Buttermilk is the liquid
left over after producing butter from full-cream milk by
the churning process. It has a slightly sour taste. It is
quite popular as a refreshment in India and a variant
called lassi is sold commercially.
From Wikibooks, the
open-content textbooks collection
Buttermilk may contain
amounts of calcium from its dairy products, such as nonfat
dry-milk powder. The sour taste, or tartness of
"cultured buttermilk" is owing to the
fermentation process, which, for buttermilk, begins with a
commercial product of buttermilk which has become
chemically active, or rather fermented.