Effects of “natural” water and “added” water on prediction of moisture content and bulk density of shelled corn from microwave dielectric properties

Authors: Trabelsi, Samir; Nelson, Stuart; Lewis, Micah

Submitted to: Journal of Microwave Power and Electromagnetic Energy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2010
Publication Date: 3/15/2010
Citation: Trabelsi, S., Nelson, S.O., Lewis, M.A. 2010. Effects of “natural” water and “added” water on prediction of moisture content and bulk density of shelled corn from microwave dielectric properties. Journal of Microwave Power and Electromagnetic Energy. 44(2):72-80.

Interpretive Summary: Dielectric properties of grain are electrical characteristics of those materials that can be used with appropriate electrical instruments to rapidly measure their moisture content. Moisture content of grain must be determined and it must be reduced to safe levels for storage to avoid spoilage. The grain moisture content must also be measured whenever it is sold, so that any necessary grain drying costs can be taken into consideration in determining the price of the grain. Traditional grain moisture meters utilize the dielectric properties of the grain samples at frequencies between 1 and 20 MHz to determine moisture content, and they must be calibrated against standard oven-drying procedures. For the calibration process, moisture content of grain samples is often adjusted to ranges of moisture content by adding distilled water to some of the samples to adjust their moisture levels. It is known that the dielectric properties of grain samples, from which moisture content is determined by grain moisture meters, may be slightly different for grain samples with natural moisture and those adjusted to the same moisture content by adding water. These differences can result in errors in moisture determination by existing meters operating in the 1- to 20-MHz frequency region. Research has shown that measurements on grain samples at microwave frequencies offers advantages in reliable determination of moisture content. Therefore this study was conducted to determine the differences in dielectric properties of corn with natural moisture soon after harvest and corn samples to which distilled water was added to increase their moisture content. Since bulk density can also be determined from the same microwave measurements, differences in bulk density between corn samples with natural water and those with added water were also compared. Results showed that dielectric properties, and consequent moisture contents determined by microwave measurements were essentially the same for the corn samples with natural water and the corn samples which had been conditioned by addition of distilled water. Bulk densities sensed by the microwave measurements were also the same for the natural water corn lots and the added water corn lots. Therefore, calibration samples for corn can be prepared by adding water without introduc ing errors in the calibration of microwave moisture meters. The findings are of value in continued research on reliable sensing of moisture content and bulk density of grain samples, which will benefit growers, marketers and consumers of grain products.

Technical Abstract: Dielectric properties of samples of shelled corn of “natural” water content and those prepared by adding water were measured in free space at microwave frequencies and 23 oC. Results of measurements of attenuation, phase shift and dielectric constant and loss factor at 9 GHz show no difference between the samples with “natural” water and those in which water was added artificially. Bulk densities and moisture contents predicted from calibration equations expressed in terms of dielectric properties of both natural and added water samples agreed closely, and standard errors were less than 1% moisture content and 5% for bulk density.