Dawn - About.com User
Shortening and margarines are created from vegetable oil, which is a liquid. Since we want our margarine and shortening to be more solid at room temperature, a process called hydrogenation is used to change the liquid oils to solids. Hydrogenation is the process by which hydrogen is forced into heated vegetable oil with the aid of a catalyst, usually nickel. Forcing hydrogen into the oil changes the chemical structure so that the oil will become more solid. Fully hydrogenating the oils will make them too solid, which makes them difficult to use for cooking. To keep the margarine or shortening slightly soft, the oils used for margarine and shortening are only partially hydrogenated. When oils are only partially hydrogenated, the molecules of the partially hydrogenated oils form trans configurations, which means the oil molecules have kind of an odd shape for fat molecules. Your body doesn't like this trans configuration, and in fact, trans fats due to partial hydrogenation are as bad, or worse, for your arteries as saturated fats.
Fully hydrogenated shortenings don't have this trans configuration, but of course they are too solid to use for cooking. This problem can be solved by blending the fully hydrogenated solid with unsaturated oils to make them softer so that they can be used like regular margarine or shortening. Crisco Trans Fat Free shortening has this hard fully hydrogenated cottonseed oil blended with liquid soy and sunflower oils which give the shortening a softer texture.
