Research On The Quality Of Hot Air And Microwave Drying Of Thin Layer Rough Rice

Drying process will affect the quality of rough rice, and its market price and theeconomic benefits of its processed products. Unsuitable drying process will not onlyincrease cost of production, but also reduce the quality of rough rice. Seeking areasonable and effective drying process was necessary for ensuring quality, improvingdrying efficiency and decreasing energy consumption. In this paper, the dryingcharacteristics of hot air and microwave drying and the effect of different dryingconditions on the quality of rough rice were studied by the combinative methods ofexperimental studies and theoretical analysis. The main conclusions were obtained asfollowing:(1). The thin layer rice drying curves and drying rate curves under differentconditions of hot air drying and microwave drying were protracted. Under higherdrying temperature and air volume rate, the time of drying to the same moisturecontent was shorter, the drying rate was greater. It need110min to dry the rough riceof initial moisture content of23.4%to safe storage moisture of14.5%at a highertemperature (70℃) and higher air volume rate (0.5m/s), while it need420min at alower temperature(40℃)and lower air volume rate (0.3m/s). The first stage of hot airdrying was a constant process, and then the second stage was the speed-down process(the moisture content was less than19.0%). Microwave drying process was essentiallyconstant, and the drying rate had been reduced when paddy moisture content was lessthan13.0%. The drying rate was positively correlated with microwave power. Ricedrying curve can be fitted with quadratic polynomial equation: MR=At~2+Bt+C,where MR was moisture ratio, t was drying time, A,B,C were constants relating withthe drying conditions.(2). The effect of hot air drying on the quality of rough rice: crack additionalpercentage was positively correlated with temperature, air volume rate, the initialmoisture content. Germination rate, germination energy and head rice yield decreasedwith the increase of the drying temperature, air volume rate and initial moisturecontent. Crack additional percentage had a significant negative correlation withgermination rate, germination energy and head rice yield(r=-0.972,-0.978,-909);and had significantly positive correlation with fatty acid value (r=0.467). It wasnecessary to choose a low-temperature and retarder drying process for high initialmoisture content of rough rice to ensure its drying quality.(3). The effect of microwave drying on the quality of rough rice: With themicrowave power increasing, crack additional percentage of rough rice increased,while germination rate, germination energy, head rice yield and free fatty acid valuedecreased. The larger intermittent drying time ratio was, the greater crack additionalpercentage was, the smaller germination rate and head rice yield were. Drying rough rice to safe storage moisture content at the same power (60W) with intermittent drying,head rice yield and germination rate were65.4%,86%respectively, while53.9%,62%with consecutive drying. Drying power had greatest impact on the quality of rice,it was rather prone to dry overly and then appear the phenomenon of char rough ricewhen the microwave power was higher than315W. It should choose the low powerintermittent and retarder drying process with the lower microwave power.(4). Compared the preferable processes: for the rough rice with the initialmoisture content of20.3%, under process of hot air drying (the temperature was50℃,the air volume rate was0.4m/s), crack additional percentage, head rice yield,germination rate and the highest viscosity of rough rice were:2.5%,66.73%,96%,1036cP respectively; while under the preferable process of microwave drying process(microwave power was60W, intermittent drying time ratio was3min/3min), crackadditional percentage, head rice yield, germination rate and the highest viscosity ofrough rice were:4.0%,86.5%,62.12%,984cP. Compared to the microwaveintermittent drying process, hot air drying process was more slowly, required moreenergy, and it had a smaller inhibition on the amount of fungal and bacteria of roughrice.