Pulsed Electric Field Treatment of Plant Tissue

Abstract: A pulsed electric field (PEF) is a newly emerged non-thermal food processing technology which has characteristic effects on the cell membrane. PEF processing involves subjecting food to short, repeated pulses of a high electric field between an anode and a cathode. The main result of this process is the disruption of the cell membrane leading to the formation of either temporary or permanent pores. The area of interest in the treatment of plant tissue with PEF is facilitation of certain processes such as extraction, drying and osmotic dehydration, non-thermally and economically. For PEF to be employed in commercial plant tissue processing, a better understanding of the changes brought about by PEF in the structure of plant tissue at the cellar level is required. After the formation of pores in the cell membranes, a number of changes are initiated inside the tissue. Most important of these is the loss of turgor, and subsequent diffusion of intracellular content out of the cell. In the study undertaken here, a microscale method was developed making the study of PEF- induced permeabilization possible at cell level. We demonstrated that permeabilization of plant tissue starts to occur with a single pulse of 100 ?s at 0.35 kV/cm field strength. The timescale for the diffusion of intracellular content into the extracellular region is in the range of 2 min. When changes in the mechanical properties of plant tissue (potato) exposed to a PEF, with or without osmotic treatment, were studied using stress relaxation, the short-term (time) modulus was found to be little affected. The residual elasticity was the parameter most affected even at lower levels of PEF treatment. Maximal PEF treatment had a similar effect to exposure to a 0.7 M hyperosmotic solution implying that the effect of PEF on relaxation behavior can be explained by loss of turgor. Neither hypoosmotic nor hyperosmotic pretreatment appeared to enhance the effect of PEF treatment. When the effect of a PEF on extraction of red pigment from red beetroot was investigated, it was found that PEF treatment level at 1 kV/cm field strength led to the extraction of about 90 % of the total red beetroot pigment following 1 h aqueous extraction. The energy consumption was 7 kJ/kg. The increase in tissue conductivity after PEF treatment correlated linearly with the extraction yield of up to an extraction level of 60-70 %. Diffusion of red pigment and ionic species in the extraction process was also studied in terms of a bimodal Fickian diffusion model. The process appeared to be governed by a slow- and a fast apparent diffusion coefficient which were independent of the treatment level. However, the fraction of the yield associated with the faster diffusion coefficient increased with treatment intensity, and at maximum PEF treatment and in frozen/thawed tissue it accounted fully for the transport. The faster apparent diffusion coefficient was 2-5 times lower than the corresponding diffusion coefficient in dilute solution.