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.
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.
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. 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.
Nutritional value and health benefitsYogurt is nutritionally rich in protein, calcium, riboflavin, vitamin B6 and vitamin B12. It has nutritional benefits beyond those of milk. Lactose-intolerant individuals can sometimes tolerate yogurt better than other dairy products, because the lactose in the milk is converted to glucose and galactose, and partially fermented to lactic acid, by the bacterial culture.
Yogurt containing live cultures has been found effective in a randomized trial at preventing antibiotic-associated diarrhea. Yogurt contains varying amounts of fat. There is non-fat (0% fat), low-fat (usually 2% fat) and plain or whole milk yogurt (4% fat). A study published in the International Journal of Obesity (11 January 2005) also found that the consumption of low-fat yogurt can promote weight loss.
Yogurt is a valuable health food for both infants and elderly persons. For children, it is a balanced source of protein, fats, carbohydrates, and minerals in a texture that kids love. For senior citizens, who usually have more sensitive colons or whose intestines have run out of lactose, yogurt is also a valuable food. Elderly intestines showed declining levels of bifidus bacteria, which allow the growth of toxin-producing and, perhaps, cancer-causing bacteria  . Yogurt may help prevent osteoporosis, reduce the risk of high blood pressure. Yogurt with active cultures helps the gut, may discourage vaginal infections, and may help you feel fuller .
SOURCE: From Wikipedia, the free encyclopedia