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United States Department of Agriculture

Agricultural Research Service

Research Project: SENSING METHODS & INSTRUMENTATION FOR RAPID DETERMINATION OF MOISTURE CONTENT... AND OTHER QUALITY ATTRIBUTES OF GRAINS, SEEDS, & NUTS

Location: Quality and Safety Assessment Research Unit

Title: Dielectric investigation of water binding in grains

Authors
item Trabelsi, Samir
item Nelson, Stuart -

Submitted to: Internation Society for Electromagnetic Aquametry Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 22, 2011
Publication Date: May 31, 2011
Citation: Trabelsi, S., Nelson, S. 2011. Dielectric investigation of water binding in grains. Internation Society for Electromagnetic Aquametry Proceedings. pp. 87-91, Kansas City, MO, May 31- June 3, 2011.

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 bound water in grain at microwave frequencies. The aim was to obtain a better characterization of bound water in wheat with time after disrupting water equilibrium by adding moisture. Samples of wheat were moistened to different moisture levels and their dielectric properties were measured over 5 to 15 GHz at different time periods as the dielectric properties returned to equilibrium. At all frequencies, the dielectric constant and dielectric loss factor decreased initially and reached a plateau within an hour or two for normal moisture contents. This behavior indicates a change in the binding modes of water over time after moistening. Moistening of grain thus can introduce errors in moisture content indicated by moisture meters for an hour or two after water is added before water binding reaches equilibrium. 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 properties of wheat samples, in which moisture equilibrium was upset by adding water, were tracked versus time in the frequency range between 5 GHz and 15 GHz at room temperature (23 oC). Results show an initial drop in the dielectric constant and loss factor, which reflects the initial stages of water binding, followed by a plateau indicating the final binding level of the water molecules inside the wheat kernels. The Cole-Cole diagram for each sample reveals a significant difference in the dielectric response at different times illustrating the different stages of water binding.

Last Modified: 11/28/2014
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