EFFECT OF DRYING AIR TEMPERATURE AND GRAIN TEMPERATURE OF DIFFERENT TYPES OF DRYER AND OPERATION ON RICE QUALITY

Quality of aromatic rice (KDML-105) and non-aromatic rice (Suphanburi I) was studied using different methods and scales of drying. At the lab scale, rough rice at high moisture (21±1% w.b.) was subjected to shade drying, sun drying, oven drying at 45 °C and 60 °C and fluidized bed drying (for different time periods) at 120 °C, with tempering for 2hrs. On a commercial scale, single stage drying (sun drying, column drying) and multi stage drying in a fluidized bed dryer coupled with column and shade drying were investigated. Head rice yield, color, hardness, stickiness and hardness ratio, water absorption, and structure of milled rice were examined for each method of drying. The results revealed that water absorption and ratio of stickiness and hardness decreased but the b-value and hardness increased with increase in dry grain temperature, longer exposure/drying time, longer tempering time, and rapid rate of water removal. Head rice yield of samples dried at high temperature (>60 °C) followed by tempering and of those dried at low temperature (<45 °C) in case of lab scale drying was increased and no significant difference (p<0.05) was seen relative control samples. For single stage drying, at low temperature, and multi stage drying in the fluidized bed drier (interment tempering), at high temperature, head rice yield was decreased and exhibited significant difference (p<0.05) relative to control sample. The yield however increased for a sample subjected to a 12hr tempering period and then shade drying stage subsequent to drying in a fluidized bed drier. Drying of rough rice at low temperature (<45 °C) did not affect cooking and eating qualities of rice which were affected by drying at high temperature (>60 °C). Cooking and eating qualities however changed at both single and multi stage drying on a commercial scale.