Inactivation of food pathogens in broth by high hydrostatic pressure treatment
A range of Gram-negative and Gram-positive bacteria (Escherichia coli, Salmonella typhimurium, S. enteritidis, Staphylococcus aureus and Listeria monocytogenes) was subjected to high hydrostatic pressures of 400 and 600 MPa at room temperature in broth adjusted at two pH values (5.3 and 6.5). The effects on viability of compression followed immediately by decompression and of different lengths of treatment (1, 5 and 10 minutes) were determined. L. monocytogenes was resistant to a compression/decompression cycle up to 400 MPa, while the other test organisms exhibited loss of viability ranging over three orders of magnitude. When the pressure was raised to 600 MPa, the viability of microbial suspensions was reduced between 2.5 and 5.9 logs. At both pressures a varying proportion of the cells exhibited sublethal injury. Experiments in which the cells were held at pressure for 1, 5 and 10 minutes showed that no Gram-negative bacteria from a population of 10 6 to 10 7 cells/ml survived 5 minutes at 400 MPa. In contrast, L. monocytogenes was not eliminated by treatment at 600 MPa for 10 minutes, although its survival fells by 5 logs from 10 7 to 10 2 cfu/ml.