Made by churning cream until it reaches a semisolid state, butter must by U.S. law be at least 80 percent milk fat. The remaining 20 percent consists of water and milk solids. AA and A grades are those most commonly found at the retail level. Unsalted butter is usually labeled as such and contains absolutely no salt.
Unsalted butter is preferred by many for everyday eating and baking.
Whipped butter has had air beaten into it, thereby increasing volume and creating a softer, more spreadable consistency when cold. It comes in salted and unsalted forms. Light or reduced-calorie butter has about half the fat of regular butter, possible through the addition of water, skim milk and gelatin. It shouldn't be substituted for regular butter or margarine in frying and baking. Refrigerate regular butter for up to 1 month, unsalted butter for up to 2 weeks.