Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 25, 2002
Publication Date: January 1, 2003
Citation: NELSON, S.O. FREQUENCY- AND TEMPERATURE-DEPENDENT PERMITTIVITIES OF FRESH FRUITS AND VEGETABLES FROM 0.01-1.8 GHZ. TRANSACTIONS OF THE AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS. 2003. V. 46(2). P. 567-574. Interpretive Summary: The permittivities, or dielectric properties, of materials are electrical characteristics that determine how the materials interact with electromagnetic fields. For example, the dielectric properties of foods determine how rapidly they will heat in a microwave oven. These properties can also be sensed by properly designed instruments to indicate other characteristics of materials, such as moisture content. Therefore, knowledge about the dielectric properties of agricultural products may provide means for rapid nondestructive measurements of quality factors or other properties that may be important. In the study reported, the dielectric properties of nine different fresh fruits and vegetables were measured over the frequency range from 10 MHz to 1.8 GHz. The new data show how the dielectric constant and loss factor (two dielectric properties of interest) vary as the temperature of these fruits and vegetables changes from 5 degrees C to 95 degrees C (41 to 203 degrees F) at all the frequencies in the range from 10 to 1800 MHz. The new data are of use to those interested in dielectric and microwave heating of food materials. The data for tissues of fresh apple, avocado, banana, cantaloupe, carrot, cucumber, grape, orange, and potato provide basic background information that will be of value to scientists interested in the dielectric behavior of these products and utilizing such data for nondestructive testing of fruits and vegetables.
Technical Abstract: The relative complex permittivity, consisting of the dielectric constant and loss factor, were measured for samples cut from nine fresh fruits and vegetables over the frequency range from 10 MHz to 1.8 GHz at temperatures ranging from 5 to 95 degrees C. Permittivity 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 of several hundred to a few thousand at 10 MHz to much less than 100 at 1.8 GHz. The dielectric loss factor generally increased with increasing temperature. The dielectric constant increased with temperature at lower frequencies, but it decreased with temperature at the higher frequencies. There was a point in the frequency range between 10 MHz and 100 MHz where temperature dependence of the dielectric constant was minimal. At frequencies below this point, ionic conduction dominated the dielectric behavior, but above that point dipolar relaxation appeared to control the behavior. Moisture content, density, and total soluble solids content of the fruit and vegetable samples were measured, but poor correlation was observed between these characteristics and the dielectric properties across the selection of fruits and vegetables. The data provide new information useful in understanding dielectric heating behavior and evaluating dielectric properties of such agricultural products for quality sensing applications.