FACTORS INFLUENCING SURVIVAL OF LISTERIA-MONOCYTOGENES IN MILK IN A HIGH-TEMPERATURE SHORT-TIME PASTEURIZER

Heat resistance experiments were carried out with Listeria monocytogenes which had been grown at three different temperatures 30, 39, and 43-degrees-C). Heated whole milk was inoculated with L. monocytogenes and then passed through a high-temperature short-time system at 72, 69, 66, and 63-degrees-C for a minimum holding time of 16.2 s. Heated cells were recovered both aerobically and anaerobically using four different methods: direct plating, most probable number, cold enrichment, and warm enrichment. Significant differences in recovery of L. monocytogenes were observed depending on the growth temperature. Cells grown at 43, 39, or 30-degrees-C, held 1 d at 4-degrees-C, and then heated at 69-degrees-C showed an overall decrease in numbers of approximately 2.1, 2.8, and 4.1 logs, respectively. Cells grown at 39-degrees-C and then held 3 d at 4-degrees-C appeared to be the most heat sensitive. Although cells grown at 43 and 39-degrees-C were capable of surviving at the minimum high-temperature short-time temperature (72-degrees-C), those grown at 30-degrees-C were not. In some instances, anaerobic incubation enhanced the recovery of L. monocytogenes, as compared to cells recovered aerobically, although these differences were not statistically significant. While L. monocytogenes can survive minimum pasteurization treatment (71.7-degrees-C/16 s) under certain conditions, common methods of handling, processing, and storing fluid milk will provide an adequate margin of safety.