Developed 100 years ago as a less costly substitute for butter, this hydrogenated vegetable oil (usually soy or corn oil) has become widely used in recent years, because it is a polyunsaturated fat (good fat) rather than a saturated fat (bad fat) and is lower in cholesterol than butter. However, recent research indicates that the hydrogenation process creates trans fatty acids. These apparently act like a saturated fat, thus negating the supposed benefits of margarine. Since the current debate over which product (margarine or butter) is less damaging to overall health remains unresolved, margarine is sometimes preferred over butter because of its lower cost.

By law, stick margarine must be 80 percent fat. It is sold salted and occasionally unsalted. Margarine can be used in place of butter for all cooking and baking uses, although flavor will vary and in some cases the texture of baked goods may be different.

A margarine-type product with less than 80 percent fat must be labeled as a spread. Spreads contain more water than margarine, which limits the way they can be used. Spreads with at least 70 percent fat can be substituted for margarine in cooking and baking with two exceptions. Dough for shaped cookies, such as spritz, will not hold the shape, and buttercream-type frostings may be too soft. Frosting can be adjusted by adding additional powdered sugar. Spreads with less than 70 percent fat will produce less tender cakes and less crisp cookies, which will stale quickly. Some spreads may not be satisfactory for sautéing because of the added water.

Some products may be blends of margarine and butter (usually 60 percent margarine, 40 percent butter). They have the flavor of butter but less cholesterol.

Soft margarine and spreads, which come in tubs, are spreadable at refrigerated temperature. Liquid margarine and spreads, which come in squeeze bottles, are a convenient way to add flavor to vegetables, popcorn and quick breads. These products are generally not acceptable as ingredients in baked goods.