Formation of chemical species and their effects on microorganisms using a pulsed high-voltage discharge in water

The primary mechanism for sterilization of microorganisms by high-voltage pulses has been considered to be an electrical breakdown of the cell membrane. However, it is expected that many kinds of chemically active species would be generated by an electrical discharge in a needle-plate or rod-rod electrode system. Therefore it is necessary to identify the chemical species produced by the discharge and to investigate lethal effects of the active species on microorganisms. Using a nozzle-plate electrode configuration, the authors previously reported that magenta colored streamers propagated from the nozzle tip during a pulsed discharge in water with various conductivities. The authors also investigated the generation of ozone from oxygen bubbled through the discharge. In the present study, the formation of active species in water (without O-2 flow) and their effects on yeast cells were investigated using needle-plate electrodes. In the presence of the streamer discharge, H and OH radicals were detected hy means of emission spectroscopic analysis of the discharge light. Hydrogen peroxide (H2O2) was also detected by absorption spectrophotometry using a reaction of peroxidase and catalase. The effect of the electrical conductivity of the water on the formation of the active species was investigated. Maximum . OH and H2O2 concentrations were obtained at a water conductivity of about 10(-5) S/cm. The H2O2 formation mechanism was considered to be a recombination reaction of . OH. The lethal effects on beer yeast of . OH and H2O2 generated by the pulsed electrical discharge in water were also investigated. It was found that . OH had almost no effect in reducing the survivors. However, the H2O2 did kill the yeast cells: the logarithm of the survival ratio decreased linearly with increasing H2O2 concentration.