Buttermilk

Buttermilk is the liquid left over after producing butter from full-cream milk during the churning process. It has a slightly sour taste. It is quite popular as a refreshment in Northern Europe and South Asia, particularly in Afghanistan, Punjab and the Pashtoon belt in Pakistan and in India. Many breads are made with buttermilk, and it is also used in creamy soups and sauces.

 The sour taste, or tartness of "cultured buttermilk" is a result of a fermentation process in which the bacteria turn lactose into lactic acid.  At this point, casein, a milk protein, precipitates as it is no longer soluble under acidic conditions, causing what is called clabbering or curdling. The acidity of buttermilk inhibits bacterial growth, and this gives it a long refrigeration life. This process can be repeated when making sour cream with slight alterations.

Traditional buttermilk is quite different from cultured buttermilk: it is thin and slightly acid, while cultured buttermilk is thick and tart.