Sterilization of bacterial spore using intense nanosecond pulsed electric fields with a pressurized flow system

There have been many attempts to kill spores in liquid by pulsed electric field (PEF) for last half century. However, only a few successful experiment has been reported. In conventional treatment systems, electrical power into the exposure cell, which usually consists of two parallel plane electrodes, is limited by the arcing across the electrodes because of the local vaporization. This paper proposes nanosecond PEF sterilization of Bacillus subtilis spores in liquid by using a pressurized flow system, which enables to increase the power level to be sufficiently large to kill spores. The exposure cell is a conventional parallel electrode with inlet and outlet for the bacterial suspension. The electrode separation and the cell volume are 4 mm and 0.92 ml, respectively. A no-fluctuation compressor and a flow control valve were used to keep the treatment pressure constant. The temperature of the suspension was monitored at the outlet of the cell by using an electrically isolated device based on temperature sensitive fluorescent dye. The suspension was cooled down to room temperature after passing through a metal tube between the cell and the cooler. Passing through the tube took 15 seconds and caused the temperature drop of 10degC. The pulsed power generator based on a magnetic compression scheme repetitively delivers 100 ns long, 60 kV voltage pulses to the treatment cell. As a result of the PEF application to Bacillus subtilis spores at the pressure of 0.6 MPa, the number density of spores were successfully reduced by 10^-5 at the temperature of 110degC, which is 10degC lower than that in the heating sterilization process. The lower treatment temperature is supposed to prevent nutrition from being destroyed in the food sterilization process. From the PEF treatment for different conductive suspensions, the bacteria reduction ratio tends to depend on the treatment temperature at the cell but not on the electric field strength or the current density. The cumul- - ative energy density for 5 log reduction was 5 J/ml for the optimum condition.