High voltage pulse application for the destruction of the Gram-negative bacterium Yersinia enterocolitica

Abstract

High voltage pulses of peak voltages U = 10–75 kV and rise times t p = 500 −1300 ns were applied with frequency f = 1 Hz in order to cause the irreversible electroporation of the Gram-negative bacterium Yersinia enterocolitica. The bacteria were suspended in a NaCl solution of pH = 7.2 and conductivity γ=0.8–1.3 S m −1. The suspension was placed in a glass tube immersed in the gap of a cylindrical electrode system to which high voltage pulses, generated by a Marx bank, were applied. Such an electrode system will protect the bacteria suspension from the chemical processes at the electrode-liquid interface due to prebreakdown phenomena. A current chopping electrode system was connected in parallel with the sample in order to avoid heat generation from direct discharge of the pulse through the suspension. The maximum temperature rise was only 2–3°C with the application of 70 pulses of 45 kV magnitude. The dependence of the survival ratio s = N N 0 (the number of bacteria per cm 3 after pulse treatment, N, divided by the number of bacteria per cm 3 before treatment, N 0) of Y. enterocolitica on the peak voltage of the pulse, the number of pulses applied and the rise times of the pulses have been measured. A reduction by ∼7 orders of magnitude of Y. enterocolitica viable cells per cm 3 was achieved. The lethal effect on the bacteria strongly depends on the peak voltage and the number of pulses applied; however, the effect of the rise time on the killing of Y. enterocolitica tested was not significant witt in the range of rise times used (500–1300 ns). The results show that a considerable inactivation of the microbes can be achieved by the application of short ( t p <1000 ns) high voltage pulses for the bacteria suspension without directly exposing the suspension to the electrodes. It is therefore possible to use this non-thermal method as a means for sterilization of liquid foods where the effects due to electrolysis must be minimized.