Effect of pulsed electric fields on water distribution in apple tissue as monitored by NMR relaxometry

Pulsed electric fields (PEF) technology is a promising innovative non-thermal process to improve mass transfer in food sector. PEF treatments induce a partial cell membranes electroporation which extent depends on electric field strength, number, duration and shape of the pulses and application time. The present work aimed at highlighting the effect of the application of PEF on mass transfer phenomena in apple parenchyma tissue, by evaluating the water distribution across cell compartments by means of NMR relaxometry. Pulsed electric fields treatments were carried out using nearrectangular shaped pulses with fixed 100 μs pulse width and 10 ms repetition time at three different specific voltage (100, 250 and 400 V cm-1) and two different pulse number series (n=20 and n=60). Results showed different trends according to the applied voltage. The lowest (100 V cm-1) was not able to induce significant changes in plasma membranes, so that no water redistribution was achieved between cytoplasm and extracellular space. At the opposite, a marked redistribution was registered inside the cellular compartments, namely vacuole and cytoplasm, showing an alteration of the tonoplast. The total number of pulses was found to influence the amount of water migrating, from vacuole to cytoplasm, from 15% with 20 pulses to 40% with 60 pulses. Medium and high voltage (250 and 400 V cm-1, respectively) removed the possibility to distinguish the different cell compartments, probably due to intense damage of both plasma membrane and tonoplast. By observing water transverse relaxation time an additive effect of both voltage and total number of pulses was demonstrated. Interestingly, by considering water distribution during 120 minutes after PEF treatment time-dependent trends were found in the effects of each experimented protocol.