Buttermilk refers to a number of dairy drinks. Originally, buttermilk was the liquid left behind after churning butter out of cream. This type of buttermilk is known as traditional buttermilk.
The term buttermilk also refers to a range of fermented milk drinks, common in warm climates (e.g., Middle East, Pakistan, India, and the Southern United States) where unrefrigerated fresh milk otherwise sours quickly, as well as in colder climates such as Germany, Poland, Scandinavia and the Netherlands. This fermented dairy product known as cultured buttermilk is produced from cow's milk and has a characteristically sour taste caused by lactic acid bacteria. This variant is made using one of two species of bacteria—either Streptococcus lactis or Lactobacillus bulgaricus, which creates more tartness.
The tartness of buttermilk is due to acid in the milk. The increased acidity 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.
Buttermilk is usually drunk straight, but it can also be used in cooking. Soda bread is a bread where the buttermilk acidifies the rising agent, sodium bicarbonate, to produce carbon dioxide.
Read more about Buttermilk: Traditional Buttermilk, Cultured Buttermilk, Acidified Buttermilk, Powdered Buttermilk