Environmental factors influencing the inactivation of Listeria monocytogenes by pulsed electric fields

Aims: To investigate the influence of the growth phase, growth temperature, storage time, pH and a(w) of the treatment medium on the resistance of Listeria monocytogenes to pulsed electric fields (PEF). Methods and Results: Square wave pulses of 2 mus at a frequency of 1 Hz and 25 and 28 kV cm(-1) were used. Cells were more PEF resistant in the stationary than in the exponential phase at both incubation temperatures investigated (4 and 35degreesC). Cells grown at 4degreesC were more PEF sensitive than cells grown at 35degreesC independent of the growth phase. After a treatment of 25 kV cm(-1) and 800 mus, 1.48, 3.86 and 5.09 log(10) cycles of inactivation were obtained at pH 7.0, 5.4 and 3.8, respectively. A reduction in the a(w) of the treatment medium protected cells against PEF treatments. Conclusions: The PEF resistance of L. monocytogenes depended on different environmental factors. The influence of growth conditions and treatment medium characteristics should be known and controlled to obtain reproducible and reliable PEF inactivation data. Significance and Impact of the Study: Erroneous conclusions and misinterpretation of results are possible if factors affecting the PEF resistance of L. monocytogenes are not considered during PEF inactivation studies.