1

^ yeasts, micrococci cheese aging

Figure 5-5. General steps for manufacture of hard cheeses.

large U.S. manufacturers to use pasteurized milk for most cheeses, even those that are aged.

Pasteurization, however, not only kills pathogens and undesirable spoilage bacteria, but it also inactivates much of the endogenous microflora and enzymes ordinarily present in raw milk. Since the microflora and enzymes both contribute to the overall flavor and texture properties of the finished cheese, especially if the cheese is aged, quality differences between cheese made from raw or heat-treated milk can be significant. Thus, there is now a debate between those who believe that pasteurization should be required and those who contend that pasteurization is detrimental to cheese quality (Box 5-4). It should be emphasized, however, that whether the milk is raw or pasteurized, it should still be of high microbiological quality, free of antibiotics, and within the standards specified by government regulations.

Finally, one of the most obvious ways to treat milk to distinguish one cheese from another is by the color. Milk, and the finished cheese, can be made more yellow by adding the natural coloring agent, annato, or made

Box 5-4. Safety of Cheese Made From Raw Milk

In the United States, only cheese that is aged for more than sixty days can be made from raw milk; all other cheeses must be made from milk that has been pasteurized.The rationale for this policy, which was established in the 1930s, is based on early research that indicated any pathogenic bacteria present in the raw milk would die during a sixty-day ripening period. Factors believed to contribute to cell death included low pH, low water activity, low Eh, high salt-in-moisture levels,the presence of organic acids and competing microflora, and the absence of fermentable sugars. In other words, the collective effect of these inhospitable conditions were thought to create an environment that would result in the eventual lysis and death of any pathogenic bacteria that may have been present in the original cheese.

The sixty-day requirement was really nothing more than an approximation based on a rather limited number of studies. Moreover, these studies were done long before Listeria monocytogenes, Escherichia coli O157:H7, and other pathogenic organisms were recognized as potential contaminants of raw milk.That these pathogens could be inherently more resistant to environmental extremes or that they could induce stress resistance defense systems were possibilities that could hardly have been imagined.

It is now well-known that some of these pathogens can indeed tolerate all sorts of inhibitory conditions. For example,L. monocytogenes is particularly capable of surviving, even growing, at acid pH and in the presence of high salt concentrations. Furthermore, when exposed to various shocks, such as high, sub-lethal temperatures, this organism synthesizes a set of proteins that enable it to tolerate other subsequent stresses.These observations led researchers to challenge the conclusions made seventy years ago, i.e.,that a sixty-day aging process would result in raw milk cheese that is free of pathogens. Recent research, in fact, now indicates that these "newly" recognized pathogens could survive aging and, if present at sufficiently high numbers in the raw milk, could be present in the aged cheese. For example, several publications from the Marth laboratory at the University of Wisconsin showed that L. monocytogenes could survive more than one year in Cheddar cheese and that levels greater than 107 cfu/g could be reached in Camembert cheese (summarized by Ryser, 1999).

In response to these findings, the FDA announced in 2002 that this policy would be reviewed and left open the possibility that all cheese, regardless of whether it was aged or not, would have to made from pasteurized milk.Although manufacturers of aged cheese were undoubtedly concerned about this announcement, the greatest furor of opposition came from gourmet-type consumer groups.Their position, as articulated on various Web sites, is that cheese made from raw milk is superior in flavor, texture, and overall sensory appeal to that of pasteurized milk, and that any perceived food safety risks are exaggerated or non-existent. Indeed, despite the research showing that pathogens might, at least in theory, survive sixty days of aging, the food safety record for aged cheese is extraordinarily good.There are few published accounts of aged raw milk cheese having been responsible for foodborne disease, and in those cases where the cheese was implicated, other extenuating circumstances were involved (e.g., post-aging contamination by workers or equipment).

Despite the excellent record for aged raw milk cheese, fresh raw or pasteurized milk cheese has been the cause of several serious foodborne disease outbreaks (Table 1). In the majority of these outbreaks, poor handling of the milk or cheese or poor sanitation was responsible. Post-pasteurization contamination of milk, in particular, is the most frequent cause of these outbreaks. However, there are some fresh or non-aged cheeses that, even when properly made, may still pose a risk to certain consumers. Those at risk include the very young and very old, pregnant women, and other immuno-compromised populations. Brie cheese, for example, is on the "do not eat" list for pregnant women and HIV-positive individuals, due to the potential (but infrequent) occurrence of L. monocytogenes in this cheese.The use of sub-pasteurization treatments, such as "thermization" (performed by heating milk to about 65°C for fifteen to twenty

Box 5-4. Safety of Cheese Made From Raw Milk (Continued)

seconds), may be somewhat effective against potential pathogens and provides a margin of safety at which some cheese makers may be comfortable.

Table 1. Safety record of cheese.

Year

Cheese

Cases

Organism

Cause

1970s

Brie

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