Submitted to: Journal of Microwave Power and Electromagnetic Energy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2005
Publication Date: 3/27/2006
Citation: Nelson, S.O. 2006. Dielectric spectroscopy of fresh fruit and vegetable tissues from 10 to 1800 mhz. Journal of Microwave Power and Electromagnetic Energy 2005. 40(1):31-47 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 intrinsic 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 spectroscopy over the frequency range from 10 MHz to 1.8 GHz, was used to study the dielectric properties of tissue samples cut from nine fresh fruits and vegetables over the temperature range from 5 to 65 degrees C. Dielectric constant and loss-factor data are presented graphically 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.