Electrical impedance spectroscopy analysis of eggplant pulp and effects of drying and freezing–thawing treatments on its impedance characteristics

Abstract

The electrical impedance characteristics of eggplant pulp were investigated; the effects of drying and freezing–thawing treatments on the pulp samples were discussed by means of electrical impedance spectroscopy analysis. Fresh eggplant pulp was subjected to vacuum drying, hot-air drying and freezing–thawing treatments respectively, the electrical impedance of the non-treated, partially dried and frozen–thawed pulp tissue was measured using a LCR tester over a frequency range of 42 Hz to 5 MHz; the impedance spectra were analyzed with two lumped element models and a distributed element model based on the Cole–Cole impedance equation; the circuit model parameters representing physiological properties of the tissue were calculated using non-linear regression analysis. Results showed that the impedance loci of the fresh as well as partially dried samples could be described by the models, and the distributed model fitted the experimental data significantly better than the two lumped models did. After drying, the impedance of the samples increased dramatically across the test frequency range due to the loss of moisture; the increase in the impedance of hot-air dried sample was statistically higher than that of vacuum dried sample under the present experimental conditions. After freezing–thawing treatment, the impedance spectrum of the sample lost its original character and became almost independent of the test frequencies, which verified that the membranes of the cells were severely damaged during the freezing process.