Caseins constitute almost 80% of milk proteins. They occur in the form of micelles, small aggregates of the four types of casein, alfa-S1, alfa-S2, beta, and kappa casein, together with inorganic ions (see Figures 184.108.40.206 and 220.127.116.11). The micelles don't precipitate because of the steric repulsion due to their hairy surface, because the electric charge on their surface is negative, and because they are surrounded by water molecules linked to their surface. The micellar colloidal system is also responsible for the white color of milk.
Coagulation means destabilization of the milk colloidal structure and consequent switch from sol to gel. If the gel doesn't form, a flocculation takes place (e.g., by acidification of the milk). к-casein can be considered as formed by two distinct parts: a hydrophobic part (residues 1-105), associated with the other caseins; and a hydrophilic one (residues 106-169), containing complex sugar groups esterified to Thr residues, protruding into the watery environment, hence stabilizing the micelles for steric reasons. Rennet acts only on к-casein—the other forms of casein remain in their original status. к-casein, which is at the surface of the micelles, is attacked by rennet, which hydrolyzes the bond Phe105-Met106 to produce para-к casein and glycomacropeptide (which represents about 30% of original k-casein). The release of the glycomacropeptide causes the reduction of the surface potential from -20 mV to about -10 mV and also removes the steric stabilizing layer. When about 85% of the total к-casein is hydrolyzed, the colloidal stability of the micelles is reduced so much that they coagulate at temperature above 20°C in the presence of Ca2+.
Classically, the clotting events can be divided in two phases: cleaving and clotting. Cleaving is enzymatic (the enzyme cleaves the peptide bond of к-casein), doesn't require calcium ions, and is not dependent on temperature (even at 0°C, its speed is high). The second phase (clotting) is dependent on temperature and on availability of Ca2+ ions. The highest speed of curdling is reached at temperatures higher than 30°C, the optimum being between 35°C and 40°C, as can be expected for a reaction that in nature takes place inside the stomach of mammals.