Basically, Basic Cheese is Acidic. What?!
The basic steps in cheesemaking are: adding beneficial bacteria to milk, coagulating the milk into a soft white substance called curd, and pressing and cutting curd into the finished cheese shape. But to get a delicious final product, the milk must be at the right temperature and the right pH at the right time.
“You cannot fully understand cheese and cheesemaking unless you understand acidity and pH,” says Paul Kindstedt, professor at the Vermont Institute for Artisan Cheese, in Burlington, Vt.
The pH measures the concentration of hydrogen ions (H+) in a solution, with most solutions between the range of 0 and 14. The more acidic the solution, the lower the pH, with 7 considered “neutral” and above 7, “basic.” Milk has a pH between 6.6 and 6.7. pH is a logarithmic scale, so one unit differs by a factor of 10. A pH of 6 has ten times the H+ concentration than a solution with a pH of 7. This logarithmic scale is why even apparently subtle pH differences can be quite significant.
To make cheese, milk is pumped into a large tank and warmed to the right temperature. Two different bacteria are used in this process: mesophilic and thermophilic bacteria. Mesophilic bacteria grow best in moderate temperatures, typically between 20 and 45°C (68 and 113 °F). They are used to make mellow cheeses such as cheddar, gouda, and Colby. Thermophilic bacteria thrive between 45 and 122 °C (113 and 252 °F) and are used to make sharper cheeses such as Gruyère, Parmesan, and romano.
In the tank, the bacteria ferment the sugar present in milk, called lactose (C12H22O11), into lactic acid (CH3CHOHCOOH), as follows:
C12H22O11 + H2O ⇾ 4 CH3CHOHCOOH
As more lactic acid is produced, the milk’s pH lowers. “pH is our indicator of activity,” says Jeremy Stephenson, cheesemaker at Spring Brook Farm in Reading, Vt. “When the pH changes, we know the bacteria are alive and well. By measuring the pH, we are measuring the activity of these bacteria and assuring that the fresh curd is on the right path to becoming cheese.”
After the bacteria replicate and culture the milk at the optimal temperature, the milk coagulates and changes from a liquid into a firm, rubbery material. This change takes an hour or two and is possible because of the casein proteins in milk. Casein molecules aggregate into spheres called micelles. The outer layer is negatively charged, which allows the micelles to remain dispersed in liquid milk. To form cheese, the proteins must coagulate, or stick together (Fig. 2).