Study on moisture adsorption rate and effective moisture diffusivity in soybean kernels by gravimetric method

The soybean samples with low ( 4.33% ~ 5.85%) , normal ( 11.74% ~ 12.65%) , and high ( 17.58% ~ 17.89%) initial moisture content ( IMC) were used to determine the rate of moisture adsorption by gravimetric method at 10, 20, 25, 30, 35 ℃ under 65%, 86% and 100% RH, respectively. A moisture diffusion equation was modified to fit for the relationship between moisture ratio of samples and exposed time. The soybean was considered as a sphere with radius symmetry, and its effective moisture diffusivity and activated energy was calculated using the method of slope.In the range of RH 65% to 100%, the lower the sample IMC, the higher moisture adsorption rate at temperatures of 10 to 35 ℃, and the moisture adsorption rate of samples were increased with an increase in temperature.Under the same IMC, the moisture sorption rate of samples were increased with an increase in exposed RH. For soybean samples with normal IMC, The moisture desorption rate at 20 to 35 ℃ were quickly deceased within 72 h under three kinds of RHs, but at 10 ℃, the moisture adsorption rate was slowly decreased within 96 h.During the sorption process of 10³5 ℃, the effective moisture diffusivity of two soybean varieties with normal moisture content ranged from 1.920 × 10- 8m2·h- 1to 5.253 × 10- 8m2·h- 1, the activated energy ranged from10.711 k J·mol- 1to 23.358 k J·mol- 1. For the samples with similar moisture, their effective moisture diffusivity were increased with an increase in temperature, their activated energy tended to be increased with an increase in relative humidity.For the same soybean variety, its effective moisture diffusivity and activated energy tended to be increased with the raise of initial moisture.