Whitish and with a sour taste, buttermilk is, in theory, the liquid that separates from the cream when making butter. Buttermilk found in supermarkets today is manufactured differently: it is skim milk or partly skimmed milk with added bacterial culture. It comes in a quart container: if you buy it to only make this mashed potato recipe, you can use the remaining to make a dressing with fresh herbs, add to smoothies or to prepare pancake batter. It is also used in the preparation of several cakes and muffins. Once opened, the buttermilk will keep for one week in the refrigerator or three months in the freezer.