Investigating Moisture, Density, and Temperature Dependence of Dielectric Properties of Unshelled and Shelled Peanuts at Microwave Frequencies

Authors: Trabelsi, Samir; Lewis, Micah; Nelson, Stuart

Submitted to: Microwave Power Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2009
Publication Date: 7/8/2009
Citation: Trabelsi, S., Lewis, M.A., Nelson, S.O. 2009. Investigating Moisture, Density, and Temperature Dependence of Dielectric Properties of Unshelled and Shelled Peanuts at Microwave Frequencies. 43rd Annual International Microwave Power Symposium Proceedings. pp. 95-100.

Interpretive Summary: Electrical characteristics, known as dielectric properties, of grain and oilseeds, are closely related to the amount of water that they contain. Consequently, instruments can be designed to sense these dielectric properties and be calibrated to read moisture content. Such instruments, called moisture meters, are widely used in the grain and seed trade for rapid determination of moisture content. Moisture content is especially important in the sale and storage of peanuts so that spoilage and development of toxins can be avoided. If kernel moisture contents of peanut lots exceed 10.5 percent, they cannot be sold, because they must be dried for safe storage. Microwave dielectric properties of peanuts are being studied for use in development of microwave moisture meters for use in the peanut industry. In particular, the microwave measurement technique provides a method for reliably measuring the moisture content of the peanut kernels from measurements on the unshelled peanut pods. In these studies, the dielectric properties of unshelled and shelled peanuts were determined by measurement at microwave frequencies from 2 to 18 GHz over wide ranges of bulk density, moisture content, and temperature to provide data for better understanding the variation of the dielectric properties of pod peanuts and shelled peanuts with these four variables, frequency, bulk density, moisture content and temperature. Understanding the dependence of peanut dielectric properties on frequency, bulk density, and temperature, as well as moisture content will be helpful in developing better peanut moisture meters. Ability to rapidly sense the bulk density of peanut pod samples should also be helpful in determining the value of different peanut lots. The new microwave moisture meters offer promise for improving efficiencies of grading and processing as well as preventing losses due to spoilage and will be useful tools for peanut farmers, handlers, and processors, thus providing safe and high quality products for consumers.

Technical Abstract: Dielectric properties of unshelled and shelled peanuts were measured with a free-space transmission technique between 2 and 18 GHz over wide ranges of bulk density, moisture content, and temperature. For better accuracy a pair of horn/lens antennas providing a focused beam was used; the sample was placed in a tunnel shaped enclosure made of radiation-absorbing material to isolate it from surroundings; and the sample thickness was selected to keep the attenuation between 10 dB and the maximum attenuation measurable by the vector network analyzer. Typical variations of the dielectric properties with frequency, bulk density, moisture content, and temperature are shown.