Pulsed electric fields inactivation of attached and free-living Escherichia coli and Listeria innocua under several conditions

The use of pulsed electric fields (PEF) is considered as a mild process in the inactivation of microorganisms present in liquid food products. PEF treatments of Escherichia coli and Listeria innocua suspended in milk and phosphate buffer, with same pH and same conductivities, yielded to similar inactivation. Reduction rates obtained in distilled water indicated that conductivity of the food product is a main parameter in bacterial inactivation. Bacteria attached to polystyrene beads were inactivated by PEF at a greater (E. coli) or equal rate (L. innocua) than free-living bacteria. Base on the use of selective and non-selective enumeration media, no clear indications were obtained for sublethal damage of microorganisms surviving the PEF treatment. E. coli cells subjected to 60 pulses at 41 kV/cm were examined by transmission and scanning electron microscopy. Changes in the cytoplasm were observed and the cell surface appeared rough. The cells outer membranes were partially destroyed allowing leaking of cell cytoplasm.