According to a study published in Food Microbiology, raw milk aged cheeses are relatively safe. The cheeses must be aged at least 60 days, at temperatures not less than 35 degrees F. The aging process produces low pH, low water activity, and high salt content that renders the cheese inhospitable to bacteria, yeast, and molds.
The regulations regarding raw milk aged cheeses were enacted in 1950, ”long before contemporary pathogens such as Listeria monocytogenes, E. coli 0157:H7, and Campylobacter had been recognized,” according to the study. Those bacteria can tolerate inhospitable environments such as low pH and low water activity.
In the last 20 years, “many small and several large foodborne outbreaks linked to consumption of cheese made from unpasteurized milk have occurred, both in Europe and in the U.S.” according to the study.
So the FDA is reviewing the raw milk aged cheese regulations and may extend the 60-day aging period or may rescind the regulation altogether. In 1950, the regulation was based on observation that raw milk aged cheese was not associated with many foodborne illness outbreaks. There were few, if any, studies at that time confirming that assumption.
The study sampled 41 raw milk cheeses from farmers markets, retail specialty shops, and on-line sources in twelve different states. The sampled cheeses included Cheddar, blue, Gouda, Gruyere, Romano, and Monterey Jack.
The researchers analyzed the cheeses for Listeria monocytogenes, Salmonella, E. coli 0157:H7, Staphylococcus aureus, and Campylobacter. They also tested for coliform, yeast/mold, and aerobic plate counts.
None of the enteric pathogens were detected in any of the samples. But five samples contained coliform bacteria, which are used as a marker for the presence of fecal material. And three cheeses contained S. aureus, although the strains of that bacteria found do not produce enterotoxins.
However, some studies have found that when milk was intentionally inoculated with pathogenic bacteria, some of those bacteria do survive the aging process.
The study’s authors conclude that “the 60-day aging rule for unpasteurized milk cheeses appears adequate for producing microbiologically safe products.” But they also state that “risk assessment models suggest that efforts aimed at improving hygiene and on-farm milk practices can be even more effective at reducing risks of foodborne disease.”
And if you or a member of your family is in a high risk group (very young, elderly, pregnant, with a chronic disease or suppressed immune system), you may want to avoid raw milk cheeses, aged or fresh, altogether.