Influence of Nonequilibrated Water on Microwave Dielectric Properties of Wheat and Related Errors in Moisture Sensing

Dielectric properties of wheat samples, in which moisture equilibrium was upset by adding water, were tracked versus time in the frequency range between 5 and 15 GHz at room temperature (23 degC). The results show an initial drop in the dielectric constant and loss factor, which reflects the initial stages of water binding, followed by a plateau indicating the final binding level of the water molecules inside the wheat kernels. The complex-plane representation of the dielectric properties divided by bulk density at a given frequency compares well to that obtained for well equilibrated wheat samples over the same moisture range. With application of a density-independent calibration function, the apparent moisture content was predicted in each sample from measurement of the dielectric properties. For each wheat sample, the predicted initial moisture content was higher than the reference oven moisture content and decreased as time increased. The Cole-Cole diagram for each sample reveals a significant difference in the dielectric response at different times, illustrating the different stages of water binding