Cheese is a nutritious food made from the milk of various animals, with a high protein, calcium, and vitamin content. Cheesemaking was already common by 2000 BC. It involves first the curdling of milk by adding an acid or rennet, so that the fat and protein (mostly casein) coagulate to form the solid curds. After excess liquid whey has been drained off, the curds are compressed and enough moisture is removed to give the cheese the desired degree of hardness. Most cheeses (but not cottage cheese) are then subjected to a period of fermentation, from two weeks to two years, called ripening or curing, during which they are salted and perhaps flavored. The consistency and flavor of the cheese depend on the time, temperature, and humidity of storage and on the microorganisms present. Camembert, for instance, is ripened with two molds, Penicillium candidum and P. camemberti, which make it soft.