基于损伤力学的稻谷干燥品质的模拟和控制机理研究

Our main objectives of this project are to determine how micro-fissures caused by improper above and below glassy transition temperature (Tg) drying and tempering procedure lead to the occurrence of macroscopically visible fissures and the significant decrease in Head Rice Yield (HRY), especially providing a more rational explanation about the tempering treatment difference of above-Tg drying procedure on the amount of rice fissure and HRY, as well as the sensitivity of cracking rate using below-Tg drying methods. The changing of rice cell during above- and below- Tg drying and tempering process could be verified and analyzed by tissue section method. An experimental study of rice heat and moisture stress field physics parameter based on damage mechanism would be conducted and analyzed by finite element method (FEM) . The relationship among rice breaking energy (or maximum tensile stress) and drying condition could be obtained by regression of FEM simulation results and verified by three point bending breaking experiments. According to brittle fracture kinetics of polymer, a new term, a time interval ratio of either stress or internal work, would be defined to represent the process of rice kernel damage in the glassy state. Furthermore, a general thin-layer drying regression model could be gained through the combination of those regression models based on FEM and the time interval ratio of either stress or internal work. Meanwhile, the gelatination kinetics of rough rice during temporary storage and drying would also be studied. Finally, the physical and chemical state of rice kernel in thin-layer drying could be incorporated into the stochastic deep-bed drying model so that a new deep-bed drying model with regard to rice damage mechanism is established.

稻谷干燥后品质与干燥生产率、能耗之间的矛盾一直是困扰稻谷干燥机推广和发展的瓶颈问题，系统研究高温快速、低温慢速干燥后谷物品质劣化机理，提炼新的干燥品质劣化预测模型是取得突破的关键。本项目运用组织切片的方法，对稻谷干燥（缓苏）过程中籽粒内部组织结构的变化进行显微观察；测量干燥过程中谷粒的损伤力学特性参数以及热湿梯度场物理特性参数；有限元分析单粒稻谷的热湿应力场和梯度场，回归拟合断裂能（或极限拉应力）与干燥温度和谷物含水率的经验公式，并依据高分子脆性断裂动力学理论，定义谷粒承受应力时间比概念，用以表征干燥条件对薄层谷物的损伤程度。引入稻谷糊化动力学方程，并结合深床干燥模型和稻谷原始水分的随机分布特性，建立稻谷深床干燥品质劣化模型。

本项目测定了不同品种稻米的淀粉分子量分布；利用三种方法测定了稻谷的玻璃化转变温度(Tg)；利用低场核磁成像技术研究缓苏过程中颗粒内部水分分布及扩散过程；运用组织切片的方法，对稻谷干燥（缓苏）过程中颗粒内部微观结构的变化进行显微观察；解析稻谷内部组织的损伤机理，系统研究了高温快速干燥后稻谷品质的劣化机理。主要结论如下：.⑴ 籼米和粳米主要有三个主要级分，即直链淀粉、中间级分和支链淀粉；而糯米主要由支链淀粉组成，中间级分及直链淀粉含量很少。支链淀粉平均分子量分布范围为8.96×107 g/mol-1.56×109 g/mol。直链淀粉平均分子量变化范围为5.63×106g/mol-8.15×106g/mol。.⑵ 水分含量变化差异大的稻谷用差式扫描量热仪测出的Tg并无明显差异。而低场核磁和热机械分析仪法测定稻谷的Tg过程中，由于升温过程引起的水分不断流失，并不能测出Tg。.⑶ 低场核磁共振成像技术研究了缓苏过程中稻谷颗粒内部水分分布情况，干燥后颗粒内部的水分呈不均匀分布，水分平衡主要发生在缓苏的前1~2h内。将传统的干燥模型与颗粒内部信号比的变化趋势相拟合，得出双指数模型拟合效果最好，拟合结果为： ，说明稻谷的缓苏过程存在两种水分传递过程，即颗粒内部的水分扩散与颗粒表面的对流，两者共同作用于颗粒内部水分梯度的消退。 .⑷ 研究了缓苏温度对稻谷爆腰率的影响，并结合玻璃化转变理论对裂纹形成机理进行了研究。缓苏温度高于Tg时，淀粉链节、链段可自由旋转，内应力易消退，导致微观裂纹不易形成。高温缓苏时，还可适当的延长干燥时间、缩短缓苏时间，而不会引起爆腰率的增加。.⑸高温缓苏后稻谷的微观裂纹细长，而低温缓苏后稻谷微观裂纹粗大。低温干燥稻谷的淀粉颗粒保持原有的多边形形状。而颗粒温度过高时，淀粉颗粒发生熔融，失去原有的不规则的多边形形状。.⑹ 对低水分稻谷（18.5%w.b.）进行干燥-通风联合干燥后，整精米率无明显下降。脂肪酸的积累，大分子的变性使稻谷在干燥过程中品质发生劣化。.⑺ 一次加湿量、均质温度对裂纹增率影响极显著。一次加湿量增加，均质温度升高，裂纹增率增加。