Microwave dielectric properties of cereal grains

Authors: Trabelsi, Samir; NELSON, STUART - Collaborator

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2012
Publication Date: 10/29/2012
Citation: Trabelsi, S., Nelson, S.O. 2012. Microwave dielectric properties of cereal grains. Transactions of the ASABE. 55(5):1989-1996.

Interpretive Summary: Dielectric properties of materials are electrical properties that determine how materials interact with electric fields such as those of high-frequency and microwave electromagnetic energy. Therefore, the dielectric properties of materials determine how rapidly they will heat in microwave ovens and lower radio-frequency dielectric heating equipment. Dielectric properties are also important in low power applications, such as the rapid measurement of moisture content in grain. Electronic moisture meters use radio-frequency electric fields to sense moisture content in products such as grain, because the dielectric properties are highly correlated with the moisture content of the grain. Moisture content is the most important characteristic of cereal grain affecting suitability for harvesting, storage, transport, and processing. It is also an important factor affecting the price paid for grains. Electronic moisture meters have been developed and used for many years to provide quick moisture determinations. However, instruments to monitor grain moisture content in moving grain for on-line applications have not been entirely satisfactory, because changes in the grain packing or grain temperature can give large errors in moisture content. Research on microwave measurements for sensing grain moisture content has provided means for determining moisture content from its dielectric properties independent of changes in packing density. More basic data are needed on the microwave dielectric properties of cereal grains. Such data have been measured and are reported in this paper for wheat, corn, barley, oats,, and grain sorghum. These data will be useful in determining relationships between the dielectric properties and other variables, such as density and moisture content, and in modeling the dielectric behavior of such grains. Therefore, they have value in further research on improving the utility and accuracy of microwave measurements for determination of moisture content. These advances will encourage the commercial development of techniques for practical use and provide new tools for managing moisture content and quality of grain and grain products for consumers.

Technical Abstract: Dielectric properties of five cereal grains (wheat, corn, barley, oats, and grain sorghum) were measured at 23°C over a range of moisture contents and over microwave frequencies from 5 to 15 GHz with a free-space transmission technique. Resulting dielectric constants and loss factors are tabulated for each material for moisture ranges of interest to the grain industry at bulk densities that are close to the corresponding test weight of those cereal grains. Bulk densities of all grain lots decreased linearly with increasing moisture content. Both the dielectric constant and loss factor of all grain lots increased nearly linearly with increasing moisture content at all frequencies. For given moisture contents, the dielectric constants of all grains tested decreased with increasing frequency, and the loss factor changed relatively little over the frequency range. The new data will be useful in grain moisture sensing work and in microwave heating applications.