The Diffusion of Moisture in Paddy During Hydration and Dehydration Processes

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Hydration and dehydration behavior and the effective diffusivity of paddy during the process of parboiling were studied. Hydration of three different paddy samples (Sherpa low and high head rice yield and Reiziq) were performed below (60°C) and above (90°C) the gelatinization temperature. The hydration period ranged from 5 to 300 minutes at 60°C and 5 to 90 minutes at 90°C. All of the paddy samples showed different hydration behavior below and above the gelatinization temperature, discerned with two different stages at 60°C and three stages at 90°C. Dehydration was carried out at 40°C just after hydration (without tempering the kernel), which mostly took place at the falling rate period. The hydration and dehydration pattern was not different between the high HRY and low HRY paddy, indicating a limited contribution of microfissures to the diffusion rate in the paddy. Five commonly used semi-empirical models were used to predict the hydration and dehydration behavior of paddy and, among them, the Page model was found to be the most suitable. The effective diffusivity during hydration was dependent on the temperature of hydration, which was 1.83 × 10−11 to 2.11 × 10−11 m2/s at 60°C and 6.68 × 10−11 to 7.94 × 10−11 m2/s at 90°C. The effective diffusivity during dehydration depended on the soaking temperature and period of soaking; it was lower for high-temperature-hydrated samples than low-temperature-hydrated samples. The study concluded that the mass water diffusivity was not affected by the microfissures within the paddy kernel, and the hydration pattern was strongly dependent on whether the temperature was above or below the gelatinization temperature.