Buttermilk is a dairy drink made by adding lactic acid bacteria to milk to produce a fermented dairy drink. Traditionally, it was made from the nearly fat-free milk remaining after churning butter from cream, which was cultured with natural bacteria prior to and during churning, giving a slight sour taste to the buttermilk. However, with the ubiquity of refrigeration in industrialized countries, butter in those areas is typically made from uncultured or "sweet" cream. Therefore, most modern buttermilk is specifically produced by inoculating fresh, pasteurized milk, and is available in different levels of fat content.
Buttermilk is consumed as a beverage and used in cooking. Drinking buttermilk remains common in warmer climates where unrefrigerated milk sours quickly, as the fermentation prevents further spoilage. In making soda bread, the acid in fermented buttermilk reacts with the leavening agent, sodium bicarbonate, to produce carbon dioxide. Buttermilk is used in marination, especially for chicken and pork.