Submitted to: Measurement Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 2006
Publication Date: July 17, 2006
Citation: Trabelsi, S., Nelson, S.O. 2006. Temperature-dependent behavior of dielectric properties of bound wataer in grain at microwave frequencies. Measurement Science and Technology. 17:2289-2293. Interpretive Summary: The moisture content of cereal grains and oilseeds 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 water held in grain. The aim was to obtain a better characterization of water in wheat with changing frequency and temperature. Samples of wheat were cooled down to –80 degrees C and their dielectric properties were measured periodically at several microwave frequencies as their temperature increased gradually to room temperature. At all frequencies, the dielectric constant and dielectric loss factor increase linearly with temperature, with a faster increase beginning around –20 degrees C. This behavior indicates a change in the binding modes of water similar to that observed when ice changes to liquid water. Thus, the freezing temperature for bound water in wheat is most likely about -20 degrees C. With better understanding of the electrical response of grain, 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 was investigated through measurement of the dielectric properties of hard red winter wheat at microwave frequencies over a wide temperature range between –80 degrees C and +10 degrees C. Measurements were performed in free space between 2 and 18 GHz on two wheat samples of 15.7% and 23.6% moisture content, respectively. Cole-Cole plots reveal the existence of different distributions of relaxation times at different temperatures for the lower moisture sample. In contrast, the Cole-Cole plot of data corresponding to the higher moisture sample indicates the existence of a single distribution of relaxation times for all temperatures. Analysis of data obtained at a single frequency for each wheat sample shows that for both samples epsilon prime and epsilon double prime increase linearly with temperature with a characteristic slope change at around –20 degrees C, which might be interpreted as the freezing temperature of bound water in wheat.