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. 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 by 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