buttermilk The residue left after churning butter, 0.1–2% fat, with the other constituents of milk proportionally increased. It is slightly acidic, with a distinctive flavour due to the presence of diacetyl and other substances. Usually made by adding lactic bacteria to skim milk; 90–92% water, 4% lactose with acidic flavour from lactic acid, it is similar to skim milk in composition. Dried buttermilk is used in bakery products and ice cream.

but·ter·milk / ˈbətərˌmilk/ • n. the slightly sour liquid left after butter has been churned, used in baking or consumed as a drink. ∎  a pale yellow color (used esp. to describe paint or wallpaper): [as adj.] buttermilk paintwork.