Impact of Pulsed Electric Fields (PEF) on post-permeabilization processes in plant cells

I Abstract The exposure of biological cell material to Pulsed Electric Fields (PEF) leads to a spectrum of biophysical and biochemical responses. The most important effect, the electrical breakdownThe exposure of biological cell material to Pulsed Electric Fields (PEF) leads to a spectrum of biophysical and biochemical responses. The most important effect, the electrical breakdown of cellular membranes, realizes the temporary or permanent pore formation in cell membranes, which induces an increase in membrane permeability. The loss of semipermeability enables the transport of non-permeating molecules across the cell membrane. The disintegration of the cell membrane as well as the alteration of structural properties offers numerous options to apply this novel, non-thermal and short-time technique in foodand bioengineering. In this thesis the impact of PEF on plant single cells as well as on vegetable tissues was investigated. In order to understand underlying mechanisms at cellular level and to clarify the influence of cell wall on the degree of cell membrane disintegration, protoplasts from cultured tobacco cells (Nicotiana tabacum b.y.-2) and cells with cell wall were compared during and after reversible as well as irreversible PEF treatment. Results showed higher sensitivity of protoplasts to electric fields related to native cells. Protoplasts sizes were measured before and after different treatment intensities and protoplasts shrinkage was used as an indicator for cell rupture. It could be demonstrated that cell volume decrease is influenced by PEF intensity, initial cell size, cell orientation in the electric field and nucleus position. Focus was also put on the potential of PEF to gentle disintegrate plant tissue and thus to apply this technique in food industry. Hence, the enhancement of mass transfer after irreversible membrane permeabilization from potato and asparagus tissue was examined. Results showed the enhanced release of intracellular molecules from permeabilized tissue as well as improved uptake of low molecular substances into the sample. Sugar, one substrate for the Maillard reaction, was decreased in PEF treated potatoes due to membrane permeabilization and the subsequent release of cell vacuole sugar, while conductivity increased after electroporation and soaking in sodium chloride solution, indicating the improved diffusion of salt caused by PEF. Higher release of cell liquid during drying was noticed additionally. This effect increased with the treatment intensity. Furthermore, it was revealed that PEF application leads to a significant reduction of fat content after deep fat frying of potato stripes and thus provides a potential for the production of low-fat French fries. It was noticed additionally that PEF treatment decreases the content of the biopolymer lignin in white asparagus in order to improve macroscopic characteristics and gain softer texture of the spears. It can be presumed that PEF is a capable assistance to thermal treatments in the processing of potato snack products or in the preserving of asparagus for the achievement of structural modifications and the improvement of process conditions.