Hydration Kinetics and Energy Analysis of parboiling Indica Paddy

Abstract

Hydration of Indica paddy (variety IR-20) in hot water was studied as a function of time, pressure and vacuum followed by pressure at temperatures ranging from 25 to 75°C and time of soaking not exceeding 6 h. Becker's diffusion equation was used to predict the soaking behaviour of paddy. Soaking at 196 kPa and above, maintaining temperatures of 65°C, shows complete hydration within 2·5 h. Within the experimental range studied, conditions such as hot water soaking under atmospheric pressure, soaking under vacuum alone for 6 h and temperatures of 60°C and below, will not result in the reduction of the soaking period as well as not being sufficient for complete hydration necessary for parboiling. The activation energies of the diffusivity through the grain were calculated using Arrhenius-type equation for diffusion dependence on temperature. For the soaking conditions selected, the values of activation energy and diffusion coefficients of paddy were found to be in the range of 19·49–23·09 kJ mol −1 and 0·406–0·647 m 2 s −1, respectively. The heat energy required for the existing and proposed method of soaking amounts to 1·105 and 1·005 GJ, respectively. The study showed that soaking of paddy involving heating of water to 75°C, soaking at 65°C, maintaining a gauge pressure of 196 kPa and above, a reduction of soaking time from 4 to 2·5 h is possible over the existing process, with a single-stage compression. Assuming perfect inter-cooling and neglecting inter-cooler pressure drops, the power input can be further reduced by 5% with two-stage compression. The economic aspects of the suggested method of parboiling as compared with the traditional method adopted indicated a saving of 4·4% besides realising higher head rice yield and lower broken grains.