Effective moisture diffusivity in wheat kernels during adsorption

• A modified moisture diffusion equation fits the plots between moisture ratio and time.
• The moisture sorption rate of grain is showed by differential form of d(Mt)/dtd(Mt)/dt.
• Moisture diffusivity in wheat tends to increase with temperature raise at certain RH.

Wheat samples with low (4.97–6.08%), normal (10.11–11.04%), and high (20.7–22.72%) initial moisture content (IMC, wet basis) were used to determine the rate of moisture adsorption by 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 of samples and exposure time. From 65% to 100% RH, the IMC of wheat samples was inversely related to the moisture adsorption rate at temperatures from 10 to 35 °C. Moisture adsorption rates of samples increased with increasing temperatures. The moisture adsorption rate of samples with the same IMC increased with increasing RH at a given temperature. A single wheat kernel was considered geometrically to be a finite homogeneous slab shape, and the analytical solution of the partial differential equation for moisture diffusion was given. The effective moisture diffusivity was calculated using the 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 wheat kernels with normal moisture was 1.681 × 10−8–1.516 × 10−7 m2 h−1, and their activation energy was 23.651–28.434 kJ mol−1. For the same IMC, the effective moisture diffusivity of wheat kernels tended to increase with increasing temperature at a given RH, but decreased with an increase in RH at a given temperature. Activation energy and the pre-exponential factor of Arrhenius equation (D0) tended to decrease with increasing RH. With similar initial moisture content, winter wheat and spring wheat exhibited similar effective moisture diffusivity.