Thermal resistance of Listeria monocytogenes in inoculated and naturally contaminated raw milk.

Raw whole milk inoculated with 10(5) CFU/ml of Listeria monocytogenes was thermally processed at 60-72 degrees C for a minimum holding time of 16.2 s with survival being observed at temperatures up to 67.5 degrees C. In addition, milk naturally contaminated with L. monocytogenes serotype 1 (around 10(4) CFU/ml) was pooled for 2 to 2.5 days and then run through an HTST pasteurizer at temperatures ranging from 60-78 degrees C. Viable L. monocytogenes were detected in the temperature range of 60-66 degrees C. No viable Listeria were detected after treatment at temperatures of 69 degrees C and above in any of five trials. Efficacy of pasteurization and widespread use of processing conditions well above the minimum HTST guidelines ensure the absence of Listeria in pasteurized milk products. However, survival of Listeria at sub-pasteurization temperatures (60-67.5 degrees C) is of concern with regard to heat-treated or raw-milk cheeses.