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item Nelson, Stuart

Submitted to: Transactions of the ASAE
Publication Type: Proceedings
Publication Acceptance Date: 7/5/2005
Publication Date: 7/17/2005
Citation: Trabelsi, S., Nelson, S.O. 2005. Microwave dielectric methods for rapid, nondestructive moisture sensing in unshelled and shelled peanuts. In: Proceedings of the American Society of Agricultural Engineers, St. Joseph, MI. Paper no. 056162.

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 determination 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. Microwave dielectric properties of both unshelled and shelled peanuts of different moisture contents were measured at different bulk densities, or packing densities, and results were analyzed for usefulness in rapidly sensing the moisture content of the peanuts. This paper reports details of the microwave measurements of the dielectric properties and reveals that moisture content is determined with accuracies of about one half of one percent. In addition, it was determined that the moisture content of peanut kernels in the pods could be determined without need for shelling them. With further development, the microwave measurements offer good potential for saving considerable time and labor in the moisture testing required when drying peanuts and testing them for safe storage and marketing. Microwave moisture 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: Two microwave dielectric methods were used to determine nondestructively moisture content in unshelled and shelled peanuts from measurement of their relative complex permittivities in free space at 8.0 GHz and 24 degrees Celsius. The first method is based on direct relationships between the two components of the relative complex permittivity, the dielectric constant and dielectric loss factor, and bulk density and moisture content in unshelled and shelled peanuts. The second method allows moisture content in unshelled and shelled peanuts to be determined independent of bulk density changes with the use of a density-independent permittivity calibration function. Statistical analysis provided moisture content calibration equations along with corresponding standard errors of calibration over wide ranges of bulk density and moisture content. Also, an equation was established for determining moisture content in peanut kernels directly from measurements of microwave dielectric properties of unshelled peanut pods.