The effect of growth stage and growth temperature on high hydrostatic pressure inactivation of some psychrotrophic bacteria in milk.

The effect of high hydrostatic pressure on the survival of the psychrotrophic organisms Listeria monocytogenes, Bacillus cereus, and Pseudomonas fluorescens was investigated in ultrahigh-temperature milk. Variation in pressure resistance between two strains of each organism were studied. The effect of growth stage (exponential and stationary phase), growth temperature (8 and 30 degrees C) on pressure resistance, and sublethal pressure injury were investigated. Exponential-phase cells were significantly less resistant to pressure than stationary-phase cells for all of the three species studied (P < 0.05). Growth temperature was found to have a significant effect at the two growth stages studied. Exponential cells grown at 8 degrees C were more resistant than those grown at 30 degrees C, but for stationary-phase cells the reverse was true. B. cereus stationary-phase cells grown at 30 degrees C were the most pressure resistant studied. L. monocytogenes showed the most sublethal damage compared to B. cereus and P. fluorescens. B. cereus spores were more resistant to pressure than vegetative cells. Pressure treatment at 400 MPa for 25 min at 30 degrees C gave a 0.45-log inactivation. Pressure treatment at 8 degrees C induced significantly less spore germination than at 30 degrees C. This study indicates the importance of the history of a bacterial culture prior to pressure treatment and that bacterial spores require more severe pressure treatments, probably in combination with other preservation techniques, to ensure inactivation.