Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: May 20, 2006
Publication Date: July 9, 2006
Citation: Trabelsi, S., Nelson, S.O. 2006. Temperature-dependent behavior of microwave dielectric properties of bound water in grain and seed. American Society of Agricultural and Biological Engineers Paper Series. Paper Number 066077. Interpretive Summary: The moisture content of cereal grains is very important in determining how long such commodities can be safely stored without spoilage and loss of value. Electrical moisture meters determine grain and seed moisture content by sensing their electrical characteristics, called dielectric properties, which are closely correlated with moisture content. There is need to improve the accuracy of moisture meters, and measurements at microwave frequencies, much higher than the frequencies used by current moisture meters, offer some advantages. Therefore the way in which the dielectric properties depend on the water in the grain and seed is important. The research reported was carried out to gain new knowledge about the nature of bound water held in grain. The aim was to obtain a better understanding of the way microwave dielectric properties of grain change with temperature, and by studying this behavior at very low temperatures, minus 70 degrees to plus 21 degrees C, new information was obtained on the dielectric behavior of bound water. A change in slope of the line representing dielectric properties at different temperatures revealed that bound water is most likely frozen at about minus 20 degrees C, whereas liquid water freezes at about 0 degrees C. With better understanding of the dielectric behavior of water in grain, the accuracy of rapid sensing of moisture content by microwave moisture meters can be improved. Better moisture sensing can be achieved to preserve quality of grain and seed and provide higher quality products for consumers.
Technical Abstract: Dielectric behavior of bound water in grain and seed was investigated through measurement of the dielectric properties at microwave frequencies over a wide temperature range between –70 degrees Celsius and +21 degrees Celsius. Samples of wheat and soybeans were cooled to –70 degrees Celsius and then their dielectric properties were tracked with a free-space transmission measurement system between 2 and 18 GHz while they warmed up to room temperature. Results showed that both the dielectric constant, epsilon prime, and loss factor, epsilon double prime, increased with temperature with a characteristic slope change and a higher increase rate at around –20 degrees Celsius.