Buttermilk is a rich, thick, and sour dairy product made from milk. Buttermilk tastes somewhat like plain yogurt and its thickness lies somewhere in between milk and heavy cream. The name "buttermilk" is actually deceiving, as there is no butter in modern buttermilk. Several decades ago buttermilk was made when heavy cream was churned into butter. Over the course of churning the fats solidified into butter and a liquid remained that was called buttermilk. The buttermilk was slightly sour due to the presence of airborne bacteria. Today, buttermilk is made by combining regular milk with several kinds of bacteria. This mixture is left at room temperature for about half a day to allow the bacteria to create the buttermilk. One gallon of milk makes about a half-pint of buttermilk.
How do the bacteria create buttermilk from milk? Producers use certain kinds of bacteria named Streptococcus lactis and Leuconostoc citrovorum. These bacteria use the lactose sugar in the milk as food and produce lactic acid, which lowers the pH of the milk. This acidic pH does two things; it gives the buttermilk its characteristic sour flavor and it causes some of the proteins in the milk, including the predominant protein called casein, to precipitate. This precipitation gives buttermilk its thick consistency.
Buttermilk is generally lower in fat than regular milk. It is high in potassium, vitamin B12, calcium, and riboflavin. Buttermilk also has less lactose than milk because the bacteria have used the lactose as food. People who are lactose intolerant often find buttermilk easier to digest than milk. Besides the nutritional benefits there are some old wives' tales as to other benefits of buttermilk. The Irish thought drinking buttermilk prevented hangovers and American settlers thought a glass of buttermilk protected a person from poison ivy. Women settlers often washed their faces in buttermilk for a soft and creamy complexion.
Buttermilk is commonly used in baked goods such as pancakes, muffins, and biscuits. The acidity of the buttermilk gives these products a more tender texture and a mild tangy taste. Buttermilk (an acid) is often mixed with baking powder (a base) to create little gas bubbles that make pancakes fluffier. Because buttermilk is so acidic it also makes an excellent marinade to tenderize meats. It is used to make salad dressings and can serve as a substitute for sour cream on baked potatoes. Buttermilk keeps longer than milk in the refrigerator because its acidity wards off contaminating microorganisms. It is generally best for drinking no more than a week after purchasing and good several weeks longer if it used in baked goods.
It is possible to make homemade buttermilk. Take milk that has not been pasteurized (the pasteurization process destroys buttermilk-producing bacteria present in the milk) and let it sit out at room temperature for 24 hours. After this time the milk should smell and taste sour and be visibly thicker. Buttermilk can also be made using other buttermilk as a starter. Simply take a small amount of purchased buttermilk and combine it with regular milk in a sealed container. Leave the container out at room temperature for about a day. At this point the batch should be all buttermilk. This works because the bacteria in the buttermilk use the lactose in the fresh milk to convert it to buttermilk.
Buttermilk is widely available today in the milk section of supermarkets. However, if there is no buttermilk available there are several easy substitutions that can be used instead. The easiest substitution is to make "sour milk". Add one tablespoon white vinegar or lemon juice to a measuring cup and top off to the one cup line with milk. Let this stand at room temperature for about ten minutes to allow the acid to curdle the milk. Plain, unsweetened yogurt can also be directly substituted for buttermilk. The yogurt can either be used straight or mixed with a bit of milk to thin it. Dehydrated buttermilk powder, much like dry milk, can also be found in specialized grocery stores.
BlueDragon adds:"It's not widely available in the UK, in face I dont recall seeing it anywhere. Possibly in Health Food stores, but I rarely go there.."