Rapid and Nondestructive Determination of Moisture Content in Peanut Kernels from Microwave Measurement of Dielectric Properties of Pods

Authors: Trabelsi, Samir; Nelson, Stuart

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 4/4/2009
Publication Date: 6/21/2009
Citation: Trabelsi, S., Nelson, S.O. 2009. Rapid and Nondestructive Determination of Moisture Content in Peanut Kernels from Microwave Measurement of Dielectric Properties of Pods. ASABE Annual International Meeting.

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. Sale of peanuts is illegal in the United States if moisture content is 10.5% or higher. 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 better than one half of one percent moisture content for shelled or unshelled peanuts at temperatures between 0 and 60 degrees C (32 and 140 degrees F). In addition, it was determined that the moisture content of peanut kernels in the pods can be determined from measurements on the pods without need for shelling them. With further development, the microwave measurements offer good potential for saving considerable time and labor in the peanut grading process and for 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: A method for moisture determination in peanut kernels from measurement of the dielectric properties of peanut pods at microwave frequencies is presented. The dielectric properties of peanut kernels and pods were measured in free space with a vector network analyzer and a pair of focused beam horn-lens antennas. A density-independent algorithm was used to determine moisture content in peanut kernels and pods. Moisture calibration equations with temperature correction were determined from a three-dimensional representation, and an explicit relationship between peanut pod moisture content and kernel moisture content was identified. Results presented at 6 GHz and temperatures ranging from 0.5 oC to 58 oC show that kernel moisture content can be predicted with a standard error of calibration of 0.79% and that of pods with a standard error of calibration of 0.95%.