Effects of combined exposure of micrococcus luteus to nisin and pulsed electric fields.

Death and injury following exposure of Micrococcus luteus to nisin and pulsed electric field (PEF) treatment were investigated in phosphate buffer (pH 6.8, sigma = 4.8 ms/cm at 20 degrees C). Four types of experiment were carried out, a single treatment with nisin (100 IU/ml at 20 degrees C for 2 h), a single PEF treatment, a PEF treatment followed by incubation with nisin (as before) and addition of nisin to the bacterial suspension prior to the PEF treatment. The application of nisin clearly enhanced the lethal effect of PEF treatment. The bactericidal effect of nisin reduced viable counts by 1.4 log10 units. Treatment with PEF (50 pulses at 33 kV/cm) resulted in a reduction of 2.4 log10 units. PEF treatment followed by nisin caused a reduction of 5.2 log10 units in comparison with a 4.9 log10 units reduction obtained with nisin followed by PEF. Injury of surviving cells was investigated using media with different concentrations of salt. Sublethally damaged cells of M. luteus could not be detected by this means, following PEF treatment.

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.

Effects of pulsed electric fields on inactivation kinetics of Listeria innocua.

The effects of pulsed electric field (PEF) treatment and processing factors on the inactivation kinetics of Listeria innocua NCTC 11289 were investigated by using a pilot plant PEF unit with a flow rate of 200 liters/h. The electric field strength, pulse length, number of pulses, and inlet temperature were the most significant process factors influencing the inactivation kinetics. Product factors (pH and conductivity) also influenced the inactivation kinetics. In phosphate buffer at pH 4.0 and 0.5 S/m at 40 degrees C, a 3. 0-V/microm PEF treatment at an inlet temperature of 40 degrees C resulted in > or = 6.3 log inactivation of strain NCTC 11289 at 49.5 degrees C. A synergistic effect between temperature and PEF inactivation was also observed. The inactivation obtained with PEF was compared to the inactivation obtained with heat. We found that heat inactivation was less effective than PEF inactivation under similar time and temperature conditions. L. innocua cells which were incubated for a prolonged time in the stationary phase were more resistant to the PEF treatment, indicating that the physiological state of the microorganism plays a role in inactivation by PEF. Sublethal injury of cells was observed after PEF treatment, and the injury was more severe when the level of treatment was increased. Overall, our results indicate that it may be possible to use PEF in future applications in order to produce safe products.