RF Analysis of Fruit and Vegetables using Equivalent Circuits Deduced from S-parameters

Abstract

We deduced equivalent circuits of the electrical properties of five fruit and eight vegetable samples from the S-parameters. The S-parameters were measured at a frequency range from 1 MHz to 100 MHz using a portable VNA and a tiny three-pin SMA probe. Admittance values calculated from the S-parameters were plotted on the admittance Smith chart, and the equivalent circuits with four parallel sub-circuits were synthesized by curve fitting. The admittance curves for the fruit and vegetable samples have two parts with different lengths split at a frequency of about 30 MHz. The admittance in the low-frequency range was determined mainly by two large capacitive and two small resistive elements of the equivalent circuit related to the plant cell wall and membrane. When the plant cell wall and membrane were decomposed by grating and 30µm-filtering, the specific part of the admittance curve was eliminated, and the admittance value increased. The measured and calculated admittance curves were in good agreement. About 30 samples of plant cultivars were classified as fruit and vegetables according to four criteria deduced from the RF analysis. S-parameter spectroscopy (SPS) is an effective analytical method for evaluating the electrical properties of plant tissues.