With the development of hydrogenation in 1905 it became possible to shift from animal to vegetable fats as a substitute for butter. Hydrogenation combines fatty acids with hydrogen in order to solidify oil, raise its melting point, and prevent it from going rancid. Hydrogenation also creates "trans" fatty acids in the product, which function similarly to saturated fats in raising blood cholesterol levels. Generally, the harder the margarine, the more trans fatty acids it contains and also the longer its shelf life. Margarines may contain animal products to lessen the need to hydrogenate; strict vegetarians should check the label. By law, only products that contain 80 percent fat, the same as butter, can be labeled margarine. Butter and margarine contain the same amount of calories as well.
