TWO-STAGE DRYING As A DIFFERENT STRATEGY FOR PADDY RICE

摘要

Drying is a key operation in maintaining the quality of rice, most especially in the humid tropics. As part of a study to explore the drying characteristics of modem cultivars, a series of experiments was conducted to determine the effects of drying air temperature on grain quality, including head rice, whiteness, milling recovery and seed viability. A thin layer-drying simulation of PSB Rc54 at various temperatures and relative humidity was undertaken using a laboratory dryer, an environmental chamber fitted with a fan and a bin. The ultimate objective of the study was to develop a drying strategy which will result in faster drying rates while maintaining grain quality. Samples of 750 grams of PSB Re 54 grain were dried in the laboratory dryer. Results showed that from 25% moisture content (MC) on a wet basis to 14% MC, with continuous single-temperature drying the higher air temperature dries the grains faster but reduces grain quality in terms of percent head rice. In contrast, with two-stage drying, a higher drying rate can be achieved without significant loss in grain quality. Tlie first stage could utilize higher drying air temperature (80 deg C) to remove surface moisture at high moisture content (above 18% MC wet basis), while the second stage should use lower temperature (40 deg C) to reduce internal moisture to the desired level. As moisture is reduced on the swface of the grain, the internal moisture diffusion becomes significant and diying air temperature should be reduced accordinglyto maintain grain quality. Results also showed that grains subjected to air temperature of 60 deg C and above, whether in the first or second stage of diying, lost their viability. Therefore, for seed production purposes, drying air temperature should be below 60°C. Milling recovery and whiteness were not significantly affected by drying air temperature. Experimental results suggest that with the existing dryers in the market, simple adjustments of the drying process can be made to increase drying rate, ie, drying capacity, while maintaining grain quality.