Investigating Effect of Temperature on Dielectric Response of Bound Water in Grain

INTRODUCTION Electromagnetic wave interaction with materials containing water at microwave frequencies is dominated by water [1]. Water is an important component of many biological and nonbiological materials. It is sometime divided into two categories: free water and bound water. From an electrical standpoint, behavior of free water is well described by the Debye dielectric model [1] with a single relaxation taking place at microwave frequencies, while little is known about the dielectric properties of bound water. They are described as lying somewhere between those of liquid water and those of ice depending on the degree of binding of the water molecules. In granular materials such as cereal grain and oilseed, bound water is expected to have several modes of binding depending on the inner structure of the kernels, the amount of water available, and the nature of surrounding molecules. One way to study bound water behavior is to examine changes in dielectric properties resulting from temperature changes. In this study, samples of wheat of given bulk density and moisture content were cooled to –80 C, and their dielectric properties were measured in free space between 2 and 18 GHz as their temperature increased gradually at room temperature. The data collected constitutes the basis for better understanding and modeling of the electrical behavior of bound water. As an example, variations of the two components of the effective complex permittivity, and , with temperature are shown at 8 GHz.