Disintegration Efficiency of Pulsed Electric Field Induced Effects on Onion (Allium cepa L.) Tissues as a Function of Pulse Protocol and Determination of Cell Integrity by 1H-NMR Relaxometry

The influence of electrical pulse protocol parameters on cell rupture of onion tissues was investigated in order to improve fundamental understanding and to enhance the processing of plant tissues with pulsed electric fields (PEFs). The impact of PEF parameters on cell integrity of 20 mm dia, 4-mm thick disks of Don Victor onions (Allium cepa L.) was determined by ion leakage measurements. Electric field strength, pulse width, total pulse duration, and frequency effects were determined in relation to their effects on cell damage as a function of pulse protocol. Electric field strengths up to 500 V/cm increased the damage efficiency but there was no significant difference in efficiency beyond this field strength. Larger pulse widths increased the degree of tissue disintegration at a constant pulse number. Higher PEF efficiency was achieved with shorter pulse widths and a larger number of pulses at a constant total treatment time. Lower frequencies caused a greater degree of disintegration at constant number of pulses. 1H-NMR experiments were performed to determine the proton relaxation components of the PEF-treated onion samples and to obtain cell damage information nondestructively. Paramagnetic ion uptake by the onion sample was used to identify different proton relaxation components. Five different proton relaxation components were observed and changes in the 2 components representing different proton environments showed high correlations with ion leakage results (R2 = 0.99), indicating that T-2 distributions can be used to obtain information about cell membrane integrity in PEF-treated samples. 1H-NMR proved to be an effective method for nondestructive quantification of cell membrane rupture in onions.