Old-fashioned buttermilk was exactly what the name implies, the thin and milky liquid left over after cream was churned for butter. It was often tangy, because the cream itself had been colonized by beneficial bacteria that converted the milk’s natural sugars into lactic acid. Modern buttermilk cuts out the intermediate steps, adding bacterial cultures directly to — usually — low-fat or non-fat milk. It’s a simple process, so if you’re running low on buttermilk, you can easily make your own.

Culturing Your Own

Cheesemaking-supply shops stock powdered bacterial cultures for making buttermilk or sour cream, but it’s simplest to just use the cultured buttermilk that’s already in your fridge as a starter. The live bacteria in the buttermilk die over time as they consume its lactose, so use fresh buttermilk. Just combine buttermilk and fresh milk in a sterile Mason jar, using one part of starter for every three parts of milk. Leave the sealed jar at room temperature — 75 to 80 degrees Fahrenheit — for about 24 hours, and your milk should visibly thicken to the familiar buttermilk consistency. Refrigerate and use it as usual, and save back a portion to act as the starter for your next batch.

Faking It

In baking and some cooking tasks, such as pancake-making, buttermilk is used as an acidic ingredient to activate the baking soda and help your foods rise. If you’re out of buttermilk and don’t have 24 hours at your disposal, you can replace it by deliberately souring an equivalent quantity of fresh milk. For every cup of buttermilk your recipe calls for, pour a tablespoon of vinegar or lemon juice into your measuring cup, and then top it up with milk. After a few minutes, the milk will sour and clabber and can be used in place of the buttermilk. It won’t have buttermilk’s distinctive flavor, but it will provide the necessary leavening power.