Buttermilk

Originally, buttermilk was the by-product of the butter-making process — the liquid left after cream is churned into butter. Today it is made from pasteurised skim milk to which an acid-producing bacteria is added, thickening it and giving it its characteristic tanginess. Using buttermilk instead of milk in baked goods such as muffins or scones results in a softer texture and a good crust, and also adds to the flavour.