Traditionally, buttermilk (a low-fat mixture of milk and cream) was the liquid that was left after churning butter. That liquid, which was allowed to ferment, had a thick texture and acidic flavor. It had a subtler, more complex, and smoother flavor and texture than the buttermilk sold in most supermarkets today; traditional buttermilk is, however, a less-consistent product and spoils more easily.
The buttermilk widely available today is made in a similar way to yogurt and is known as “cultured buttermilk.” Bacterial cultures are added to milk, and the mixture is heated at a low temperature for several hours. The bacteria cause the milk to ferment and thicken, resulting in buttermilk’s creamy texture and its slightly tart flavor. The fermentation process is stopped earlier than yogurt’s, however, resulting in a final product that is still liquid.
Buttermilk is primarily used in baked goods, though it can be used in ice cream, and some people enjoy drinking it straight. Buttermilk is also available in powdered form, which is mostly used for baking.