Food Protection Trends - March 2009 - 152

ability to develop adaptive responses that allow the organisms to survive in cheeses, together with the emergence of multidrug-resistant strains of Salmonella (15), makes it important for the farmstead cheesemaker to remain vigilant for the presence of Salmonella on the farm. The presence of E. coli O157:H7 was not detected in any of the samples from this study. The incidence of this pathogen in cheeses is very low, although a number of other studies have attempted to detect it in dairy products (1). Even though E. coli O157:H7 is not often found in cheeses, taking steps to prevent contamination of products with this foodborne pathogen remains important, because it is infectious at very low doses (37) and causes severe illness, with symptoms of bloody diarrhea and acute renal failure sometimes resulting in death, especially in children (39). Ruminants, especially cattle, can be carriers of E. coli O157:H7 (20), so the organism can be present on the farm with no obvious signs that it is there, as it is not pathogenic in ruminants as it is in humans. Because of its extreme pathogenicity, the ability of E. coli O157:H7 to survive in cheeses and cheese brines has been investigated (12, 20, 25, 26, 35, 37). The results of these studies showed that E. coli O157:H7 can often tolerate the cheese production process (12, 26), the salinity of brines used in cheese making (20), and the acidity of some cheeses (25, 37), as well as surviving the mandatory 60-day aging period in some raw milk cheeses (35). L. monocytogenes is commonly found in the dairy farm environment (19, 22, 23), but none of the farmstead cheese samples from this study were positive for this important foodborne pathogen. Human infection with L. monocytogenes is relatively rare, with about 3 or 4 cases per million people annually in the United States (36) and between 3 and 7 cases per million annually in Europe (8). But the symptoms of invasive disease caused by L. monocytogenes (listeriosis) — encephalitis, meningitis, and miscarriage in pregnant women — are severe, and results in death in about 20% of those infected (8, 22, 28, 29). A L. monocytogenes risk assessment conducted in the United States in 2003 concluded that cheeses have a moderate to low relative per-serving risk of causing foodborne listeriosis, with soft unripened cheese being classi- fied as moderate risk and other types of cheeses classified as low risk (36). For the sake of comparison, it will be mentioned that the high relative risk ready-toeat foods include deli meats, frankfurters (not reheated), and unpasteurized fluid milk, and that pasteurized fluid milk is in the same moderate-risk category with soft unripened cheese (36). Studies have shown that L. monocytogenes can still be viable in some types of cheeses made from raw milk after 42 days of ripening and storage (31), and even after the federally required 60-day aging period (7). Other studies have shown that L. monocytogenes can develop a tolerance to acidic environments (17) and can survive in the high salinity of cheese brines, where many other species of bacteria perish (23). It would appear that with the high incidence of L. monocytogenes in the dairy farm environment, and the ability of this pathogen to adapt to acidic environments and to tolerate salinity, the very low incidence of listeriosis attributed to contaminated cheeses, indicates that United States cheesemakers are working successfully to control the contamination of their products, perhaps as the result of effective regulations and a zero tolerance for the presence of L. monocytogenes in foods. Staphylococcal food poisoning, caused by enterotoxins produced by some strains of S. aureus, is a very common and relatively mild foodborne illness, causing nausea, vomiting and diarrhea that normally subside within one to three days (9). S. aureus survives quite well in milk, and dairy products (including cheeses) have been implicated in cases of illness caused by enterotoxins produced by these bacteria (30). Another reason to be concerned about the presence of S. aureus in foods is the increasing presence of methicillin-resistant S. aureus (MRSA) in settings other than hospitals. The presence of MRSA in food producing animals has been demonstrated by others, with about 3-4% of S. aureus isolates from two studies possessing the mecA gene characteristic of MRSA (24, 32). Because S. aureus can be shed in the milk of animals infected with this organism, and because S. aureus is destroyed by pasteurization, raw milk and cheeses made from raw milk are more likely than products made from pasteurized milk to be contaminated with S. aureus that could be MRSA. Consistent with this statement, the three cheese samples from the present study that were unacceptable because of high S. aureus counts were made from raw milk (isolates were not tested to determine if they were MRSA). However, pasteurized products can become contaminated with S. aureus through post-pasteurization contamination. Because MRSA is resistant to most commonly used antibiotics and infections with MRSA can consequently be difficult to treat, cheese makers should take steps to control the presence in their products of S. aureus, some of which could be MRSA. Lastly, the packaging, shipping, and labeling of the farmstead cheeses tested in this study must be considered. Most of the samples were received cold or cool, having been shipped in insulated coolers, as perishable products should be, but six samples were shipped in plain boxes with no ice packs and they were in transit for four days before they were received. Interestingly, two of these six samples were among the total of seven that were found to be microbiologically unsafe (one was positive for presence of Salmonella, and the other had a high S. aureus count). Proper packaging and shipping of perishable products such as cheeses are necessary to help minimize the potential risk to consumers of microbial food safety hazards. Also, of the 61 samples in this study, only 22 included some sort of date or lot information that could be useful in a traceback, and 2 samples were received with no label at all. Virginia regulations require that all packages containing a finished dairy product (a product ready for sale) be labeled with the name of the product (a name that does not mislead the consumer), the name and address of the processor, and a “sell by date” (38). The proper labeling of these products appears to be an area where much improvement could be made. With the desire of some individuals to go back in time to produce farmstead cheeses in the way of the artisan, and the desire of others to consume these highly flavorful cheeses full of character, it is likely that the trend of farmstead cheese production and consumption will continue. The results of this study indicate that most farmstead cheesemakers are successfully preventing contamination of their cheeses with Salmonella, L. monocytogenes, E. coli O157:H7, and S. aureus, but a wealth of scientific data shows that these harmful pathogens are in the dairy 152 FOOD PROTECTION TRENDS | MARCH 2009

Food Protection Trends - March 2009

Table of Contents for the Digital Edition of Food Protection Trends - March 2009