Inactivation of Yersinia enterocolitica Gram-negative bacteria using high-voltage pulse technique

High voltage pulses of peak voltages U=5-75 kV and rise times t/sub p/=500-1300 ns were applied with repetition frequency f=1 Hz in order to cause the irreversible electroporation of Gram negative bacteria Yersinia enterocolitica (Y. enterocolitica). The bacteria were suspended in NaCl solution of pH=7.2 and conductivity /spl gamma/=0.8-1.3 S/m. The suspension was placed in glass tube immersed in the cylindrical electrode system gap filled with distilled water. Such an electrode system will protect the bacteria suspension from the chemical processes at the electrode-liquid interface due to conduction and prebreakdown phenomena. The current chopping electrode system was connected in parallel to the sample in order to avoid heat generation from direct discharge of the pulse through the suspension. The dependence of the survival ratio s=N/N/sub 0/ (the number of bacteria per cm/sup 3/ after pulse treatment, N, divided by the number of bacteria per cm/sup 3/ before treatment, N/sub 0/) of Y. enterocolitica on peak voltage of the pulse, number of pulses applied and on various rise times of pulses have been measured. The reduction by 6 orders of magnitude of Y. enterocolitica living cells per cm/sup 3/ was achieved. The results show that considerable inactivation of microbes can be achieved by the application of short (t/sub p/<1000 ns) high voltage pulses for bacteria suspension without directly exposing the bacteria suspension to the electrodes. It is therefore possible to use the electrode system proposed as a means for sterilization of liquid foods.