While bacteria spoil many foods, there are a few foods that they actually preserve instead. Old-fashioned pickles, sourdough bread starters and buttermilk provide just a few notable examples. These foods are colonized by bacteria that produce lactic acid, which deters the growth of less-desirable bacteria and molds. Buttermilk, gains its tang and its shelf life from these bacteria. Ordinary milk can spoil in a matter of days, but buttermilk can remain usable well past its best-by date.
Understanding the Dates
Buttermilk isn't packaged with an actual expiration date, which is reserved for foods that can become dangerous over time. Instead, buttermilk packaging is imprinted by the dairy with a variety of less-stringent dates. Some employ a "sell by" date, to let retailers know when it should be discarded. Others print a "use by" or "best before" date, which represents the dairy's estimate -- usually conservative -- of how long the buttermilk will retain its best flavor. None of these are hard and fast, because the shelf life of your buttermilk depends largely on how carefully you keep it cold. You can typically use buttermilk freely for one to two weeks past that date, and sometimes longer.
Check It First
Newly purchased buttermilk has a distinctive aroma of its own. Although it's definitely tangy and has cheese-like overtones, its smell is clean and fresh. If you detect a sudden change in its smell, that's usually a sign of spoilage. Molds and unwanted bacteria can give your buttermilk a funky note reminiscent of blue cheeses, or a sharply unpleasant odor of well-aged gym socks. If your buttermilk begins to discolor, or to separate into thin liquid and solid chunks, it's also past its prime and should be discarded. A few lumps are normal in buttermilk, but the surrounding liquid should still be thick and smell fresh.
Doughs and Batters
Buttermilk's flavor can become stronger with time, so using it in doughs and batters -- where its flavor will be masked -- is often a good option. Many recipes for pancakes, biscuits, muffins and cakes call for buttermilk or soured milk to help them rise, or lend them a pleasantly tangy flavor. If your favorite recipe calls for sweet milk, you can readily adapt it. Just replace each cup of sweet milk with buttermilk, and each teaspoon of baking powder with 1/4 teaspoon of baking soda. The soda interacts with your buttermilk, creating carbon dioxide bubbles that leaven your recipe.
Although buttermilk's flavor isn't universally appreciated, it's a refreshing cold beverage in its own right. If you enjoy buttermilk but find its flavor too strong after it passes its freshness date, consider using it to make Indian-style "lassi" instead. This traditional Indian drink calls for thinning buttermilk or plain yogurt, then mixing it with spices or fruit and serving it with ice. Alternatively, incorporate it into smoothies in place of yogurt or sweet milk. It's also frequently incorporated at the last minute into hot and cold soups to lend body and flavor, another use that dilutes and masks the stronger flavor of old buttermilk.