buttermilk
Also known as: Butter Milk
A tangy, slightly thick cultured dairy product -- most sold today is milk cultured with bacteria rather than the traditional byproduct of churning butter -- used in baking to add tenderness and tang and to activate baking soda. Its acidity reacts with baking soda to produce carbon dioxide, giving biscuits and pancakes made with it extra lift, and it tenderizes gluten for a softer crumb.
Buying guidance
Full-fat buttermilk gives a richer result than low-fat, but either works for leavening purposes. Check the sell-by date and that the container is cold and sealed -- buttermilk is cultured and slightly sour by design, but it should smell pleasantly tangy, not sharply off.
Substitutions
- Milk plus lemon juice or vinegar (about 1 tablespoon acid per cup of milk, rested 5-10 minutes) -- close enough for most baking, though thinner-bodied than real buttermilk.
- Plain yogurt thinned with a little milk -- closer in thickness and tang than the lemon-juice trick.
- Kefir -- similar tang and acidity, works interchangeably in most baking.
Tips & common mistakes
Pouring it directly into a very hot pan or liquid can make it curdle and look broken -- whisk a little of the hot liquid into the buttermilk first to bring it up to temperature gradually before combining the rest. A homemade stand-in of milk with vinegar or lemon juice supplies the acidity for leavening but not real buttermilk's thickness, so batters made with it tend to come out noticeably thinner.
Storage
It keeps meaningfully longer past its printed date than regular milk, since it's already cultured and acidic -- a bit more tang than usual isn't a sign it's gone bad. Visible curdling into distinct lumps and liquid, or a sharp, off smell, is what actually means it should be discarded.