In the “old days”, buttermilk was the liquid left over from churning the butter. Lovely stuff it was, with flecks of butter floating around in it–a tart thirst quencher when served icy cold.
Today’s buttermilk is made by adding a culture to non-fat or low-fat milk to give it a little tang and thicken the texture. It still makes a good beverage, and is especially nice for cooking. Try it for pancakes, and cornbread. Biscuits will get a nice rise from the acidity in buttermilk.
Buttermilk has quite a long shelf life in the fridge. For those of you who just guzzle it straight, you’ll want it freshly opened. But you can cook with the stuff well past the expiration date. Even though it’s “sour”, you’ll know when it’s gone bad —it’ll get watery, separate, and acquire unsavory chunks of gunk.