The exact processes in the making of cheese varies between different varieties. However, all cheeses are made by essentially the same method. Initially, the milk is usually pasteurised by heating at 72°C for 15 seconds to destroy potentially harmful bacteria. The milk is then cooled to around 30°C and a starter culture of lactic acid bacteria is added to help souring. These convert lactose into lactic acid and help in the coagulation process. In addition, they also have a beneficial effect on the eventual quality, taste and consistency of the cheese. Some cheeses are coagulated entirely by lactic acid bacteria and are known as lactic-curd or acid-curd cheeses. However, some cheeses sold as lactic-curd cheese may have had rennet added.
The next stage is the addition of rennet, containing the enzyme chymosin. Rennet is usually sourced from the abomasum (fourth stomach) of newly-born calves. Here, chymosin aids the digestion and absorption of milk. Adult cows do not have this enzyme. Chymosin is extracted by washing and drying the stomach lining, which is then cut into small pieces and macerated in a solution of boric acid or brine at 30°C for 4-5 days. Pepsin may sometimes be used instead of chymosin. This is usually derived from the abomasum of grown calves or heifers, or less commonly pigs. Pepsin may be mixed with calf rennin. Rennet coagulates the milk, separating it into curds and whey. This is called curdling.
Chymosin breaks down the milk protein casein to paracasein which combines with calcium to form calcium paracaseinate, which separates out. Milk fat and some water also becomes incorporated into this mass, forming curds. The remaining liquid is the whey. The strength of different rennets can vary, though usual strength varies between 1:10,000 and 1:15,000 i.e. one part rennin can coagulate 10-15,000 parts milk.
Other substances may also be added during the cheese making process. Calcium chloride is added to improve the curdling process, and potassium nitrate is added to inhibit contaminating bacteria. Dyes (e.g. annatto, beta-carotene), Penicillium roquefortii mould spores to promote blue veining, or propionic acid bacteria to encourage hole formation may be added.
Following curdling, the curds are cut and drained. The size of the cut and the methods used vary for different cheese varieties. For soft cheeses, the curds are sparingly cut and allowed to drain naturally. For hard cheeses, the curds are heated and more whey is drained off. The curds are then cut into small pieces, placed in vats and pressed.
After pressing, the curds may be treated in a number of ways. They may be moulded into different shapes, soaked in a saltwater solution, be sprayed with mould forming spores or bacteria, washed in alcohol
, or covered in herbs.
The final stage is ripening, or maturation. This can vary in length from 4 weeks to 2-3 years, depending on the type of cheese. During ripening flavours develop, the cheese becomes firmer and drier, and special characteristics such as holes, blue veining and crust formation occurs.