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.

Influence of several environmental factors on the initiation of germination and inactivation of Bacillus cereus by high hydrostatic pressure.

The influence of pH, aw, L-alanine, and fat concentration of milk on the initiation of germination and inactivation by high hydrostatic pressure (HHP) (250 mPa at 25 degrees C for 15 min and 690 mPa at 40 degrees C for 2 min) of Bacillus cereus sporulated at 20, 30 and 37 degrees C was investigated. B. cereus sporulated at the lowest temperature was found to be the most resistant to the initiation of germination and inactivation by HHP. At ambient pressure, the rate and extension of germination induced by L-alanine were also lower in B. cereus sporulated at 20 than 30 or 37 degrees C. The optimum pH for the germination and inactivation of B. cereus depended on the sporulation temperature. At 250 mPa the extent of germination for the three suspensions increased with higher pH. At 690 mPa, the pH barely affected the germination of B. cereus sporulated at 20 degrees C (3 log cycles), but the inactivation increased as the pH of the medium was lowered. After the same treatment, pressure optimally initiated the germination of B. cereus sporulated at 30 and 37 degrees C (6-7 log cycles) around neutral pH. Higher inactivation was obtained at pH 6. High concentrations of sucrose protected the spores from the germinating and inactivating effect of HHP. At aw 0.92, no germination was detected when the spores were pressurized at 250 mPa, and only 1 log cycle of B. cereus sporulated at 20 and 30 degrees C and 2 log cycles of B. cereus sporulated at 37 degrees C were germinated at 690 mPa. In addition, no inactivation was observed at aW 0.92 even at 690 mPa. The presence of L-alanine in the medium of pressurization increased the germination initiated by HHP at 250 mPa, but not at 690 mPa. A combination of 250 mPa at 25 degrees C with L-alanine (100 mM) was found to give an additive response. The initiation of germination and inactivation by HHP were not affected by the fat concentration of the milk.