Evaluation of Freshness of Fresh-cut Lettuce based on Electrical Impedance Analysis

Abstract

The impedance of the fresh-cut lettuce over a frequency range of 10 Hz-1 MHz were analyzed in order to obtain the fundamental knowledge for the application of electrical impedance spectroscopy (EIS) to the evaluation of freshness of vegetables. The measured impedance characteristics were analyzed with two equivalent circuit models: the Hayden model that represents a cell structure and the model modified with constant phase element (CPE). As a result of comparison of goodness of the equivalent circuit model fitting, the modified model showed significantly higher accuracy of fitting than the Hayden model. The mass ratio and CO₂ and O₂ concentrations in the film package containing the samples greatly changed during storage at the temperature of 20°C, indicating the loss of freshness. Also, the extracellular resistance R_e and the intracellular resistance R_i that are the parameters in the modified model decreased with decreasing freshness during storage at 20°C, while R_e and R_i did not change during the storage at 5°C and 10°C. The decreases of R_e and R_i are attributed to the decline in osmoregulatory function of cell membranes and the increase in concentration of electrolyte of cell fluid due to the physiological changes during storage. Therefore, the present results suggested that the EIS has the potential use in evaluation of freshness of vegetables.