Buttermilk is a fermented dairy drink. Traditionally, it was the liquid left behind after churning butter out of cultured cream; most modern buttermilk is cultured, however. It is common in warm climates (including the Balkans, India, the Middle East and the Southern United States) where unrefrigerated fresh milk sours quickly.Buttermilk can be drunk 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, which the lactic acid helps to tenderize, retain moisture and allows added flavors to permeate the meat.