Should you suddenly find yourself confronted by a recipe that demands buttermilk, but don’t have time to run to the store, there are several possible substitutions. Modern buttermilk is just skim or low-fat milk that’s been curdled slighty with lactic acid-producing bacteria. But since acid is really acid (as long as it’s edible, of course) you can easily replicate the process at home.
To replicate a cup of buttermilk, just stir a tablespoon of fresh lemon juice or vinegar, OR two teaspoons of cream of tartar, into a cup of 2% or nonfat milk and let it sit for 10 minutes or so. Bingo. If truly pressed you can also use an equal amount of slightly thinned sour cream or yogurt in place of buttermilk.