Buttermilk refers to a number of dairy drinks. Originally, buttermilk was the liquid left behind after churning butter out of cream. It also refers to a range of fermented milk drinks, common in warm climates where fresh milk would otherwise sour quickly.
Buttermilk may also refer to a fermented dairy product produced from cow's milk with a characteristically sour taste caused by lactic acid bacteria. This variant is made in one of two ways:cultured buttermilk is made by adding lactic acid bacteria to milk; Bulgarian buttermilk is created with a different strain of bacteria called Lactobacillus bulgaricus, which creates more tartness.
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 byproduct naturally produced by lactic acid bacteria while fermenting lactose, the primary sugar found in milk.
