Breakage of Cellular Tissue by Pulsed Electric Field: Extraction of Polyphenols from Fresh Tea Leaves

Pulsed electric field (PEF) is commonly understood as a fast, non-thermal, and highly effective method for extraction of intracellular compounds. This treatment involves discharge of high voltage electric pulses of a few microseconds into the food product which is placed or passed between two electrodes (Angersbach et al., 2000). Results of experimental investigation of the electric breakage of a cellular material in pulsed electric fields and the influence of electric fields on polyphenols (PPs) extraction from fresh tea leaves are presented. Thin slices of the tissue are subjected to PEF treatment at different intensities and time. The disintegration of the cellular membrane is detected indirectly by a total PPs content measurement. When the cellular membrane is permeabilized, the overall amount of extracted PPs increases. Electric field strength is an important parameter that controls the efficiency of electroporation of the cellular tissue. Moreover, electroporation of the cellular tissue also depends on other parameters such as pulse duration or pulse width (PD), number of pulses (N) and pause between pulses (PBP). Dependencies of the electric field strength (E), the total treatment time (which considers the number of pulses (N) and the pulse width (PD) applied in the system) and the extraction yield of PPs at various modes of PEF treatment are studied. Protocol I (N=30, PD=0.05 s and PBP (EXA)=0.5 s) results in a maximum value for the extraction yield of 27% when the electric field strength is 0.9 kV/cm. However, when the interval between pulses is equal to 3 s an electric field of 1.1 kV/cm is needed to obtain the same extraction yield (protocol II: N=30, PD=0.05 s and PBP (EXB)=3 s). The extraction yield (EY) values of the samples subjected to both pulse protocols increase with increasing electric field strength. The effect of the total treatment time on the extraction yield is studied for two different electric field strengths, i.e. E=0.4 kV/cm and E=0.9 kV/cm. Experimental results show that longer pulses are more effective and their effect is particularly pronounced at a moderate electric field, E=0.4 kV/cm. When the total treatment time is 5 s (for both electric field strengths), experiment could not be performed due to problems with the PEF unit itself (low conductivity was detected in the PEF chamber). The PEF treatment accelerates kinetics of the extraction of polyphenolic compounds from fresh tea leaves.