Years ago,a friend of the family said that when he saw that a recipe called for buttermilk, he just went on to the next one. Buttermilk just wasn’t something he usually had in the fridge. It may not be for you, either, unless you make your own butter. After all, unlike “normal” sweet milk, buttermilk doesn’t taste very good to drink (though Wikipedia says it can be done). But if you bake from scratch, it’s worth keeping around.
Buttermilk started out as the liquid left over from churning cream into butter. It was sour because the cream had typically stood for a while at room temperature. These days, the buttermilk you can buy is almost always “cultured” — deliberately soured under safe conditions.
So — why does the dairy industry make a point of souring some of its milk? According to The Joy of Cooking, when the sugar in the milk breaks down, lactic acid is produced, and this causes “a tenderer curd” and eventually “a tenderer crumb in baking”. In other words, buttermilk biscuits, pancakes, and so forth are fluffier than those without buttermilk. That alone makes it worth using.
That said — there are a number of things you can substitute for it. I told our buttermilk-free friend about a trick I’d learned in Home Ec: If you stir a tablespoon of lemon juice or vinegar into a cup of milk and let it sit for at least 10 minutes — though more is probably better — it’ll thicken into something a lot like buttermilk. (I’ve had slightly better results with lemon juice.) Sour cream thinned with a little milk will work, too, and so does plain yogurt thinned the same way. (And so does flavored yogurt, but you have to make sure the flavor won’t clash with the other ingredients.) Though I’m no health expert, I’d recommend you use the buttermilk substitute as soon as it’s ready rather than trying to keep it around for later use.
Another thing you have to make sure of when cooking with buttermilk is to add something that will balance its acidity — most often baking soda. Without this, the original sour taste of the buttermilk will be everywhere in the dish. I forgot to add baking soda to my buttermilk pancake batter — once. Since then I’ve been known to get up in the middle of the night to throw in a spoonful, just in case. Baking soda will also work on milk that’s a little sour but not quite spoiled yet.
So there’s no need to skip recipes that call for buttermilk, even if you don’t have any.