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, South Asia, 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.
Originally, buttermilk referred to the 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.
Traditional buttermilk is still common in many Indian, Nepalese, and Pakistani households, but rarely found in Western countries. In Nepal, buttermilk is called mohi and is a common drink in many Nepalese homes. It is served to family members and guests, and can be taken with meals or snacks. In many families, it is most popularly served with roasted maize.
Cultured buttermilk was first commercially introduced in the United States in the 1920s. Commercially available cultured buttermilk is milk that has been pasteurized and homogenized, and then inoculated with a culture of Lactococcus lactis or Lactobacillus bulgaricus plus Leuconostoc citrovorum to simulate the naturally occurring bacteria in the old-fashioned product. The tartness of cultured buttermilk is primarily due to lactic acid produced by lactic acid bacteria while fermenting lactose, the primary sugar in milk. As the bacteria produce lactic acid, the pH of the milk decreases and casein, the primary milk protein, precipitates, causing the curdling or clabbering of milk. This process makes buttermilk thicker than plain milk. While both traditional and cultured buttermilk contain lactic acid, traditional buttermilk tends to be less viscous, whereas cultured buttermilk is more viscous.
When introduced, cultured buttermilk was popular among immigrants, and viewed as a food that could slow aging. It reached peak annual sales of 517,000,000 kilograms (1.140×109 lb) in 1960. Buttermilk's popularity has declined since then, despite an increasing population, and annual sales in 2012 reached less than half that number.
However, condensed buttermilk and dried buttermilk remain important in the food industry. Liquid buttermilk is used primarily in the commercial preparation of baked goods and cheese. Buttermilk solids are used in ice cream manufacturing, as well as being added to pancake mixes to make buttermilk pancakes.
Acidified buttermilk is a substitute made by adding a food-grade acid such as vinegar or lemon juice to milk. It can be produced by mixing 1 tablespoon (0.5 US fluid ounces, 15 ml) of acid with 1 cup (8 US fluid ounces, 240 ml) of milk and letting it sit until it curdles, about 10 minutes. Any level of fat content for the milk ingredient may be used, but whole milk is usually used for baking. In the process which is used to produce paneer, such acidification is done in the presence of heat.
Buttermilk is comparable to regular milk in terms of calories and fat. One cup (237 ml) of whole milk contains 157 calories and 8.9 grams of fat. One cup of whole buttermilk contains 152 calories and 8.1 grams of total fat. Low fat buttermilk is also available.  Buttermilk contains vitamins, potassium, calcium, and traces of phosphorus.