The inactivation of Escherichia coli O157:H7 during pulsed electric field (PEF) treatment in a static chamber

The inactivation of Escherichia coli O157:H7 by pulsed electric field (PEF) processing as a function of electric field strength (15-30 kV cm^-1), pulse number (1-20), temperature (5-65°C) and pH (3.5 and 6.8) was studied using a commercially available pulser, a static chamber and gellan gum gel as a suspension medium. The custom-designed static chamber achieved near-isothermal treatment conditions while eliminating flow field effects. Gellan gum gel was used to suspend the bacteria for treatment. It allowed uniform distribution of bacteria and neither inhibited nor promoted bacterial growth. The combination of equipment design and experimental protocol allowed the contribution of electrical (PEF) and thermal energy to be measured separately. In water-based gel, a maximum of 3 log reductions were achieved by PEF energy. Greater inactivation was observed at a treatment temperature of 55°C, but the additional inactivation was attributable entirely to thermal energy. Microbial injury was also observed at this temperature. At 60°C and above, complete inactivation was achieved, but this was attributable entirely to thermal energy. In water-based gellan gum gel adjusted to pH 3.5, again a 3 log reduction was achieved by PEF. In gel made from freshly squeezed apple juice naturally having the same pH, however, a maximum of 1.5 log reduction was observed.