cheese, nutritiousfood consisting primarily of thecurd, the semisolid substance formed whenmilk curdles, or coagulates. Curdling occurs naturally if milk is not used promptly: it sours, forming anacid curd, which releaseswhey, a watery fluid containing thesoluble constituents; and it leaves semisolid curd, or fresh cheese. In some areas, cheese is still made simply by allowing milk to curdle naturally, or by mixing milk with juices or extracts that reduce it to curds and whey. A brief treatment of cheese follows. For full treatment,seedairy product: Cheese andcheese making.

Cheese making probably originated soon after humans first took milk from wild or domesticated animals. The Bible refers to “cheese of the herd” being given to King David. Milk from cows, and presumably from other animals, was used for cheese making by about 1000bce. The ancient Greeks and Romans knew and valued cheese, as did early people in northernEurope. Methods for cheese making have often been kept secret.

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Important as a preservation technique, cheese making reduces about 10 volumes of milk to one volume of cheese. The curd, or soft gel, is formed by the chiefprotein in milk,casein, when enoughlactic acid is developed fromlactose (milk sugar) by the microorganisms present in the milk or when it is acted upon byrennin. Rennin is anenzyme usually obtained from the stomach of young calves in a brine extract called rennet. At the moment of coagulation, all the milkconstituents, including most of thefat, casein, and other water-insoluble substances, are contained in the curd. The curd is cut or broken to release the whey, but a portion of the whey is always retained in the curd.

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Hundreds of varieties of cheese are made from the milk of cows, goats, sheep,water buffalo, horses, llamas, and yaks (see alsolist of cheeses). Products vary according to the selection and treatment of the milk; adjustment of its fat content; heating or pasteurizing; and addition of enzymes orcultures ofbacteria, molds, or yeasts. Curd formation varies with changes in temperature, time, acidity for coagulation, proportions of rennet and acid, and the speed and extent of removal of the whey. Ripening and curing consists of biological and chemical changes that occur in the cheese and are affected by the moisture content, acidity, texture, shape, size, and microorganisms in the cheese. These changes alter the consistency as well as the flavour of the cheese. Before ripening, cheese is said to be fresh or green; after ripening it is called cured, aged, or ripened. The chemical changes may be classified broadly as the breakdown of fats to fatty acids; proteins to amino acids; and lactose to such products as lactic, acetic, and propionic acids, diacetyl, andcarbon dioxide. Flavourful products of ripening include volatile fatty acids, ketones, esters, alcohols, peptides, amino acids, ammonia, andhydrogen sulfide.

In modern factories, cheese is mass-produced according to standardized recipes and techniques that result in a more uniform product. It is not necessarily of higher quality, and there are fewer varieties. Cheese making has been of primary economic importance for hundreds of years in the Netherlands,France,Italy, andSwitzerland. Certain cheeses are historically associated with particular areas. Keeping quality isenhanced by a variety of procedures, including heating and kneading (Italianprovolone) and soaking in brine (Egyptian Domiati). The loosely knit structure of FrenchRoquefort develops bluemold only in a cool, wet atmosphere such as occurs naturally in the caves of Roquefort.