Buttermilk is a fermented dairy product produced from cow's milk with a characteristically sour taste. The product is made in one of two ways. Originally, buttermilk was the liquid left over from churning butter from cream. Today, this is called traditional buttermilk. Buttermilk also refers to cultured buttermilk, a product where lactic acid bacteria called Streptococcus lactis have been added to milk. Whether traditional or cultured, the tartness of buttermilk is due to the presence of acid in the milk. The increased acidity is primarily due to lactic acid, a by-product naturally produced by lactic acid bacteria while fermenting lactose, the primary sugar found in milk. As lactic acid is produced by the bacteria, the pH of the milk decreases and casein, the primary protein in milk, 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 thinner whereas cultured buttermilk is much thicker.
In the early 1900's, cultured buttermilk was labeled artificial buttermilk, to differentiate it from traditional buttermilk, which was also known as natural or ordinary buttermilk.
Acidified buttermilk is a related product that is made by adding a food-grade acid to milk.
Commercially available cultured buttermilk is pasteurized and homogenized milk which has been inoculated with a culture of lactic acid bacteria to simulate the naturally occurring bacteria found in the old-fashioned product. Some dairies add colored flecks of butter to cultured buttermilk to simulate the residual pieces of butter that can be left over from the churning process of traditional buttermilk.
Buttermilk solids have increased in importance in the food industry. Such solids are used in ice cream manufacture.
Today, traditional buttermilk is rarely found. Adding specific strains of bacteria to pasteurized milk has allowed for more consistent production.
Buttermilk is lower in fat and calories than regular milk because the fat from buttermilk has already been removed to make butter. It is high in potassium, vitamin B12 and calcium. Buttermilk is more easily digestible than whole milk and it also contains more lactic acid than skim milk. Due to being more easily digestable (a result of the bacteria added to the milk), protein and calcium can be taken up more easily by the body. There are 99 calories and 2.2 grams of fat in one cup of buttermilk (fat content may be different with some buttermilk brands), as opposed to whole milk that has 157 calories and 8.9 grams of fat.
A low-fat substitute can be made by mixing equal parts of skim milk and low-fat yogurt.