Although buttermilk is not as widely used as it once was it is often used in recipes to create a light and tender baking result, often with a slightly tangy flavour. Buttermilk has more acid and is usually thicker than regular milk.
Originally buttermilk was the naturally fermented liquid left behind after churning butter out of cream ("traditional buttermilk"). More common now is "cultured" buttermilk, milk to which a lactic acid culture has been added, converting the lactose into lactic acid and creating the soured milk.
The lactic acid in buttermilk is responsible for both the characteristic sour flavour and the leavening power. When the acid is combined with baking soda (sodium bicarbonate) and heated, carbon dioxide is produced and the gas bubbles cause the rising.
If you find yourself with a recipe calling for buttermilk and none on hand, there are a few substitutions which produce similar texture but not identical flavour.
Possible substitutions for 1 cup buttermilk:
- Take 1 tablespoon of fresh lemon juice and add milk to make 1 cup (skim, low fat or whole milk). Let the mixture stand for 5-10 minutes before using in recipe. Vinegar can be used instead of lemon juice.
- Add 1 1/2 teaspoon Cream of Tartar (an acid) to dry ingredients or stir carefully into 1 cup milk. This will create the desired rising effect in the baking but not necessarily the tangy flavour.
- Replace with an equal amount of yogurt.