Just like its name describe, powdered milk is milk with almost every drop of water in it removed to create a powder. You can buy powdered milk in three forms: (1) dried whole milk, (2) non-fat dry milk, and (3) powdered buttermilk. Dried whole milk must always be refrigerated. Nonfat dry milk comes in both regular (tastes better) and instant (mixes easier) forms. Powdered buttermilk is used mostly for baking. Once opened, powdered milk should be stored in a refrigerator to retain freshness. Except for dried whole milk powder, if unopened, the powdered milk can be stored in a cool, dry place for up to 6 months.