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
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. References
Bryan FL. Listeria monocytogenes. In: Riemann H, Bryan FL, eds.
Food-borne infections and intoxications. 2nd ed. New York: Academic
Press, 1979:266-8.
2.Lovett J, Francis DW, Hunt JM. Listeria monocytogenes in raw
milk:
detection, incidence, and pathogenicity. J Food Protection
1987;50:188-92.
3.Hayes PS, Feeley JC, Graves LM, Ajello GW, Fleming DW. Isolation
of
Listeria monocytogenes from raw milk. Appl Environ Microbiol
1986;51:438-40.
4.WHO Working Group. Foodborne listeriosis. Bull WHO 1988;66:421-8.
5.Bearns RE, Girard KF. The effect of pasteurization on Listeria
monocytogenes. Can J Microbiol 1958;4:55-61.
6.Fleming DW, Cochi SL, MacDonald KL, et al. Pasteurized milk as a
vehicle of infection in an outbreak of listeriosis. N Engl J Med
1985;312:404-7.
7.Bradshaw JG, Peeler JT, Corwin JJ, et al. Thermal resistance of
Listeria monocytogenes in milk. J Food Protection 1985;48:743-5.
8.Donnelly CW, Briggs EH, Donnelly LS. Comparison of heat
resistance
of Listeria monocytogenes in milk as determined by two methods. J
Food
Protection 1987;50:14-7.
9.Beckers HJ, Soentoro PSS, Delfgou-van Asch EHM. The occurrence of
Listeria monocytogenes in soft cheeses and raw milk and its
resistance
to heat. Intl J Food Microbiol 1987;4:249-56.
10.Farber JM, Sanders GW, Emmons DB, McKellar RC. Heat resistance
of
Listeria monocytogenes in artificially-inoculated and
naturally-contaminated raw milk (Abstract). J Food Protection
1987;50:893.
11.Food and Drug Administration. Grade A pasteurized milk
ordinance.
Washington, DC: US Department of Health and Human Services, Public
Health Service, 1985; FDA publication no. 229.
12.Doyle MP, Glass KA, Beery JT, Garcia GA, Pollard DJ, Schultz RD.
Survival of Listeria monocytogenes in milk during high-temperature,
short-time pasteurization. Appl Environ Microbiol 1987;53:1433-8.
13.Food and Drug Administration. FDA's dairy product safety
initiatives: 2nd year status report to the states. Washington, DC:
US
Department of Health and Human Services, Public Health Service,
1988.
14.Ryan CA, Nickels MK, Hargrett-Bean NT, et al. Massive outbreak
of
antimicrobial-resistant salmonellosis traced to pasteurized milk.
JAMA
1987;258:3269-74.
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