SENSING MOISTURE CONTENT AND QUALITY OF GRAIN AND OTHER AGRICULTURAL PRODUCTS BY DIELECTRIC PROPERTIES
Location: Quality and Safety Assessment Research Unit
Title: Frequency, temperature, density and moisture dependence of dielectric properties of unshelled and shelled peanuts
Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: June 10, 2008
Publication Date: June 30, 2008
Citation: Trabelsi, S., Lewis, M.A., Nelson, S.O. 2008. Frequency, temperature, density and moisture dependence of dielectric properties of unshelled and shelled peanuts. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE).
Interpretive Summary: Dielectric properties of materials are those electrical characteristics that determine how they interact with electromagnetic fields. For example, some materials are heated much more rapidly in a microwave oven than others. Those materials that heat rapidly have a higher dielectric loss factor than materials that do not absorb much energy from the microwave fields. The dielectric properties of materials such as grain and oilseeds are also 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 determinations of moisture content. Research has shown that microwave frequencies may offer advantages for the development of new reliable moisture meters. Moisture content is especially important in the sale and storage of peanuts so that spoilage and development of toxins can be avoided. This paper reports measurements of the dielectric properties of unshelled and shelled peanuts in the microwave frequency range from 2 to 18 GHz. These properties are being studied for use in development of microwave moisture meters for use in the peanut industry. Thus the variation of the dielectric properties with frequency, temperature, density and moisture content must be determined, and this research has accomplished this objective. Linear relationships between the dielectric properties and each of the three variables, moisture, temperature, and density, are shown graphically in this paper. These relationships will be useful in the development of microwave moisture meters for peanuts. Such moisture meters will be helpful in preventing losses due to spoilage and will be useful tools for peanut farmers, handlers, and processors, thus providing high quality products for consumers.
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 (VNA). Typical variations of the dielectric properties with frequency, bulk density, moisture content, and temperature are shown.
Keywords. Dielectric properties, free space, microwaves, shelled peanuts, unshelled peanuts, bulk density, moisture content, temperature.