Microwave Permittivities of Fresh Fruits and Vegetables from 0.2 to 20 GHz

Procedures are described for microwave permittivity measurements taken on 23 kinds of common fresh fruits and vegetables with an open-ended coaxial-line probe used in conjunction with a microwave network analyzer. Plots of the dielectric constant and loss factor at 41 frequencies between 200 MHz and 20 GHz are illustrated for apple, lemon, carrot, cucumber, and avocado, and condensed data for all, along with moisture content, tissue density, and total soluble solids, are tabulated at six frequencies covering the same range. Although specific values differ, the dielectric constant decreases steadily with increasing frequency, dropping more rapidly at frequencies above 5 GHz. Values for the loss factor decrease as frequency increases above 200 MHz to a broad minimum in the 1- to 3-GHz region and then increase again as the frequency approaches 20 GHz. The dielectric behavior of the fruit and vegetable tissues appears to be influenced by ionic conductivity and bound water relaxations at the lower frequencies and by free water relaxation at the higher end of the frequency range.