The Influence of Pulse Duration on the Inactivation of Bacteria Using Monopolar and Bipolar Profile Pulsed Electric Fields

In recent years, a number of new applications have emerged where pulsed power is being used in the treatment of waste and effluent, foodstuffs, and beverages. One of these emerging applications is pulsed electric field (PEF) inactivation of microorganisms in liquid media. There are several ways in which PEF can be applied, including oscillatory, double exponential, and square wave pulses. Of these, the square-wave pulse is considered to be the most energy-efficient form of PEF delivery. It has been reported that the bipolar square-wave pulse, involving polarity reversal half way through the pulse, provides superior inactivation when compared to the monopolar pulse. However, results from the authors have shown that this is not always the case and that monopolar PEF is at least as effective for bacterial inactivation under the conditions investigated (Beveridge, et. al., 2004). Further results are presented here on the effect of changing the pulse duration of the monopolar and bipolar pulse, using total pulse durations of 1, 2, 3, and 4 s. These results, obtained from an improved system, demonstrate how the relative effectiveness of inactivation is a function of the pulse duration. When inactivation is plotted against energy delivered, they show the superiority of monopolar over bipolar pulses for 1- and 2- s pulse profiles. For a pulse duration of around 3 s, there is no significant difference, and with still longer pulses, bipolar pulses appear to be superior to monopolar. It is postulated that electric-field-induced orientation may, in part, be responsible for this effect. These comparative experiments are conducted with complete control of the fluid temperature, which is maintained below 30 C.