Yogurt is a fermented milk product produced by bacterial fermentation of milk. The bacteria used to make yogurt are known as "yogurt cultures". Fermentation of lactose by these bacteria produces lactic acid, which acts on milk protein to give yogurt its texture and its characteristic tang.
Worldwide, cow's milk, the protein of which mainly comprises casein, is most commonly used to make yogurt, but milk from water buffalo, goats, ewes, mares, camels, and yaks is also used in various parts of the world.
In Western culture, the milk is first heated to about 80 °C (176 °F) to kill any undesirable bacteria and to denature the milk proteins so that they set together rather than form curds. In some places, such as parts of India, curds are a desired component and milk is not pasteurized but boiled. The milk is then cooled to about 45 °C (112 °F). The bacterial culture is added, and the temperature is maintained for 4 to 7 hours to allow fermentation.
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