Johnson (2001) reviewed the microbiology of cheese products and noted the complexity of the subject because of the great diversity in cheese manufacturing and ripening protocols, as well as composition of the different cheese types. The 60-day aging rule is based on the theory that pathogens, if present, will die-off to levels below the infectious dose during the aging process. However, the effectiveness of this system depends on the initial microbiological quality of the milk and other ingredients used, and the hygienic practices used during cheese processing (Donnelly 1990). No amount of curing or aging or even pasteurization will compensate for poor quality milk or lack of hygiene during manufacturing and storage.
The intrinsic properties of the cheesemaking process that affect pathogen survival and growth include:
- salt content
- redox potential
- cheese microbial flora including starter culture (microbial community)
Individually and in combination, these factors can have significant impacts on whether a foodborne pathogen survives or grows in cheese during curing. The effectiveness of these natural processes is ultimately dependent on the initial contamination level of the cheese. A high inoculum of a pathogen, especially one with a low infectious dose, will overwhelm these control systems. The soft and semi-soft surface-mold-ripened cheeses are at the greatest risk of contamination due to their higher pH and moisture content (D’Amico 2008a).
The presence of pathogens in milk used for production of raw milk cheeses represents a risk for consumers. Oliver (2009) reviewed the literature on pathogen prevalence in US bulk tank milk and found these levels.
- Campylobacter: 2 – 9.2%
- E. coli O157:H7: 0 – 0.75%
- Listeria monocytogenes: 2.8 – 7.0%
- Salmonella spp: 0 – 11%
- Shiga-toxin E. coli: 2.4 – 3.96%
- Yersinia enterocolitica: 1.2 – 6.1%
D’Amico (2008b and 2010) surveyed milk used to produce small-scale farmstead cheese in Vermont and found an overall low level of contamination, but documented variations from farm-to-farm indicating that some operations practice strict hygienic controls while other need improvement in their food safety practices.
Experimental studies of the behavior of pathogens in aged cheese show mixed results (Bachmann 1995; Back 1993; D’Amico 2008a; D’Amico 2008b; D’Amico 2010; Govaris 2002; Marth 1969; Reitsma 1996; Schlesser 2006). The studies are difficult to compare because of different experimental methods, and variations in how the cheese was manufactured for the experiments. For example, Reitsma (1996) found viable E. coli O157:H7 in cheddar cheese at 158 days, but used pasteurized milk in their comparisons. Schlesser (2006) inoculated cheddar cheese with a 5-strain E. coli O157:H7 cocktail and demonstrated an inadequate reduction at 60 days (1 log) and 120 days (2 logs); in contrast, heat treating the milk resulted in a 5-log reduction. D’Amico (2010) examined the behavior of E. coli O157:H7 in aged Gouda and stirred-cured cheddar cheeses manufactured from raw milk and was able to recover viable cells for more than 270 days in both cheese types using selective enrichment.
Listeria monocytogenes can be a pervasive problem in the dairy processing environment. There is evidence that L. monocytogenes can survive aging in both pasteurized and surface-mold-ripened cheeses if the pathogen is introduced post-processing (D’Amico 2008b). These findings underscore the importance of hygienic practices at cheesemaking facilities regardless of pasteurization status. D’Amico (2008a) provides a more comprehensive review of experimental studies using different pathogens and cheese types.
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