Cheese Safety 101

Previous ‘episodes’ in this series described the concept of risk assessment (Part 1), and the types (Part 2) and sources (Part 3) of pathogenic bacteria that may occur in cheese. This article is the first of three on process control tools that importantly influence cheese safety, namely, pH history (acidity), temperature history and salt. Here we consider several principles of cheese safety assurance based on cheese acidity measured as pH. This article includes a diagrammatic explanation of pH.

Lactic acid is a natural preservative. Changes in acidity during natural fermentation of warm raw milk are illustrated in Figure 1. Manufacture of Cooked cheese (German) involves this entire cycle, but most fermented milk products include only part of this cycle. Fermented milks and yoghurts are chilled to control pH in the range of 4.4-4.8. Typical cheese pH values measured at 3-7 days after manufacture are 4.9-5.5 in most firm and hard ripened varieties, and 4.4-4.8 in fresh lactic and most soft ripened varieties. For comparison, color zones in Figure 1 show generalized pH (acid) tolerance of pathogenic bacteria. The point is that lactic acid is critical to cheese safety and quality.

It’s about pH history. For most cheese varieties, the pH profile (pH values at critical points during manufacture and ripening) is valuable for assessing safety. For examples: (1) A ‘slow vat’ allows more time at high pH for undesirable bacteria to grow; and (2) Bacteria from the milk or post pasteurization which survive but don’t grow during lactic acid fermentation, may grow when acidity is neutralized (pH increases) during cheese ripening, especially varieties with bloomy rinds. Typical pH profiles for representative varieties are in Table 1.

Pasteurization is not the final safety solution. Because many pathogens survive low pH (see Part 2 in this series), pasteurization of fresh cheese is an absolute must. But, proper acid development is also important to inhibit growth of post pasteurization contaminants, especially for varieties such as cottage cheese where open vats allow environmental exposure. For most ripened varieties (pasteurized or not) the combination of reduced pH and time along with other factors causes numbers of most bacteria to gradually decline during ripening. The key word is gradually, meaning that adequate acid development is important to minimize growth during manufacture and, therefore, minimize the reduction required during ripening.

Good hygiene is especially important to minimize post pasteurization contamination of non or minimally fermented varieties such as ricotta, Latin American white cheese and Halloumi. High pH (low acidity) is important to prevent melting of Latin American white cheese. However, we observed that even relatively minimal fermentation to pH 6.0 importantly inhibited the growth of Staphyloccoci on Queso Blanco cheese. Some low acid varieties, such as Halloumi are brined, which helps keep bacterial counts low.

The final principle, for now, is that, good cheese is not always safe cheese. Cheese pH profiles have developed as means to achieve certain cheese properties with respect to texture and flavour. But as illustrated by several examples cited above, fermentation and pH control alone are insufficient to assure safety. Part 5 of this series will discuss the role temperature history in cheese safety assurance.