I stick pretty tight to the expiration date on my buttermilk containers. I'll give it a few days--after all, I've had expired buttermilk at my grandparents' house and I'm still here to tell about it. Still, I often find myself needing a half cup here or a few tablespoons there for a recipe. And instead of going out to buy a quart of buttermilk, I've taken to making my own after I realized how you can "sour" regular milk.
The substitute process works much the same way commercially-produced buttermilk does: Dairy companies add lactic acid to regular milk to produce buttermilk. (This is the same bacteria that makes yogurt and cheese.) You can mimic the process with lemon juice or vinegar.
A Simple Substitute for Buttermilk
Add 1 tablespoon of vinegar or lemon juice to 1 cup of milk. Let it sit for 15 to 20 minutes for the milk to "sour."
It works like a charm. As a matter of fact, I used the buttermilk substitute this weekend in our Whole-Wheat Buttermilk Pancakes. You can't tell the difference.
Have you ever used this trick as a substitute for buttermilk? Tell us about your experience in the comments.