HIGH-PRESSURE STRESS AND INACTIVATION OF LISTERIA-INNOCUA IN INOCULATED DAIRY CREAM

Listeria innocua, ''model'' microorganism for Listeria monocytogenes w as added to a liquid UHT dairy cream (35% fat) at a level of about 10( 7) CFU/ml. The cream was pressurized at 450 MPa and 25-26-degrees-C fo r 10 to 30 min. The vegetative cells underwent alterations, some being lethal, some others being reversible after a resuscitation step on a nutritive broth at 37-degrees-C. For a pressurization time of 10 min, microbial reduction reached 0.987 and 1.10(-5) of the initial cells gr ew on a selective medium without a resuscitation step. With pressuriza tion times of 20 and 30 min, microbial reduction reached 0.9995 and 0. 9999, respectively, all survivors being stressed (unable to grow on a selective medium without a resuscitation step). Microbial inactivation followed an apparent first order kinetics, with a decimal reduction t ime of 7.4 min at 450 MPa (and 25-26-degrees-C). After 20 min under 45 0 MPa, a resuscitation step of 16 h at 37-degrees-C was necessary for the growth of a maximum number of stressed bacteria.