Numerical Modeling of Heterogeneous Moisture Diffusion in Milled Rice Grains: Diffusion Coefficient as a Function of Moisture, Temperature and Time

Abstract

The heterogeneous moisture diffusion of milled rice grains at 25, 35 and 45℃ was simulated in 3D using the finite element method. The objectives of the study were to investigate the effects of temperature on moisture absorption and to predict the heterogeneous moisture diffusion in milled rice grains. Major routes and cracks channeled and facilitated rapid moisture movement in the grain. High temperature increased the rate of moisture movement.The moisture diffusion coefficient expressed as a function of moisture, temperature and time adequately described the heterogeneous moisture diffusion characteristics in milled rice. The integration of the logistic function of time reduced the root mean square error (RMSE) values between the experimental data and the numerical solution.Moreover, the RMSE values were much lower than the values modeled with moisture and temperature only.Modeling moisture transfer using the major routes of moisture revealed an increase in the modified frequency factor of the moisture diffusion coefficient.Keywords: Heterogeneous moisture diffusion, milled rice, finite element modeling, diffusion coefficient, 3D modeling IntroductionThe cooking of rice is initiated by two concurrent processes: the hydration process and the heating process. The purpose of grain hydration is to bring the moisture within the grain to an appropriate level to cause partial disruption in the crystalline structure of the starch. On the other hand, heating raises the temperature to a sufficient level to gelatinize the starch granules. Whether the cooking of rice is done at home or commercially, these two processes are required. Usually, the cooking process requires a longer duration because the starch granules need time to become hydrated in order to be gelatinized (van den Doel et al., 2009).Thus, researchers and food processors alike are investigating ways to accelerate water uptake, thereby shortening the cooking time and reducing costs. Hence, this study aims to provide relevant data that will be useful in the design and optimization of the cooking of moisture with moving boundaries. Their findings revealed that the "differential" diffusion coefficient increased as the mean moisture content of rice increased with time. Perez et al. (2011) simulated the moisture diffusion and hygroscopic swelling in rice in three dimensions (3D) using Fick's or Fickian diffusion and Peleg's equation. The results of these studies were promising in understanding the hydration of rice. However, due to the complexity of grain geometry, which renders the diffusion process heterogeneous, it remains necessary to simulate the heterogeneous diffusion of moisture in milled rice in order to realistically reproduce the conditions occurring during moisture absorption. By definition, heterogeneous denotes variety and indicates nonuniformity. Ogawa et al. (2003) showed that the morphology of the starch particles in the grain is made up of different sizes. Voids and microcracks also served as microchannels for moisture migration.They also reported...

Three-Dimensional Numerical Modeling of Convective Heat Transfer During Shallow-Depth Forced-Air Drying of Brown Rice Grains

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