The effects of high field DC pulse and liquid medium conductivity on survivability of Lactobacillus brevis

Abstract

Survivability of Lactobacillus brevis cells in suspensions of phosphate buffer solutions of different conductivities (170 μS/cm to 2230 μS/cm) using electric pulse application has been investigated under varied test conditions. Survivability decreased rapidly with the application of the first few pulses (approx. 25 to 50 depending on the test conditions). However, the destruction performance decreased with increased number of pulse applications. Hence to obtain a maximum reduction in survivability, the electrical conditions should be so selected that effective killing is achieved with the fewest number of pulses applied. The maximum reduction in survivability (N/N0, approx. 10−7) was obtained in liquid possessing the lowest conductivity (170 μS/cm) with an application of 150 pulses of 160-μs pulse width. Despite the increase in liquid medium temperature during pulse treatment, the killing was significantly due to pulse as the maximum temperature rise (22° C) during treatment was insufficient to cause any synergistic effect of temperature and pulse treatment. In this work we have shown for the first time that if the pulse width is kept constant, the higher reduction in survivabilities observed in liquids with lower conductivities was primarily due to conductivity influencing the membrane permeability. The small change in test liquid pH (<0.5) indicated that the killing of cells was affected primarily by high field pulses rather than by-products of electrolysis in the medium of different conductivities.