Estimation of the degree of electroporation at low electric field strengths: apple tissue as a case study

Recently, the interest on the application of Pulsed Electric Field (PEF) technology on solid foods is growing considerably mainly due to its potential on increasing extraction yields. The PEF technology has been proposed as an effective method to induce electroporation of cellular tissues and so, to increase the degree of disintegration of cellular systems. The complexity of the optimization procedure is mainly characterized by the difficulty in obtaining a correlation between the processing protocol and the damage degree measurements of biological systems. Different methods of cell disintegration measurement have been proposed in order to understand the influence of the process parameters on the efficiency of the treatment. The most commonly applied method to determine the degree of cell permeabilization is based on changes in electrophysical properties, i.e. the electrical conductivity of samples, σ, that gives information on the cell damage degree of a sample when compared to both an untreated and completely destroyed sample (Angersbach et al., 1999 ; Lebovka et al., 2002). Another method presented is based on the measurements of water distribution inside and outside cells by time domain nuclear magnetic resonance (TD-NMR) (Dellarosa et al., 2016). Alternatively, changes of texture can be quantitative determined as a measurement of mechanical destruction of cells (Lebovka, Praporscic, & Vorobiev, 2004). The aim of this study is to compare different proposed methods for the evaluation of the degree of cell disintegration (z) in apple tissue applying low electric field strengths, which could induce both reversible and irreversible electroporation. Electrical conductivity according to Angersbach et al., 1999 and Lebovka et al., 2002, proton transverse relaxation time (T2) according to Dellarosa et al., 2016 and the quantification of the effect on tissue structure based on texture analysis evaluating differences of deformation gradient (N mm-1) of stress-strain curves, were compared. Different PEF treatments were applied to apple cylinders (1.5 cm diameter; 2.0 cm length) using an in-house developed pulse generator equipment providing monopolar pulses of near-rectangular shape using following parameters: E = 100, 250, 400 V cm-1; pulse width = 100 ± 2 μS, frequency = 100 Hz and 60 pulse series. Four different methods to evaluate the cell disintegration degree were compared. Results showed the same trends for all methods used. However, the results of z index obtained by the texture analysis and those obtained by NMR relaxometry seemed to better describe the effect of reversible and irreversible electroporation occurred at these low intensity PEF treatments compared to methods based on changes in electrophysical properties. Therefore, proposed methods could be used to improve the assessment of PEF treatment protocols in order to increase the effectiveness of novel possible applications.