HIGH-VOLTAGE PULSE APPLICATION FOR THE DESTRUCTION OF THE GRAM-NEGATIVE BACTERIUM YERSINIA-ENTEROCOLITICA

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 t he irreversible electroporation of the Gram-negative bacterium Yersini a enterocolitica. The bacteria were suspended in a NaCl solution of pH = 7.2 and conductivity gamma = 0.8-1.3 S m(-1). The suspension was pl aced in a glass tube immersed in the gap of a cylindrical electrode sy stem to which high voltage pulses, generated by a Marx bank, were appl ied. Such an electrode system will protect the bacteria suspension fro m the chemical processes at the electrode-liquid interface due to preb reakdown phenomena. A current chopping electrode system was connected in parallel with the sample in order to avoid heat generation from dir ect discharge of the pulse through the suspension. The maximum tempera ture rise was only 2-3 degrees C with the application of 70 pulses of 45 kV magnitude. The dependence of the survival ratio s = N/N-0 (the n umber of bacteria per cm(3) after pulse treatment, N, divided by the n umber of bacteria per cm(3) before treatment, N-0) of Y. enterocolitic a on the peak voltage of the pulse, the number of pulses applied and t he rise times of the pulses have been measured. A reduction by similar to 7 orders of magnitude of Y. enterocolitica viable cells per cm(3) was achieved. The lethal effect on the bacteria strongly depends on th e peak voltage and the number of pulses applied; however, the effect o f the rise time on the killing of Y. enterocolitica tested was not sig nificant within the range of rise times used (500-1300 ns). The result s show that a considerable inactivation of the microbes can be achieve d by the application of short (t(p) < 1000 ns) high voltage pulses for the bacteria suspension without directly exposing the suspension to t he 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. (C) 1997 Elsevier Science S.A.