Effect of Various Drying and Tempering Conditions on Whiteness Index of Rice in a Two-Stage Drying Method

In multi-stage drying of various rough rice varieties, determination of the optimum drying and tempering conditions is important to accelerate the operation and reduce energy consumption, as well as produce a high quality product. In this research, two rice varieties namely, Shafagh (long-grain) and Sazandegi (medium-grain) were dried under high (air temperature and relative humidity of 60°C and 17%) and low (air temperature and relative humidity of 40°C and 12%) drying conditions at four various durations of 1.5, 3, 4.5 and 6 points percentage moisture content removal (PPMR) in the first stage of drying. Immediately after drying, the tempering operation was performed at the same temperature of drying in sealed bags at seven durations of 0, 40, 80, 120, 160, 200 and 240 min. Finally, all samples were dried to 12.5% (w.b.) moisture content as the second drying stage. After hulling and milling, the indices of mass degree of milling (DOMm), and the optical degree of milling (DOMmm) determined by a milling meter were measured and statistically compared for each sample. The experiments were conducted in factorial split with a completely randomized design. The results revealed that during dehydration of the tested varieties under low condition in the first stage, the tempering operation had not significant effect on DOMm and DOMmm. But during drying under high drying condition in the first stage, the tempering effect was significant on both quality indices. So that, for Sazandegi and Shafagh varieties, 6 PPMR in the first drying stage followed by 240 and 200 min tempering for the highest DOMm, and 160 and 240 min tempering for the maximum DOMmm, is recommended, respectively.