Periodic hydration of paddy rice in mass transfer, physical and thermodynamic properties

Abstract

Hydration is an essential stage in the rice parboiling process, and it can be a time-consuming. We evaluated the effects of periodic operation in paddy rice hydration to reduce the processing time. Periodic hydration with temperature modulation (10 °C amplitude and 12 min periods) was compared with isothermal hydration (conventional) at mean temperatures of 25, 35, 45, 55, and 65 °C. The process comparison was based on mass transfer, process time reduction, change in grain microstructure and thermodynamic properties. The periodic process increased water absorption, with a 41.47 % average reduction in process time. The Diffusion model satisfactorily predicted the moisture uptake experimental data (R2 > 0.97). The significant increase (p < 0.05) in the 27.38 % average in the model parameter indicated that the periodic operation favored the mass transfer phenomenon. Micrographs of hydrated grains revealed that the hydrations at 55 e 65 °C favored starch gelatinization, being able to reduce a subsequent step in the process (steaming). The temperature modulation did not alter the grain microstructure when both processes were compared at the same temperature. However, it interfered with the thermodynamic properties of the system. Periodic operation revealed mean values higher to ΔH# and mean values lower to ΔS# and ΔG#.