Study of hydration kinetics and density changes of rice (Tarom Mahali) during hydrothermal processing

In this study, water absorption characteristics of white rice in distilled water was determined at 25, 30, 40, 50, 60 and 70 degrees C by recording the weight increase in grain with respect to time. Bulk density, kernel density and porosity of rice were also evaluated as a function of soaking temperature in the range of 25-70 degrees C. Five mathematical models (Henderson and Pabis model, exponential model, Page model, modified Page model and two-term exponential model) for describing the soaking behavior of rice were investigated. Out of the five models considered, Page model was found to be the most suitable for describing the soaking behavior of the rice. The soaking temperature had great effect on the soaking kinetics of rice. We found that the water absorption increased when the soaking temperature increased from 25 to 70 degrees C and the soaking time decreased with increase in soaking temperature. Effective diffusivity of water during soaking of rice varied from 5.58 x 10-(11) to 3.57 x 10(-10) m(2)/s over the temperature range studied (25-70 degrees C), with an energy activation of 34.17 K J/mol. In addition, the temperature dependence of the effective diffusivity coefficient was expressed by an Arrhenius-type relationship. During soaking of rice, bulk density changed from 678.19 to 702.69 kg/m(3), kernel density changed from 1208.71 to 1258.71 kg/m(3) and porosity changed from 43.89% to 44.17% as soaking temperature increased from 25 to 70 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.