Food consisting of the coagulated, compressed, and usually ripened curd of milk separated from the whey. When milk sours, it forms both a protein-rich gel, or curd, and a lactose-rich fluid, or whey. Coagulation is often facilitated by adding rennin, an enzyme that acts on the milk's chief protein, casein. The resulting curd is then cut or broken to release most of the whey. Ripening and curing are affected by moisture content, acidity, presence of microorganisms, and other factors. Cheese is made from the milk of cows, goats, sheep, water buffalo, llamas, yaks, and other animals; in the West, cow's milk is most common. Products vary according to fat content of the milk, heating or pasteurization, and addition of enzymes or cultures of bacteria, molds, or yeasts. Cheese varieties include hard cheeses (e.g., cheddar, Edam, Emmental, Gouda, Provolone, Romano, Swiss), semisoft cheeses (Gorgonzola, Limburger, Muenster, Roquefort), and soft cheeses (Brie, Camembert, cottage, Neufchâtel, and ricotta). Cheese is a source of protein, fat, minerals (calcium, phosphorus, sulfur, iron), and vitamin A.