The combined effects of high pressure and nisin on germination and inactivation of Bacillus spores in milk

Aims: The aim of this work was to investigate the germination and inactivation of spores of Bacillus species in buffer and milk subjected to high pressure (HP) and nisin. Methods and Results: Spores of Bacillus subtilis and Bacillus cereus suspended in milk or buffer were treated at 100 or 500 MPa at 40°C with or without 500 IU ml^-1 of nisin. Treatment at 500 MPa resulted in high levels of germination (4 log units) of B. subtilis spores in both milk and buffer; this increased to >6 logs by applying a second cycle of pressure. Viability of B. subtilis spores in milk and buffer was reduced by 2.5 logs by cycled HP, while the addition of nisin (500 IU ml^-1) prior to HP treatment resulted in log reductions of 5.7 and 5.9 in phosphate buffered saline and milk, respectively. Physical damage of spores of B. subtilis following HP was apparent using scanning electron microscopy. Treating four strains of B. cereus at 500 MPa for 5 min twice at 40°C in the presence of 500 IU ml^-1 nisin proved less effective at inactivating the spores of these isolates compared with B. subtilis and some strain-to-strain variability was observed. Conclusions: Although high levels of germination of Bacillus spores could be achieved by combining HP and nisin, complete inactivation was not achieved using the aforementioned treatments. Significance and Impact of the Study: Combinations of HP treatment and nisin may be an appealing alternative to heat pasteurization of milk.