Hydration Kinetics of Rice Kernels under Vacuum and Pressure

Hydration of rough rice under vacuum (0.01, 0.03, 0.06, 0.08 MPa) and pressure (0.25, 0.4, 0.55 and 0.7 MPa) was conducted at 15 degrees, 35 degrees and 55 degrees C. As a reference, hydration tests at each temperature and atmospheric pressure were also conducted. For vacuum and pressure tests the Peleg equation was found suitable to predict hydration kinetics of rice. Peleg constants, k1 and k2, inversely related to the absorption rate (q) and the saturation moisture content (me), respectively, were estimated. The response surface method (RSM) was used to optimize the effect of temperature and pressure on q, me and saturation time (tsat) for vacuum and pressure hydration. The predicted optimum conditions required to maximize absorption rate and saturation moisture content together with the minimization of saturation time, were: a) hydration at 55 degrees C and 0.01 MPa for vacuum tests; and b) hydration at 55 degrees C and 0.59 MPa for pressure tests. For vacuum hydration the optimum responses were: q = 6.01 (g water/g dry basis) min-1, me = 0.3488 (dry basis) and tsat = 58 minutes. For pressure hydration the optimum responses were: q = 6.98 (g water/g dry basis) min-1, me = 0.3839 (dry basis) and tsat = 42 minutes. Both procedures, vacuum and pressure hydration, resulted as more convenient than hydration at atmospheric pressure. However, pressure hydration conducted to a higher hydration rate and saturation moisture content needed less soaking time.