There are many ways to curd milk, including enzymatic coagulation, acid coagulation, microbiology, salt coagulation, and thermocoagulation, etc.
Enzyme induced condensation
Rennet hydrolyzes lactose to lactic acid and then reduces the pH of the emulsion to reach the isoelectric point 4.6 of the casein protein, the main component of cow milk. At this time, the charged particles of the milk are 0, which greatly reduces the electrostatic repulsion between protein micelles. And then the binding of the colloidal calcium phosphate inside the casein also dissolves, the micelles polymerize and eventually form the gel network.
Starter microorganisms, typically Lactobacillus subsp. bulgaricus and Streptococcus thermophilus, convert lactose into lactic acid. The principle is the same as above. Simultaneously, rennet also breaks the outer layer of ?-casein hair and promotes casein structure. Collapse and re-form the gel structure. When the pH is lower than 4.6, when the pH is too low, it is not suitable for the survival of the microorganisms, so it will in turn inhibit the microorganisms from continuing to ferment and release acid.
Acid-induced methods are very common, and glucuronide (GDL), citric acid, acetic acid, lactic acid, HCl, phosphoric acid and so on are often used in labs. GDL itself is hydrolyzed to gluconic acid steadily. The rate of acid release due to hydrolysis is very stable, and it can simulate the release of acid by microorganisms. Therefore, GDL is widely used in food production and industrial applications, such as tofu production.
This method is mainly to change the gel properties by adjusting the temperature. Such as, eggs, muscle protein in pork, etc.
Its principle is that during the formation of the whey protein cold gel, and under heat-induced conditions, the hydrophobic groups of the protein expose to form protein agglomerates which rely on the electrostatic repulsive force between them to form a stable state. And salt ions can usually aggregate the protein aggregates to form a gel by blocking the hydrophobic interaction of electrons or special ions. Calcium and sodium salts are usually used.
Cheese curds are usually fermented by microorganisms, but they are not necessary. The acid induction and salt induction mentioned above can all induce the gel formation of milk. However, the fermentation of microorganisms such as lactic acid bacteria does not bring in other foreign chemicals, food quality can be better. Nowadays, microbiological fermentation is still widely used in the industry. There are special yogurt or cheese starter companies. At the same time, enzymes are highly efficient and specific, and the reaction conditions are mild, which is more suitable for food production. And rennet with chymosin rennin is not present in cheese as a component. At most, it can only be considered as a catalyst, as an aid.