Temperature-dependent behaviour of dielectric properties of bound water in grain at microwave frequencies

Dielectric behaviour of bound water in grain was investigated through measurement of the dielectric properties of hard red winter wheat at microwave frequencies over a wide temperature range between –80 °C and +10 °C. Measurements were performed in free space between 2 and 18 GHz on two wheat samples of 15.7% and 23.6% moisture content, respectively. Cole–Cole plots reveal the existence of different distributions of relaxation times at different temperatures for the lower moisture sample. In contrast, the Cole–Cole plot of data corresponding to the higher moisture sample indicates the existence of a single distribution of relaxation times for all temperatures. Analysis of data obtained at a single frequency for each wheat sample shows that for both samples ε' and ε'' increase linearly with temperature with a characteristic slope change at around –20 °C, which might be interpreted as the freezing temperature of bound water in wheat.