Low-field nuclear magnetic resonance for the determination of water diffusion characteristics and activation energy of wheat drying

Abstract

The characteristics of moisture diffusion during wheat drying were determined through low-field nuclear magnetic resonance. The feasibility of variable temperature drying was validated by identifying the variation in the characteristics of evaporable water (EW) and each water component in wheat. During drying, chemically bound water in wheat was converted into EW, which comprised physically bound water and free water components that can evaporate and gasify directly. EW was removed from wheat through diffusion. The analysis of the average rate of the reduction in EW and the variation in the characteristics of weakly chemically bound water () at different wheat drying stages showed that EW was rapidly removed from wheat at the drying temperature was set at 80 °C during the early stage of drying. The highest average drying rate of was observed during the later stage of drying at the drying temperature of 70 °C. As inferred from the behavior of chemically bound water during conversion and the behavior of EW during diffusion in wheat, the activation energy required for the removal of EW from wheat comprises the activation energy of the diffusion of EW from wheat () and the activation energy of the conversion of chemically bound water to EW (). Calculations showed that and fell within the ranges of 25.72–53.75 and 1.09–24.17 kJ/mol, respectively. The maximum value of was less than the minimum value of This result confirmed that the energy consumption of removal of chemically bound water is bigger than that of EW. A method for calculating the drying activation energy of wheat () was developed in reference to the characteristics of the stable component contents of mature wheat. The relationship between and the moisture content of biomass was revealed.