MICROWAVE DIELECTRIC PROPERTIES OF UNSHELLED AND SHELLED PEANUTS

Authors: TRABELSI, SAMIR - UNIV OF GEORGIA; NELSON, STUART

Submitted to: American Society of Agricultural Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2003
Publication Date: 11/10/2003
Citation: TRABELSI, S., NELSON, S.O. MICROWAVE DIELECTRIC PROPERTIES OF UNSHELLED AND SHELLED PEANUTS. AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS, ST. JOSEPH, MI. ASAE PAPER NO. 036169. 2003.

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 moisture meters. Moisture content is especially important in the sale and storage of peanuts so that spoilage and development of toxins can be avoided. Because very little data are available on the microwave dielectric properties of peanuts, some new research was initiated to measure accurately the dielectric properties of both unshelled and shelled peanuts to provide some data on their dielectric properties over ranges of frequency, density and moisture content. This paper reports details of the microwave measurements of the dielectric properties of peanuts and provides values for the dielectric constant and dielectric loss factor of both unshelled and shelled peanuts. The data will be useful in research on the possibility of developing rapid moisture tests for peanut kernel moisture content in unshelled peanuts as well as in shelled peanuts. Such meters would be helpful in preventing losses due to spoilage and provide useful tools to peanut farmers, handlers, and processors, thus providing high quality products for consumers.

Technical Abstract: Dielectric properties of unshelled and shelled peanuts were measured with a free-space-transmission technique at room temperature (23 degrees C) between 2.0 and 18.0 GHz over wide ranges of bulk density and moisture content. For better accuracy and optimum use of the vector network analyzer (VNA) dynamic range, a pair of horn/lens antennas was used; and for each moisture level, the sample thickness was selected to keep the attenuation between 10 dB and the maximum attenuation measurable by the VNA; and time-domain gating was applied to the main response. Also, the sample was placed in a tunnel shaped enclosure of radiation-absorbing material to isolate it from surroundings. Variations of the dielectric properties with bulk density and moisture content are shown at 8 GHz and 23 degrees C. Data, normalized to the lowest bulk density for each sample with the Landau and Lifshitz, Looyenga mixture equation, are tabulated.