Author
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NELSON, STUART |
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Submitted to: Trade Journal Publication
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/6/2005 Publication Date: 9/15/2005 Citation: Nelson, S.O. 2005. Dielectric spectroscopy in agriculture. Journal of Non-Crysitaline Solids. 351:2940-2944. Interpretive Summary: Dielectric properties of materials are those electrical properties that influence the interaction of the materials with electromagnetic fields. For example, the dielectric properties of foods determine how rapidly they will be heated in a microwave oven. These properties can also be utilized with appropriate electronic instruments for sensing moisture content in grain, oilseed, and other materials because the moisture content is highly correlated with the dielectric properties of the materials. Dielectric spectroscopy is a means for measuring the dielectric properties of materials over a broad range of radio and microwave frequencies. The agricultural applications described in this paper include the use of dielectric properties measurements to determine the best range of frequencies for selectively heating insects in grain as a means of insect control. Also included are measurements on fresh fruits and vegetables and other food materials such as whey protein gel and apple juice. Dielectric properties of materials vary with both the frequency, whether microwave frequencies or lower radio frequencies, and the temperature of the materials, and basic information on the dependence of dielectric properties of fresh fruits and vegetables on temperature is presented. With further research and understanding of the use of dielectric spectroscopy, it may be a useful tool for developing new rapid means of sensing quality factors in agricultural products that will be of benefit to producers, processors and consumers of these food materials. Technical Abstract: Reported measurements of the dielectric properties or permittivities over broad frequency ranges for some agricultural materials are cited, and graphical data from the cited literature are presented. They include 50-kHz to 12-Ghz frequency-domain permittivity data for adult rice weevils and hard red winter wheat, time-domain reflectometry data for the same insects from 30 MHz to 1 GHz, frequency-domain data from 200 MHz to 20 GHz for adults of the lesser grain borer, 200-MHz to 20-GHz data for apple juice and fresh peaches, and 10-MHz to 1.8-GHz data, as a function of temperature, for whey protein gel and navel orange tissue. Interpretation of the data showed that selective dielectric heating of rice weevils in wheat could be expected between 10 and 100 MHz but not at microwave frequencies. For fresh fruit and vegetable tissues, temperature dependence of the dielectric constant was minimal at some frequency in the 20- to 120-MHz range with a positive temperature coefficient below that frequency and a negative temperature coefficient at higher frequencies. Between 10 and 300 MHz, the loss factor can be very well expressed as a linear function of the log of frequency and temperature. With further study dielectric spectroscopy may be useful in detecting product quality factors. |
