Wiki: Acidfication

Generally, cheesemaking starts with acidification. This is the lowering of the pH (increasing acid content) of the milk, making it more acidic. Classically, this process is performed by bacteria. Bacteria feed on the lactose in milk and produce lactic acid as a waste product. With time, increasing amounts of lactic acid lower the pH of the milk. Acid is essential to the production of good cheese. However, if there is too much acid in the milk the cheese will be crumbly. If not enough acid is present the curd will be pasty.

Thus, for cheese makers, there are pH markers that you need to hit for each step of the process, for this you need a pH meter and to track the pH readings.