The process changes a liquid oil, naturally high in unsaturated fat, to a more solid and more saturated form. The greater the degree of hydrogenation, the more saturated the fat becomes. Many commercial products contain hydrogenated or partially hydrogenated vegetable oils. In the case of margarine, the process allows an oil to be partially hardened and molded into tub or stick form. Fat for shortening can be hydrogenated to give it a creamy consistency.
Hydrogenation also increases the time it takes before oils become rancid, so they stay fresh longer.
There's a great deal of misunderstanding about the effect that hydrogenation has upon the composition and nutritional properties of edible fats and oils. Recent studies suggest that these fats may raise blood cholesterol. In most oils and margarines, the hydrogenation process is controlled to achieve stability. The finished products may contain large amounts of unsaturated fats and fairly small amounts of saturated fats. Many of these products are appropriate for use in the American Heart Association diet if they have no more than 2 grams of saturated fat per tablespoon and the first-listed ingredient is liquid vegetable oil.