Moisture Adsorption Rate and Effective Diffusivity in Chinese Milled Rice

Abstract

Four varieties of Chinese milled rice samples with 11.90–15.56% initial moisture content (IMC, wet basis) were used to determine the rate of moisture adsorption using the gravimetric method at 10, 20, 25, 30, and 35°C under 65%, 86%, and 100% relative humidity (RH), respectively. A moisture diffusion equation was modified to fit the relationship between the moisture ratio (MR) of samples and exposure time. In the range of 65% to 100% RH, the lower IMC of milled rice samples corresponded to higher moisture adsorption rate at temperatures of 10 to 35°C. The moisture adsorption rate of samples increased with an increasing temperature. The moisture sorption rate of samples with the same IMC increased with increasing RH at certain temperatures. A single milled rice kernel was geometrically considered a finite homogeneous cylinder shape, and the analytical solution of the partial differential equation of moisture diffusion was given. The effective moisture diffusivity was calculated using a slope method by plotting the experimental data in terms of ln (MR) versus rewetting time. In the range of 10–35°C, the effective moisture adsorption diffusivity of milled rice kernels with normal moisture was 4.321×10-9–1.117×10-7 m2h-1. For the same IMC, the effective moisture diffusivity of milled kernels tended to increase with increases in surrounding temperature at certain relative humidity levels, but decreased with an increase in RH at certain temperatures. It was concluded that the milled rice samples from different types had similar effective moisture diffusivity when similar IMC was given.