基于玻璃化转变的稻谷变温热风干燥工艺研究

为保证稻谷干燥后品质、提高干燥效率,基于不同含水率稻谷的玻璃化转变温度,提出变温热风干燥工艺。采用三因素五水平中心组合试验方法,以稻谷温度、初始含水率和热风风速为影响因素,以稻谷爆腰指数、整精米率和干燥时间为评价指标,研究稻谷玻璃化转变温度、恒温和变温干燥特性,模拟解析稻谷干燥过程中传热传质规律,以5、10、15℃的变温幅度进行变温干燥试验。结果表明,稻谷玻璃化转变温度与其含水率呈负相关,恒温干燥最佳工艺参数为稻谷温度47℃、初始含水率22.0%、热风风速0.50 m/s,干燥后稻谷爆腰指数70、整精米率57.67%、干燥时间195 min;与恒温干燥相比,以5℃和10℃为变温幅度的变温干燥工艺,干燥后稻谷爆腰指数分别降低了20和10,整精米率提高12.6、7.7个百分点,干燥时间缩短30 min和60 min。研究表明,基于玻璃化转变的稻谷变温热风干燥工艺明显改善了稻谷干燥后品质,提高了干燥效率。

Hot-air Drying Technology of Changing Temperature for Paddy Rice Based on Glass Transition Theory

In drying of paddy,the mechanical stress inside paddy kernels is caused by moisture content gradient and glass transition temperature(GTT).The stress results inside kernel crack during the milling process.The hot-air drying method was employed to investigate the effects of stepwise changes of air temperature determining GTT on stress cracking index,head rice yield and drying duration of paddy.To achieve the objectives,the concepts of glass transition for paddy and temperature and moisture distribution of material layer were determined by using numerical modeling method.The center combination tests of three-factor with five-level were employed to analyze and optimize the parameters of drying process for paddy under constant temperature.The surface temperature,initial moisture content of paddy and the hot-air speed were selected as the influencing factors.In terms of the stepwise changes of air temperature,the drying experiments were conducted at temperatures above the GTT based on the optimal parameters determined by constant-temperature drying process.The rising amplitudes of temperature were 5℃,10℃and 15℃,respectively.The results showed that the GTT of paddy was negatively correlated with its moisture content.The optimum parameters of constant-temperature drying were developed as temperature was 47℃and the initial moisture content was 22.0%for paddy,and hotair speed was 0.50 m/s,respectively,and the evaluation indexes were as follows:the stress cracking index was 70,head rice yield was 57.67%,and process duration was 195 min,respectively.Compared with the constant-temperature drying,under the stepwise changes of air temperature of 5℃and 10℃,the quality of paddy dried was improved as the stress cracking index was decreased by 20 and 10,the head rice yield was increased by 12.6 pencentage points and 7.7 percentage points,respectively,and drying duration was shortened by 30 min and 60 min,respectively.Therefore,the stepwise changes temperature of hot-air drying can improve the drying efficiency and quality of paddy based on glass transition theory.The results may provide references for the hot-air drying process and quality control of paddy.