Sensitivity of Vegetative Pathogens to High Hydrostatic Pressure Treatment in Phosphate-Buffered Saline and Foods

The effect of high hydrostatic pressure (up to 700 MPa) at 20°C on the survival of vegetative pathogens was investigated in 10 mM phosphate buffer (pH 7.0), ultra high-temperature-treated (UHT) milk, and poultry meat. In buffer, Yersinia enterocolitica was most sensitive, with a pressure of 275 MPa for 15 min resulting in more than a 10⁵ reduction in numbers of cells. Treatments of 350 MPa, 375 MPa, 450 MPa, 700 MPa, and 700 MPa for 15 min were needed to achieve a similar reduction in Salmonella typhimurium , Listeria monocytogenes , Salmonella enteritidis , Escherichia coli O157:H7, and Staphylococcus aureus respectively. A significant variation in pressure sensitivity was observed between different strains of both L monocytogenes and E. coli O157:H7. The most resistant strains ( L. monocytogenes NCTC 11994 and E. coli O157:H7 NCTC 12079) were chosen for further studies on the effect of substrate on pressure sensitivity. In both cases the organisms were more resistant to pressure when treated in UHT milk than in poultry meat or buffer. There was evidence, assessed by differential plating using trypticase soy agar with and without additional NaCl, that sublethally injured cells were present at pressures lower than were required for death. This information may be of value if pressure is combined with preservation treatments such as mild heating. The variation in results obtained with different organisms and in different substrates should be recognized when recommendations for the pressure processing of foods are being considered.