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Buttermilk is a fermented dairy drink. Traditionally, it was the liquid left behind after churning butter out of cultured cream. As most modern butter in Western countries is not made with cultured cream but uncultured sweet cream, most modern buttermilk in Western countries is cultured separately. It is common in warm climates where unrefrigerated milk sours quickly. Buttermilk can be drank straight, and it can also be used in cooking. In making soda bread, the acid in buttermilk reacts with the raising agent, sodium bicarbonate, to produce carbon dioxide which acts as the leavening agent. Buttermilk is also used in marination, especially of chicken and pork. Originally, buttermilk referred to the thin liquid left over from churning butter from cultured or fermented cream. Traditionally, before the advent of homogenization, the milk was left to sit for a period of time to allow the cream and milk to separate. During this time, naturally occurring lactic acid-producing bacteria in the milk fermented it. This facilitates the butter churning process, since fat from cream with a lower pH coalesces more readily than that of fresh cream. The acidic environment also helps prevent potentially harmful microorganisms from growing, increasing shelf life.

Source: Wikipedia