Step 2: Milk
As a biological product, milk contains proteins, the principal milk proteins are known as casein but it also contains other proteins. It consists of 80% casein and 20% whey protein. There are four major types of casein molecules: alpha-sl, alpha-s2, beta, and kappa
Because of the negative charge of the casein, it is dispersed in milk so if we want to make cheese we need to denaturate casein to coagulate it.
Denaturation of milk proteins
When an acid is added to milk, the H+ concentration neutralizes the negatively charged casein micelles. When milk is acidified to pH 4.7, it reaches its isoelectric point (the point at which all charges are neutral) and it forms a precipitate known as acid casein. Cottage cheese and cream cheese manufacture involves an acid precipitation of casein with lactic acid or lactic acid-producing microorganisms. Acid casein is used in the chemical industry and as a glazing additive in paper manufacturing.
Casein also can be coagulated with the enzyme rennin, which is found in rennet (an extract from the stomach of calves)
Do you remember which factors affects proteins? If not, go back to step 1 because rennin as a protein is affected by these factors and is enzymatic activity can change.
Rennin works best at body temperature (37 C). If the milk is too cold, the reaction is very slow, and if the milk is too hot, the heat will denature the rennin, rendering it inactive. The mechanism for the coagulation of the casein by the rennin is different from the acid precipitation of casein. The coagulation of the casein by rennin is a two-stage process. In the first stage, rennin (a proteolytic enzyme which means it acts only on proteins) splits a specific bond in the amino acid chain of the kappa-casein macromolecule converting it into a-para-kappa-casein and a glyco-macropeptide. This causes an imbalance in the intermolecular forces in the milk system, and the hydrophilic (water-loving) macropeptides are released into the whey. Unlike kappa-casein, the parakappa-casein does not have the ability to stabilize the micellular structure to prevent the calcium-insoluble caseins from coagulation. In the second stage, colloidal calcium phosphate (this is and example of other compound affecting proteins) bridges within the casein micellular structure, resulting in the three-dimensional curd structure.
The rennin coagulum consists of casein, whey protein, fat, lactose, and the minerals of the milk, and has a fluffier and spongier texture than the acid precipitate. Rennet is used in the manufacture of cheese and cheese products (nowadays, most of the cheese producers do not use calve rennet because is very expensive they use "vegetable rennet" which is made from microorganisms), and rennet casein is used in the plastics industry.