60-Day Aging Requirement Does Not Ensure Safety of Surface-Mold-Ripened Soft Cheeses Manufactured from Raw or Pasteurized Milk When Listeria monocytogenes Is Introduced as a Postprocessing Contaminant

Abstract

Because of renewed interest in specialty cheeses, artisan and farmstead producers are manufacturing surface-mold-ripened soft cheeses from raw milk, using the 60-day holding standard (21 CFR 133.182) to achieve safety. This study compared the growth potential of Listeria monocytogenes on cheeses manufactured from raw or pasteurized milk and held for > 60 days at 4 degrees C. Final cheeses were within federal standards of identity for soft ripened cheese, with low moisture targets to facilitate the holding period. Wheels were surface inoculated with a five-strain cocktail of L. monocytogenes at approximately 0.2 CFU/ cm2 (low level) or 2 CFU/cm2 (high level), ripened, wrapped, and held at 4 degrees C. Listeria populations began to increase by day 28 for all treatments after initial population declines. From the low initial inoculation level, populations in raw and pasteurized milk cheese reached maximums of 2.96 +/- 2.79 and 2.33 +/- 2.10 log CFU/g, respectively, after 60 days of holding. Similar growth was observed in cheese inoculated at high levels, where populations reached 4.55 +/- 4.33 and 5.29 +/- 5.11 log CFU/g for raw and pasteurized milk cheeses, respectively. No significant differences (P < 0.05) were observed in pH development, growth rate, or population levels between cheeses made from the different milk types. Independent of the milk type, cheeses held for 60 days supported growth from very low initial levels of L. monocytogenes introduced as a postprocess contaminant. The safety of cheeses of this type must be achieved through control strategies other than aging, and thus revision of current federal regulations is warranted.

Outbreaks Attributed to Cheese: Differences Between Outbreaks Caused by Unpasteurized and Pasteurized Dairy Products, United States, 1998–2011

“…Because Listeria is common in the processing environment, cheese made with pasteurized milk can become contaminated during cheese-making and aging (D’Amico et al , 2008; Centers for Disease Control and Prevention, 2013). Soft cheeses can support the growth of Listeria introduced after processing, regardless of whether the milk used for the cheese was pasteurized or not (D’Amico et al , 2008).…”

“…Because Listeria is common in the processing environment, cheese made with pasteurized milk can become contaminated during cheese-making and aging (D’Amico et al , 2008; Centers for Disease Control and Prevention, 2013). Soft cheeses can support the growth of Listeria introduced after processing, regardless of whether the milk used for the cheese was pasteurized or not (D’Amico et al , 2008). Because Listeria can grow and survive at refrigerated temperatures, even in cheeses aged more than 60 days (D’Amico et al , 2008), strict adherence to controls to avoid postpasteurization contamination with Listeria is required.…”

A review of the microbiological hazards of dairy products made from raw milk

“…However, during cheese ripening, growth of this pathogen occurred in raw milk cheese, but inactivation occurred in pasteurised cheese due to the physicochemical conditions of the cheeses (Schvartzman et al, 2011). Another study found no difference in the growth potential of L. monocytogenes inoculated on the surface of cheeses made from raw or pasteurised milk during storage (D'Amico, Druart, & Donnelly, 2008). In addition, the natural microbiota can influence the behaviour of pathogens in cheese.…”

Is Food and Drug Administration policy governing artisan cheese consistent with consumers’ preferences?

“…However, on average artisan cheese consumers are making purchasing decisions based on taste, not their attitudes toward safety. The science is ambivalent on the safety of cheese made from unpasteurized milk (D'Amico et al, 2008), and this research demonstrates that affected consumers are on average not concerned about the safety of unpasteurized cheese. The heterogeneity in preferences and the importance of taste as opposed to safety attitudes in determining WTP provides justification for policy that allows two distinct markets to exist for pasteurized and unpasteurized cheese.…”

“…Federal regulation currently requires that cheesemakers using unpasteurized milk (also called raw milk) age the cheese for a minimum of 60 days before sale (Cheese from Unpasteurized Milk, 2011). The aging aspect of the regulation is based on scientific research that has found that beneficial bacteria can outcompete harmful or pathogenic bacteria as cheese ages (see discussion in D'Amico, 2008). In contrast, in Europe there is no aging requirement for unpasteurized cheese and some of the most expensive cheeses are made from unpasteurized milk without aging.…”