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Yogurt is a fermented milk product that has a custard-like consistency and a tart flavor. Yogurt is made from the milk of cows, goats, sheep, or water buffalo. Commercial yogurt is available plain or sweetened and flavored with various berries, fruits, vanilla, or coffee. Yogurt is rich in protein and calcium. Its caloric value depends on the fat content of the milk that is used.
Yogurt can be made by boiling milk until about one-third or one-half of it has evaporated. After the milk is cooled, a culture of certain bacteria or a small amount of yogurt containing the bacteria is added. As the bacteria grow in the milk, they change its lactose, or milk sugar, into lactic acid and break down the large protein molecules into smaller molecules. The milk is then either kept at about 110° F (43° C) for 3 or 4 hours or is left for about 24 hours at room temperature. Flavoring may be added. The yogurt keeps best if stored in a refrigerator.
Yogurt has been an important food in the Middle East and in southeastern Europe for many years. In areas where refrigeration was scarce, milk could be preserved by adding yogurt-making bacteria, which grow rapidly and crowd out milk-spoiling organisms.