Casein
The main protein in milk, which clumps together to form the curd from which every cheese is made.
Also: casein micelle, milk protein
Casein makes up about four fifths of the protein in cow milk. It floats in milk as microscopic clusters called micelles, held apart by a hairy outer layer of one casein fraction, kappa-casein. Cheesemaking depends on making those micelles stick together. Rennet snips off the hairy layer so the micelles join into an elastic gel, which is the rennet curd behind most aged cheeses. Acid does the same job differently, by removing the electrical charge that keeps them apart, which gives the crumbly curd of paneer and chhena. The other milk proteins, the whey proteins, stay in the liquid and can only be recovered by heat, which is how ricotta is made. Casein is also what breaks down during ripening.