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.

Three strains of Listeria monocytogenes (NCTC 11994, a poultry isolate and the Scott A strain) were exposed to a range of pressures (300, 350, 375, 400 and 450 MPa) in 10 mmol l−1 phosphate‐buffered saline (PBS) at pH 7·0 for up to 30 min at ambient temperature. Generally, increasing the magnitude and duration of compression resulted in increasing levels of inactivation, although the inactivation kinetics varied depending on the strain and pressure applied. The three strains also exhibited a wide variation in their resistance to high pressure. The resistance of the three strains to high pressure (375 MPa) was also assessed in a series of model food systems containing one of each of the three main food constituents: protein (1, 2, 5 and 8% w/v bovine serum albumin in PBS), carbohydrate (1, 2, 5 and 10% w/v glucose in PBS) and lipid (olive oil (30% v/v) in PBS emulsion). Overall, increasing the concentrations of bovine serum albumin (BSA) and glucose in the suspending medium resulted in decreasing levels of inactivation of all three strains; however, the minimum concentration of BSA and glucose required to increase survival to a level greater than that observed in PBS alone varied depending on the strain and on the duration of the treatment. The survival of all three strains was greater in the olive oil/PBS emulsion than in PBS alone at all treatment times.

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