Dielectric Study of Water Binding in Grain

Authors: Trabelsi, Samir; Nelson, Stuart

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 4/26/2007
Publication Date: 6/18/2007
Citation: Trabelsi, S., Nelson, S.O. 2007. Dielectric Study of Water Binding in Grain. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE).

Interpretive Summary: The electrical characteristics of grain known as dielectric properties are used by electronic grain moisture meters for rapid determination of moisture content. Moisture content is important in determining the potential for safe storage of grain, and the moisture content must be measured whenever a grain lot is sold. Recent research has shown that measurements on grain at microwave frequencies offer advantages for moisture testing over the lower radio frequencies currently used by commercial grain moisture meters. Experience has shown that rain or artificial wetting of grain causes errors in the moisture readings provided by grain moisture meters. These errors result because the dielectric properties of the grain are affected when moisture equilibrium in the grain kernels is upset by wetting or drying. Therefore, the effects of wetting grain samples on the microwave dielectric properties were studied to determine the extent of this influence. Samples of hard red winter wheat of 10.6 percent moisture were raised to about 14, 17 and 22 percent by spraying them with proper amounts of water. The dielectric properties at microwave frequencies from 5 to 18 GHz were then tracked over the next six hours. Results showed that the microwave dielectric properties stabilized after two to four hours, depending upon the amount of water added. They also revealed information related to the degree of binding of water to constituents of the wheat kernels. Moisture content determined from the dielectric properties therefore needs correction for changes in the water binding during nonequilibrium conditions. The new information is useful to engineers developing new moisture meters and scientists interested in moisture equilibrium in grain kernels. It will therefore contribute to the goals of providing high quality and healthful products for consumers.

Technical Abstract: Dielectric properties of wheat samples, in which moisture equilibrium was upset by adding water, were tracked versus time in the frequency range between 5 GHz and 15 GHz at room temperature (23 oC). Results presented at 10 GHz 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 in the wheat kernels. With application of a density-independent calibration algorithm, the apparent moisture content was predicted in each sample from measurement of the dielectric properties. As expected, for each wheat sample, the predicted initial moisture content was higher than the reference oven moisture content and decreased as time increased. This study shows that dielectric-based moisture sensors require correction when used for sensing moisture content in nonequilbrated materials. Keywords. Moisture content, microwaves, dielectric properties, dielectric constant, loss factor, water binding.