Some ingredients are just frustrating to buy, because you know you're always going to have leftovers. Fresh herbs are a classic example: You need a tablespoon or two that are freshly chopped, but you have to buy the huge bouquet of sprigs that'll slowly die in your vegetable crisper. Buttermilk is the same way. You'll need a cup or a half-cup for a specific recipe, and then the rest goes bad in the fridge door. If your old buttermilk isn't usable, or you just don't bake often enough to keep it around, you can sour some regular milk as a substitute.
Why It's Got to Be Sour
Part of the reason for using buttermilk in a recipe is that it just tastes good. That buttermilk tang is part of a lot of classic recipes, from waffles and pancakes to biscuits and coffee cakes. It's not there just for the flavor, though. Most recipes that call for buttermilk use it together with baking soda, to leaven the recipe and make it rise. Baking soda is sodium bicarbonate, which is alkaline. Buttermilk is acidic, and when you stir the two together in your batter, they react to form fine bubbles of carbon dioxide. Those bubbles expand in the heat of the oven – or waffle iron – to give you the light, fluffy result you're looking for.
Making and Using Sour Milk
The thing about buttermilk is that, unless you bake really, really regularly, you probably won't always have it on hand. You might happen to have some milk in your fridge that's way past its prime, and milk that sours naturally is a good substitute, but you probably can't count on that as a regular thing. Also, if it's really old, it can have unpleasant "off" flavors. Most of the time you'll have to take some regular milk and sour it deliberately, to make a substitute for buttermilk in your recipes. It's not hard – you just have to add a tablespoon of vinegar or lemon juice to each cup of milk and wait a few minutes. It's faster if you warm the milk a little. This gives you the acidity you need to work with the baking soda, but you won't get the genuine buttermilk flavor.
More Flavorful Options
If you'd like to find a substitute that tastes more like buttermilk, you may have an option or two already in your fridge. Plain yogurt doesn't taste exactly like buttermilk, but they're both cultured milk products and that gives them the same pleasant tang. If you've got extra-thick Greek-style yogurt, you might need to thin it with a bit of regular milk to keep the batter from being too dry. Another option is kefir, a lightly fermented milk beverage that tastes a lot like yogurt. If you keep unsweetened kefir around, or have a flavored version that's compatible with the flavors in your recipe, that'll work pretty well.
Keeping Buttermilk Around
If you'd really rather just use buttermilk, it's not hard to keep around to use when you need it. Your best bet is to freeze it in whatever quantity your favorite recipes call for – a cup, half-cup, quarter-cup, or whatever else – and then just thaw it when you need it. It'll keep for months this way. You can also buy buttermilk powder in bulk, and use that in place of liquid buttermilk in your recipes. It doesn't reconstitute well, so it's simpler just to add the powder to your dry ingredients and then increase your liquids with the corresponding amount of water.