Author
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Nelson, Stuart |
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Submitted to: Electromagnetic Wave Interaction with Water and Moist Substances Proceeding
Publication Type: Proceedings Publication Acceptance Date: 3/15/2005 Publication Date: 5/29/2005 Citation: Nelson, S.O. 2005. Frequency and temperature dependence of the permittivity of fresh fruits and vegetables. In: Proceedings of the 6th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances, May 29-June 1, Weimar, Germany. p. 391-398. Interpretive Summary: There is a need for nonsubjective determination of product quality in the handling of fresh fruits and vegetables. Ability to rapidly sense quality by use of instrinsic properties of the fruits and vegetables would be very helpful in the sorting and grading of these products for the market. Electrical characteristics of fruit and vegetable tissue known as dielectric properties can be sensed with appropriate instruments that use electric fields for nondestructively probing the products. Radio-frequency dielectric properties of several fresh fruits and vegetable were measured and analyzed to provide background data for further research on this problem, and results of the research are reported in this paper. The variation of the dielectric properties of samples of apple, avocado, banana, cantaloupe, carrot, cucumber, grape, orange, and potato with frequency in the range from 10 to 1,800 MHz and with temperature over the range from 5 to 65 degrees C was determined. The data provide interesting observations on the dielectric behavior of these fresh fruit and vegetable tissues, which determines how the tissues interact with electric fields that can be used to probe the quality of these products. Further studies are needed to learn whether these dielectric properties can be well related to quality factors, such as maturity, moisture content, and dry matter, which are properties of interest in working with many fruits and vegetables. Such future studies will determine whether the principles considered may be adapted to new instruments that can sense quality factors and provide advantages for producers handlers and consumers of these important agricultural products. Technical Abstract: Dielectric properties over the frequency range from 10 MHz to 1.8 GHz and over the temperature range from 5 to 65 degrees C were measured for tissue samples cut from nine fresh fruits and vegetables. Dielectric constant and loss factor data were obtained for apple, avocado, banana, cantaloupe, carrot, cucumber, grape, orange, and potato, showing dielectric constants ranging from values of several hundred at 10 MHz to less than 100 at 1.8 GHz and loss factors on the order of one thousand at 10 MHz to less than 20 at 1.8 GHz. The dielectric loss factor increased consistently with increasing temperature at frequencies below 1 GHz. The dielectric constant increased with temperature at lower frequencies, but it decreased with temperature at the higher frequencies. This reversal of the sign of the temperature coefficient occurred at some point in the frequency range between 20 and 120 MHz where the temperature dependence of the dielectric constant was zero. At frequencies below this point, ionic conduction dominates the dielectric behavior, but above that point dipolar relaxation appears to control the behavior. Multiple linear regression provided equations for calculation of the loss factor in the frequency range from 10 to 300 MHz at temperatures from 5 to 65 degrees C. The data provide new information useful in understanding dielectric heating behavior and evaluating dielectric properties of such agricultural products for quality sensing applications. |
