Dairy Sheep Symposium - the Department of Animal Sciences ...

Recent Outbreaks involving Cheddar Cheese
Four outbreaks occurring in the late 1990s were reported in the U.K. Three of these cases
involved E. coli 0157:H7 and the fourth case was caused by Salmonella (Donnelly, 2001). All
four outbreaks were caused by incomplete pasteurization or use of raw milk for cheesemaking.
An outbreak of E. coli 0157:H7 was the result of using raw milk to produce Cheddar cheese
curds that were sold fresh from the plant (Durch et al., 2000).
While outbreaks of illness have resulted from presence of L. monocytogenes in soft ripened
and Hispanic-style cheeses, no outbreaks of listeriosis have been reported as a result of Listeria
surviving in cheese aged for a minimum of 60 days.
Challenge Studies
Reitsma and Henning (1996) examined the survival of E. coli 0157:H7 during manufacture
and aging of Cheddar cheese. They used milk with two levels of E.coli, 1000 cfu/ml and 1 cfu/
ml. The cheese with 1000 cfu/ml in the milk showed a 2-log reduction after 60 days of aging but
still contained E. coli after 158 days of aging. The cheese with 1 cfu/ml in the milk had no
detectable E. coli after 60 days of aging.
Conclusions from Review
Aged hard cheeses made from raw milk are microbiologically safe when manufactured under
conditions that use milk screening procedures, good manufacturing practices and HACCP.
Legitimate concerns can be raised regarding the safety of soft and semisoft cheeses manufactured
from raw milk, as well as high moisture, low salt aged cheeses. They recommend a strategy
that involves routine testing of incoming lots of raw milk and working with producers when
infected animals are identified to allow treatment and confinement of animals to control infectious
disease.
Pathogens in Raw Milk
Surveys of various groups of producers have shown low levels of individual pathogens in
raw milk at the farm (Table 2). Some levels of pathogenic bacteria may be low but Jayarao and
Henning (1999) pointed out that 32% of milk at the farm contained at least one pathogen. When
milk from a number of farms is commingled, e.g., at a commercial cheese plant, levels of pathogens
in loads of milk are significantly increased (Ingham et al., 1997). If using commingled milk
for commercial cheese production, a cheesemaker must assume that the load of milk most likely
will contain pathogens, especially S. aureus.
Current FDA Studies on Raw Milk Cheeses
FDA is currently running studies to determine how effective the 60 day aging period is for
reducing potential pathogens in Cheddar cheese. Hard cheese was made from unpasteurized
milk inoculated with 10 3 and 10 5 E. coli 0157:H7 and aged under standard aging conditions of
7°C (45°F). Populations of E. coli were reduced by 1 log at 60 days and 1-2 logs at 90 days of
aging (Schlesser et al., 2001). Populations of E. coli in cheese aged for 180 and 240 days were
reduced by 2-logs and 3-logs, respectively. Additional studies are now being conducted with
Salmonella as the test pathogen.
Potential Future Regulations covering Raw Milk Cheeses
With the current information available to FDA on potential presence of pathogens in raw
milk and the limited reduction of pathogens during the standard 60 day aging period, FDA will
be proposing new regulations within the next 2 years. The U.S. FDA has recently stated, “a