The liquid left behind after churning cream into butter is called “traditional” or “old fashioned” buttermilk. It is tart due to the lactic acid in the cream which has begun to ferment, making churning easier. Most versions you’ll find at a grocery store, on the other hand, are “cultured” buttermilk and are not the same as “old fashioned.” Cultured buttermilk is pasteurized and homogenized, and then a lactic acid bacteria culture is added to simulate the fermentation effect. Cultured buttermilk is much thicker and more tart than old fashioned buttermilk, which is thin like regular milk. Therefore, if you’re looking for real buttermilk, ask around at the market for a farmer who makes his/her own butter.
Fun Fact: The term “buttermilk” may also refer to other fermented milk beverages (not always cow’s milk) popular in countries where refrigeration isn’t the norm, including Germany and Scandinavia, which both have significant cultural influences in Wisconsin.
Nutrition: Buttermilk is right in the middle of the caloric ratio triangle: 39 percent carbs, 32 percent fats, and 29 percent protein. It is also a very good source of calcium.