural cooling after tempering, without additional energy input. IR heating under tested conditions did not have adverse effects on rice sensory and milling quality, including total rice yield, head rice yield, and degree of milling of the dried rice. We concluded that a high heating rate, fast drying, and good rice quality can be achieved by IR heating of rough rice to about 60°C followed by tempering and natural cooling with a tested bed thickness up to 10 mm.