Yogurt, yoghurt, or yoghourt ( or ; from ; other spellings listed below) is a food 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 is mainly casein, is most commonly used to make yogurt. Milk from water buffalo, goats, ewes, mares, camels, and yaks however, is also used to produce yogurt in various parts of the world. Dairy yogurt is produced using a culture of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus bacteria. In addition, other lactobacilli and bifidobacteria are also sometimes added during or after culturing yogurt. Some countries require yogurt to contain a certain amount of colony-forming units of microorganisms.Swiss Food Law: Article 56, Yogurt, section 2: “The final product must contain a total of at least 10 million colony forming units of microorganisms under paragraph 1 or 1.2 per gram.” In Western culture, the milk is first heated to about 85 °C (185 °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 & Bangladesh curds are a desired component and milk is not pasteurized but boiled. The milk is then cooled to about 45 °C (113 °F). The bacterial culture is added, and the temperature of 45 °C is maintained for 4 to 7 hours to allow fermentation.