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Buttermilk is a fermented dairy product produced from cow's milk with a characteristically sour taste. There are two ways in which this product is made. 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 have been added to milk.^[1] 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.^[1]

In the early 1900's, cultured buttermilk were labeled as artificial buttermilk, to differentiate it from traditional buttermilk, which was also known as natural or ordinary buttermilk.^[2]

Acidified buttermilk is a related product that is made by adding a food-grade acid to milk.^[3]

The fermentation that takes place in traditional buttermilk is accomplished by wild strains of lactic acid bacteria acquired from the environment. Traditionally, before cream was skimmed from whole milk, it was left to sit for a period of time to allow the cream and milk to separate. During this time, the milk would naturally be fermented by the lactic acid bacteria in the milk. One reason this was done was to facilitate the butter churning process since cream with a lower pH will congeal more readily than fresh cream.^{[citation needed]} The acidic environment helped prevent potentially harmful microorganisms from growing, thus the soured liquid helped increase the shelf-life of the product.

Commercially available cultured buttermilk is pasteurized and homogenized milk which is 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. ^[1]

Today, traditional buttermilk is rarely found. Adding specific strains of bacteria to pasteurized milk has allowed for more consistent production.

A fresh batch of English buttermilk scones

Sour cream is produced using a similar process as the one used for cultured buttermilk. The primary difference is that sour cream begins with cream as opposed to milk, and it sometimes contains thickening agents.

For recipes, a substitute for buttermilk can be made by adding 1 tablespoon of lemon juice or vinegar or 1 3/4 teaspoons cream of tartar to each cup of regular milk. The soured milk should be allowed to sit for ten minutes before being used. ^[4]

• Whey, the liquid left over after producing cheese.

• Making Buttermilk