Inactivation of Bacillus subtilis by high intensity pulsed electric fields under close to isothermal conditions

Because of expected nutritional, sensorical and health benefits, non-thermal processing has the potential to satisfy the growing demand for minimally processed foods and can simultaneously provide safe and stable foods. Since the 60's the use of high intensity pulsed electric fields in food processing has repeatedly been demonstrated for e.g. pasteurization or cell permeabilization. The technical advance in the field of pulsed power equipment during the last years legitimates the approach of re-considering some of these processes. In this work the inactivation of vegetative Bacillus subtilis ATCC 9372 by high intensity pulsed electric fields was investigated in a batch treatment chamber. A switching system was used which allowed the application of square wave pulses with variable on-time and a turn-on rise time of less than 50 ns. Field strength from 2.5 to 52.8 kV/cm was investigated. The aim was to clearly discriminate between the effect of field strength and pulse energy without thermal interference. It was found that inactivation is a linear function of field strength. A threshold field strength which has to be exceeded to achieve cell death was not observed. However, a minimum specific pulse energy was required to affect the micro-organisms.