Effects of Pulsed Electric Fields on Plant Tissue
Abstract: The interaction between biological cells and external electric fields has been of interest by scientists for several decades. Exposure of the cells to electric fields leads to various kinds of biophysical and biochemical responses. These responses can be used to characterize cell properties and/or to manipulate cell characteristics. Electro-poration or electro-permeabilzation is probably the most important and breakthrough phenomenon that employs the virtue of the external pulsed electric fields to transiently or permanently induce cell membrane permeability in a very short period of time. The induction results in an increase in cell membrane permeability providing an extra passage for transport of non-permeating molecules across the cell membrane. The electropermeabilization can be reversible allowing cell manipulation and cell survival, on the other hand, completely cell membrane damage via irreversible electro-permeabilization, resulting in cell death, allows an emerge of number of applications in food processes. This thesis studies the effects of pulsed electric fields on plant tissue in correlation with mass transport of intracellular constituents, tissue textural changes, local permeabilization affected by cell anisotropy, reversible electroporation and electric field distribution affecting tissue permeabilization. Mass transport of intracellular components from electropermeabilized plant tissue was governed by two modes of diffusion, according to different sizes of permeabilized and nonpermeabilized tissue domain. The developed mass transfer model showed an excellent fit with the experimental data. However, with highest intensity of applied fields under the investigation, only one mode of diffusion was sufficient to explain the mass transport phenomenon. The solid-liquid expression and textural properties affected by pulsed electric fields were also investigated. The findings suggested that the yield of the expression increased by treating tissue with pulsed electric fields for all range of deformation and deformation rate investigated. Force required to compress treated tissue was reduced to one fifth at relative deformation of 0.5 when slowest compression speed of 0.1 mm/min was used. Electropermeabilization was also affected by cell size, shape and orientation. In particular, apple tissue structure was markedly anisotropic and thus selected for the investigation. Microscopic analysis and electrical measurement in apple flesh in different zones provided that the degree of permebilization induced by pulsed electric fields was distinguished significantly from the apple skin radiating towards the core. Elongated cells from the inner zone of the apple were permeabilized at lower electric field intensity if they were oriented parallel to the fields, compared to when they were perpendicular to the fields. Permeabilization on spherical cells that were in the outer zone was not affected by the orientation of the electric fields. This confirms that electropermeabilization in tissue was inhomogeneous. Based on electrical measurement, the results suggested that relatively low field intensity of 150-200 V/cm could permeabilize cell membrane in apple to about 70-80 % with only 1 % in cell death. Under reversible study in potato tissue it was revealed that measuring the electric current during electroporation pulsing could be possible to monitor the evolution of pores in real time. It was found that, based on the conductivity change during a monopolar pulse period, pores were initially formed at the applied field strength of about 200 V/cm. Pore resealing period completed in 1 second but most contribution to the resealing was in the first millisecond. Successive bipolar pulses could cause high degree of permeabilzation and it showed some degree of recovery in the time scale of tens of minutes. The most satisfied condition, in terms of degree of permeabilzation and reversibility, under the investigation was applying 5 ms of bipolar pulse with 50 kHz in succession. One of the most crucial factors controlling electropermeabilization in biological tissue was the electric field distribution. Moreover, in general, electrode configuration and electrical conductivity of the tissue domain defined this distribution. Modeling of electric field distribution in needle geometry in steady state with taking the change in electrical conductivity of tissue as a function of electric fields into consideration was possible by means of finite element method. The result of modeling showed specific tissue with permeabilized and non-permeabilized zones. The tissue exposed to sub-threshold fields was not permeabilized whereas it was if the fields were greater than the threshold. This resulted in the distribution of electrical conductivity over the tissue domain, reflecting the efficiency of permebilization when needle electrodes were used.
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