Buttermilk

Buttermilk is the liquid that remains after sweet (or sour) milk is churned to remove the fat. Commercially produced buttermilk, called cultured buttermilk, is made by adding a bacterial culture to pasteurized skim milk; this converts the milk sugar to lactic acid and gives the buttermilk its characteristic slightly tart taste. Because of its acidity, buttermilk is often used in pastry recipes containing baking soda. See Baking soda.