I think I have mentioned in an earlier posting on us using our acidity meter, the importance of acidity.
Here I am taking a quick look at it from a slightly different angle.
Milk in its raw, natural state has a pH of around 6.7. Reducing the pH to 4.6 or below results in the destabilisation of the casein structure and become solid i.e. form a curd. In other words, the casein protein in milk needs to achieve a certain acidity to form a coagulum. There are two ways to denature the casein to form a curd.
Experimenting at School!
Think back to when you were a child – did you ever do an experiment in school where you added lemon juice to milk, left it for a a few days and apart from smelling horribly, it clumped into solids and liquid. Home cheese makers make lactic cheeses in this way, by reducing the pH of milk i.e. making the milk more acidic, by adding lemon juice or vinegar. (My childhood memories are not intended in any way to be insulting to lactic cheese makers!)
These lactic type cheeses have a high moisture content and are therefore very soft: so a short shelf life.
Commercial Cheese Making
Short shelf life, high moisture content cheeses are generally no good to us cheese makes to make on a larger scale as we do not want to be stuck with the waste if unsold. To achieve that increase in acidity, we use specially prepared starter cultures to kick start a make and to give it its special characteristics e.g. a Cheddar, a Caerphilly etc.
Using Rennet to Form a Curd