Determination of membrane integrity in onion tissues treated by pulsed electric fields: Use of microscopic images and ion leakage measurements

The influence of electrical field strength and number of pulses on cell rupture of onion tissues was investigated by two different methods to understand the changes in cell viability of plant tissues after pulsed electric field (PEF) treatment. The impact of pulsed electric field parameters on cell integrity of 20 mm diameter, 4 mm thick disks of Sabroso onions (Allium cepa L) was determined by ion leakage measurements and microscopic method. The effect of treatments on cellular integrity was visualized by neutral red staining of the onion cells. Cell rupture is essential for optimal process design before extraction of desirable compounds and/or drying of plant tissues. Experimental results were obtained for onion disks treated with electrical pulses at field strengths of E approximate to 167 V/cm and 333 V/cm, pulse width of ti = 100 mu s, frequency of f=1 Hz and the number of pulses, n = 1 to 100. At 167 V/cm electric field strength treatment cell rupture was not observed however ion leakage increased and air spaces around cell walls disappeared, most likely due to changes in cell membrane permeability. Irreversible cell rupture occurred at 333 V/cm. Ion leakage values and ruptured cells count were increased with increasing pulse number. 92.2 +/- 5.9 % of the onion cells were ruptured after 333 V/cm and 100 pulse treatment. Small plant cells that are located near vascular bundles and upper epidermis showed higher resistance to pulsed electric field treatments. Industrial Relevance: The aim of this study was to determine the response of plant tissues with different cell type to the pulse electric field treatment: Two different methods, neutral red staining and ion leakage, were used to visualize and determine the cell rupture on onion tissues. Industry may choose one of these methods to evaluate treatment efficiency on the basis of cell rupture; especially ion leakage measurements which require lower investment cost and easy to apply or microscopic method for visualize the cells layer by layer. The paper stresses the importance of cell type, size and distribution of cells were found to the ability to resist cell rupture during pulsed electric field treatment. It is critical to explain overall changes caused by PEF on the structure of plant tissue at a cellular level in order to optimize the quality of PEF processed foods. (C) 2010 Elsevier Ltd. All rights reserved.