Dr. Art Hill
Department of Food Science
University of Guelph,
Guelph, ON, N1G 2W1
Previous ‘episodesâ€™ 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.
Itâ€™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 vatâ€™ allows more time at high pH for undesirable bacteria to grow; and (2) Bacteria from the milk or post pasteurization which survive but donâ€™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.
Table 1. Typical pH versus time profiles for several cheese varieties Time is in minutes unless other wise noted. MNFS means moisture as a percentage of the nonfat substance of the cheese.
|Drain or dip into forms||150||6.35||100||6.45||210||6.20||195||6.3||130||NA||360||5.0|
|Demoulding||16 h||5.30||8 h||5.40||24 h||5.20||10 h||5.20||24 h||4.6||NA||NA|
|Minimum pH||1 wk||5.20||1 wk||5.20||1 wk||5.10||1 wk||5.10||1 wk||4.4||NA||NA|
|Retail||6 mo||5.6||6 mo||5.6||24 mo||5.50||4 mo||5.3||6 wk||4.4||2-14 d||5.2|
Figure 1. Natural fermentation of raw milk: A to B. At the natural pH of milk (6.6-6.8) and temperatures greater than 20°C, lactic acid bacteria (LAB) rapidly ferment milk sugar (lactose) to lactic acid. Most other bacteria are lactose intolerant. Lactic acid lowers the pH and inhibits most bacteria, eventually including LAB. B to C. Then, acid tolerant yeasts and moulds begin to grow and utilize lactic acid, which permits further growth of LAB. This synergistic relationship continues until all the lactose is gone. C to D. Yeast and moulds are eventually joined by proteolytic bacteria. Together they consume lactic acid or neutralize it with protein break down products. The colored zones indicate very generalized pH tolerance of pathogenic bacteria. Most pathogenic bacteria will grow at pH 5.6-7.0 (blue); some will grow at pH less than 5.6 (pink); and, some may survive but few will grow at pH less than 4.6 (rose).
Appendix 1. Cheese Safety 101 Part 4. The concept of pH.
Proteins and weak acids in milk act as reservoirs for hydrogen (H+) and hydroxyl (OH-) ions. When acid is added the equilibrium shifts to the left as free hydrogen ions associate more with proteins and weak acids. Conversely, when base (hydroxyl ions) is added as in determination of titratable acidity, hydrogen ions move out of the ‘reservoirsâ€™ to react with the hydroxyl ions. These, reservoirs, therefore, act as pH buffers, in that they reduce the effect of adding acid or base on the change in pH. In other words, titratable acidity (TA) measures all hydrogen ions, including those associated with proteins and acids, but, pH measures only the activity of un-associated hydrogen ions, those that are free in solution.
Protein properties (stretchabiltiy, meltability, hardness, brittleness), color, growth of spoilage and pathogenic bacteria, and activity of cheese ripening enzymes are all dependent on the pH history of the cheese.