The resistance of Listeria monocytogenes to high hydrostatic pressure in foods

Two strains of Listeria monocytogenes (NCTC 11994, Lm1 and a poultry isolate, Lm2) were exposed to high pressure (375 MPa) in ultra high temperature (UHT) treated milk, raw chicken mince, raw beef mince, cooked chicken mince and cooked beef mince for up to 30 min at ambient temperature. Survival of the two strains in UHT milk was significantly greater than previously observed in non-nutrient phosphate buffered saline (PBS). Survival in cooked chicken mince and cooked beef mince was only marginally greater than survival in PBS, whilst survival in raw chicken and raw beef mince was generally less than observed in PBS. In all food substrates, the proportion of survivors sustaining injury increased with increasing duration of compression. The two strains were also treated at 375 MPa in cooked beef mince for up to 30 min at 45 degrees C. Inactivation of both strains was greater at 45 degrees C than at ambient temperature. Treatment at 375 MPa for 5 min at 45 degrees C resulted in approximately 4.5 log(10)-cycle reduction; however extending the treatment time resulted in only a slight further decrease in cell counts. The effects of temperature (25, 35, 45 and 55 degrees C) on survival at 200 and 375 MPa (15 min) in UHT milk was also assessed. Increasing the temperature resulted in increasing levels of inactivation. (C) 1997 Academic Press Limited.