Comparing the PEF resistance and occurrence of sublethal injury on different strains of Escherichia coli, Salmonella Typhimurium, Listeria monocytogenes and Staphylococcus aureus in media of pH 4 and 7

The effect of pulsed electric fields (PEF) on the reduction of the population and on the occurrence of sublethal injury in five strains of two Gram-positive (Listeria monocytogenes and Staphylococcus aureus) and two Gram-negative (Escherichia coli and Salmonella Typhimurium) microorganisms was investigated in media of pH 4.0 and 7.0. Samples were subjected to 50 exponential waveform pulses of 15, 20, 25 and 30 kV/cm at a repetition rate of 1 Hz.PEF resistance and occurrence of sublethal injury depended on the electric field strength and pH of the treatment medium and varied widely among the 20 strains investigated. In general, the increment of the electric field strength from 20 to 30 kV/cm had a significant effect on the viability loss of all the strains. However this increment hardly affected the viability of E. coli W3110 and O157:H7 in the medium of pH 4.0 and of L. monocytogenes 4031, 5672 and 4032 in the medium of pH 7.0. At 30 kV/cm a population reduction higher than 4-Log10 cycles was observed in two strains of S. Typhimurium (878, 4594) and in three strains of S. aureus (976, 4465, 4466) in the medium of pH 4.0. In the medium of pH 7.0, this level of reduction was observed in two strains of E. coli (471, BJ4L1), in three strains of S. Typhimurium (443, 880, 722) and in three strains of S. aureus (976, 4465, 4466).In general, important sublethal injury was not observed for the strains of the two-Gram positive microorganisms in the media of pH 4.0 and 7.0 and for the strains of S. Typhimurium treated in the media of pH 7.0. For E. coli sublethal injury was detected for all the strains investigated, except the strain BJ4L1 treated at 30 kV/cm in the medium of pH 7.0.Industrial relevance. The identification of the most PEF resistant microorganisms of public health concern is necessary to establish the treatment conditions for PEF pasteurization. Data obtained in this investigation demonstrates that the PEF resistance and the occurrence of sublethal injury may vary greatly among different strains of a bacteria and both depend on the pH of the treatment medium. Therefore the most resistant microorganisms of public health significance can be expected to vary for different foods depending of their pH.