Coagulation
The moment liquid milk turns into a soft solid gel, the first real step in making cheese.
Also: setting, curdling, clotting
Coagulation is the setting of milk. The casein proteins in milk float apart; when rennet or acid is added, they join into a network that traps fat and water, and the whole vat becomes a smooth, wobbly gel, like a set custard. With rennet, at about thirty degrees, this takes thirty to sixty minutes. With heat and acid, as in paneer, it happens in seconds and gives a looser, grainier curd. Slow lactic setting can take a whole day. The cheesemaker tests the set with a finger or knife, looking for a clean break, the moment the gel splits cleanly and clear whey runs into the gap. Cutting too early or too late changes the cheese.