THE EFFECTS OF HIGH-FIELD DC PULSE AND LIQUID-MEDIUM CONDUCTIVITY ON SURVIVABILITY OF LACTOBACILLUS-BREVIS

Survivability of Lactobacillus brevis cells in suspensions of phosphat e buffer solutions of different conductivities (170 muS/cm to 2230 muS /cm) using electric pulse application has been investigated under vari ed test conditions. Survivability decreased rapidly with the applicati on of the first few pulses (approx. 25 to 50 depending on the test con ditions). However, the destruction performance decreased with increase d number of pulse applications. Hence to obtain a maximum reduction in survivability, the electrical conditions should be so selected that e ffective killing is achieved with the fewest number of pulses applied. The maximum reduction in survivability (N/N0, approx. 10(-7)) was obt ained in liquid possessing the lowest conductivity (170 muS/cm) with a n application of 150 pulses of 160-mus pulse width. Despite the increa se in liquid medium temperature during pulse treatment, the killing wa s significantly due to pulse as the maximum temperature rise (22-degre es-C) during treatment was insufficient to cause any synergistic effec t of temperature and pulse treatment. In this work we have shown for t he first time that if the pulse width is kept constant, the higher red uction in survivabilities observed in liquids with lower conductivitie s was primarily due to conductivity influencing the membrane permeabil ity. The small change in test liquid pH (<0.5) indicated that the kill ing of cells was affected primarily by high field pulses rather than b y-products of electrolysis in the medium of different conductivities.