Production Mechanism Of The Drying-induced Fissure In Paddy Rice Based On The Glass Transition Theory

In view of the phenomenon that paddy rice easily fissures after high temperature drying,the single brown rice kernel was taken as the research object in this paper.The factors that may cause the kernel fissuring are simulated and compared with the acoustic emission experimental results.Then the mechanism of drying induced fissuring was obtained by comprehensive analysis the research results.A regulatory strategy of reducing drying induced fissuring was put forward and the high temperature drying process was optimized and verified by experiments.Firstly,the drying characteristics of the brown rice were studied by experiments and the effective diffusion coefficient of water inside the kernel was obtained.The breaking load of the brown rice kernel was measured and analyzed statistically to eliminate the influence of individual differences in kernels.Based on the threedimensional fitness geometric model,the distribution of temperature and moisture content inside the brown rice kernel during drying process was simulated,which showed that the temperature gradient only existed in the initial drying period,the moisture gradient existed in the whole drying process which increased first and then decreased and maintained at a higher value at the end of drying.The higher the drying temperature,the greater the maximum moisture gradient in the same position inside the kernel.The glass transition process of the brown rice kernel during drying was simulated based on the glass transition equation in literatures.For brown rice with an initial moisture content of 22%(w.b.),when the drying temperature is lower than 34.39?there is no glass transition inside the kernel;when the drying air temperature is between34.39?-42.19?,the kernel will transit from glassy state into rubbery state at the initial heating period,and transit from rubbery state into galssy state at the drying period;when the drying air temperature is higher than 42.19?,the kernel will transit from glassy state into rubbery state at the initial heating period and remain in rubbery state until the end of drying.The tempering process after drying only effects the moisture distribution inside the kernel,but doesn't affect the glass transition.Acoustic emission technology was used to collect and analyze the acoustic emission signal inside the brown rice kernel during drying process.The acoustic emission signal intensity in the heating period was less and lower than that in the drying period;and was higher in the60? than that in the 40? drying process.Secondly,the mechanism of drying induced fissuring was exploratory studied by comprehensive analysis of the above research results.The moisture gradient inside the kernel during drying process is the main reason for fissuring initiation and propagation.During the high temperature drying process,micro-fissure will occur due to the moisture stress caused by the moisture gradient inside the kernel.In the cooling period after drying,the micro-fissure will extend to the kernel surface under the main action of the moisture stress and form macro-fissure.The thermal stress and the edge stress caused by glass transition during heating and cooling period are very small.Finally,based on the analysis of moisture gradient inside the brown rice kernel and the mechanism of drying-induced fissure,the control strategy of reducing dryinginduced fissure was put forward: the tempering operation should be used in the drying stage with high moisture gradient and after drying;in other stages,the paddy rice should be dried continuously,which can effectively reduce the maximum moisture gradient and the fissuring kernel growth ratio.The optimization of drying process under the guidance of strategy was verified by experiments.