Epidemiologic Notes and Reports Update
Listeriosis and Pasteurized Milk
Listeria monocytogenes can be cultured from approximately 5% of raw (unpasteurized) milk samples, and case reports have shown that disease in humans can be caused by consumption of unpasteurized contaminated milk (1-4). Other reports suggesting that L. monocytogenes is relatively resistant to heat have raised concern about the effectiveness of pasteurization for eliminating this organism from milk (1,5). In an outbreak of listeriosis that occurred in Massachusetts in 1983, pasteurized whole or 2% milk was implicated as the source of infection (6). An inspection of the milk-producing plant detected no apparent breach in the pasteurization process, thereby prompting further interest in the effectiveness of pasteurization. Since then, several studies have shown that L. monocytogenes is inactivated by standard pasteurization practices (4,7-10). This report summarizes information regarding the effectiveness of pasteurization in eliminating L. monocytogenes from milk.
Current state and local regulations throughout the United States specify time and temperature conditions for pasteurization. These regulations call for milk to be heated to at least 71.7 C for 15 seconds (i.e., high-temperature short-time (HTST) process) or to 62.8 C for 30 minutes (11). In one study using milk artificially inoculated with L. monocytogenes, investigators found that 0.9 seconds at 71.7 C is needed for each one log10 reduction in the number of L. monocytogenes organisms (7). Other studies of artificially inoculated milk have concluded that at the same temperature, a period of 2.75 to 3.1 seconds is needed for each log10 reduction (4). Since the concentration of L. monocytogenes likely to be present in contaminated raw milk is estimated to be approximately 10 organisms per mL, these data suggest that there is a substantial margin of safety in the HTST pasteurization process (4). Another study found that at 62 C, each log10 reduction in the number of surviving L. monocytogenes organisms could be achieved in 6 to 20 seconds, well within the 30 minutes required for pasteurization at this temperature (8). Other investigators also have concluded that L. monocytogenes will not survive the normal milk pasteurization process (9) and have questioned previously reported methods that suggested L. monocytogenes could survive pasteurization (8,9).
In a pasteurization study designed to simulate the natural situation more closely, milk from cows that had been purposefully infected with L. monocytogenes was used, along with several different L. monocytogenes isolation procedures (12). Viable L. monocytogenes could be recovered after minimum HTST treatment (71.7 C for 15 seconds), although not after treatment at 76.4 C-77.8 C for 15 seconds. This survival was attributed in part to protection of L. monocytogenes within leukocytes in milk (intracellular L. monocytogenes organisms are found in milk from infected cows but not in artificially inoculated milk). Because this milk had L. monocytogenes concentrations of 103 to 104 per mL, higher concentrations than are usually found when L. monocytogenes is present in raw milk, these findings may not be applicable to usual production conditions. In another study in which investigators identified cows that had been naturally infected with L. monocytogenes (10), proper pasteurization was found to inactivate L. monocytogenes in milk contaminated through natural infection as well as in artificially inoculated milk.
After reviewing these studies, a World Health Organization Working Group on foodborne listeriosis recently concluded that “pasteurization is a safe process which reduces the number of L. monocytogenes occurring in raw milk to levels that do not pose an appreciable risk to human health” (4). Reported by: Milk Safety Br, Div of Cooperative Programs, Center for Food Safety and Applied Nutrition, Food and Drug Administration. Meningitis and Special Pathogens Br, Div of Bacterial Diseases, Center for Infectious Diseases, CDC.
Editorial Note: Improperly performed pasteurization and the occurrence of contamination after pasteurization are the most likely explanations for the presence of L. monocytogenes in pasteurized milk. Two percent of pasteurized milk samples from more than 700 U.S. milk-producing plants were culture-positive for Listeria species, primarily L. monocytogenes, in a survey conducted during 1987 and 1988 as part of the Food and Drug Administration’s (FDA’s) Dairy Product Safety Initiatives (13). Even when pasteurized milk is proven to be contaminated by bacteria, the exact source and mode of contamination may be difficult to identify. For example, in a recent large epidemic of salmonellosis in Illinois, the epidemic strain of Salmonella typhimurium was isolated from patients and the implicated pasteurized milk products they had drunk (14). However, an inspection of the plant by a task force of FDA officials and other experts could not prove how the milk was contaminated. Efforts to ensure that milk is safe from L. monocytogenes contamination should focus on promoting proper methods of pasteurization and on identifying and eliminating sources of postpasteurization contamination.