A novel simulation model to analyze rice intermittent drying considering glass transition concept

Abstract

During rice drying its starch exhibits a variety of thermal properties. It can be transformed from a rubbery state to a glassy state and vice versa due to temperature and moisture content gradients (MCGs) leading to the reduction in head rice yield. Intermittent drying is an effective drying method to overcome this issue. In this research, a novel mathematical model was developed for rice intermittent drying process considering the glass transition concept which has not been done before. In addition, the real 3D body-fitted geometry of the rice kernel was first utilized for simulation. To determine the required coefficients and validate the model, continuous and two-stage intermittent drying (2SID) experiments were performed. A correlation of 99% was found between the drying kinetics obtained from the experiments and those obtained from the model. Regarding the glass transition phenomenon, a quantitative S function was defined to express the local glassy-to-rubbery state at each time. The transitory temperature and moisture distributions inside the rice kernel were displayed and discussed at each stage of the 2SID. Further, the glass transition behavior of the rice kernels was investigated by using the S function's values. The glassy region extended by 28.8%, 3.3%, and 20.1% of the whole kernel rice at the first drying, tempering, and second drying stages, respectively. At the end of the 2SID, 45.6% of the entire kernel was located in the glassy region while the central layers were still located in the rubbery region. Overall, the developed model could be used to determine the optimal intermittent drying parameters for rice to improve its dehydration performance at a low cost.