In many countries where milk production is high, a significant amount of milk is used for cheese production. The most important factor determining the yield and quality of cheese in cheese making is the coagulation property of milk. For this reason, coagulation of milk with rennet, which is the basis of cheese production for the dairy industry, is of great importance. The coagulation of casein micelles forms the basis of cheese making. Coagulation occurs in two different ways, usually with acid and enzyme. In both methods, casein changes to insoluble form and forms a non-degradable network structure. In acid coagulation, the protein network is loose, whereas in enzyme coagulation, the protein network is tight and elastic.
a) Coagulation of Casein by Acid Effect
During acidification, different biochemical changes are observed in casein micelles. These changes are demineralization of casein micelles, protonation of acid groups, decrease in solubility, hydration of caseins and zeta potential. These successive events cause precipitation of caseins if acidification is rapid and gelation if acidification is slow.
Casein micelles are very sensitive to pH changes. As an increase in the amount of lactic acid in milk, Colloidal Calcium Phosphate, an important building block of casein complex, dissolves away from the structure. Degradation of casein micelles is considered to be caused by differentiation of the surface of micelles. Increased acidity causes the coagulation by neutralizing the negative charge on the micelle surface.
The easiest clotting of casein occurs at pH 4.6. This pH is the isoelectric point of casein. At this point, the positive and negative electrical charges of the caseins are equal.
Heat treatment also has an effect on the coagulation of casein with acid effect. Denaturing serum proteins by heat treatment create–casein and S-S bridges, reducing electrostatic and steric repulsion, resulting in higher pH coagulation of caseins.
b) Coagulation of Casein by Enzymatic Effect
Proteolytic enzymes from some plant, animal and microbial sources coagulate casein. All of these enzymes belong to the group of acid proteases. In particular, one of the animal proteases rennet (chymosin) enzyme is used in cheese technology.
Coagulation of casein micelles by the effect of chymosin enzyme; (a) enzymatic proteolysis, (b) aggregation (agglomeration), and (c ) gelling.