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Hydrogenation is widely applied to the processing of fats and oils. Complete hydrogenation converts unsaturated fatty acids to saturated ones. In practice the process is not usually carried to completion. Since the original oils typically contain more than one double bond per molecule (that is, they are poly-unsaturated), the result is usually described as partially hydrogenated vegetable oil, that is some, but usually not all, of the double bonds in each molecule have been reduced. Hydrogenation results in the conversion of liquid vegetable oils to solid or semi-solid fats, such as margarine. Changing the degree of saturation of the fat changes important physical properties such as melting point, which is why the liquids become semi-solid. Semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product. Since partially hydrogenated vegetable oils are much less expensive than most other fats with similar characteristicts, and because they have other desirable characteristics leading to longer shelf life, they are the predominant fat used in most commercial baked goods.

Processes accomplishing the reverse are called "dehydrogenation" or "partial dehydrogenation."

A side effect of incomplete hydrogenation, which has implications for human health, is the isomerization of the remaining unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats of most foods. But hydrogenation often converts these molecules to transIn chemistry, a double bond in which the greater radical on both ends is on the opposite side of the bond is called trans . Compare with cis''. In the diagram on the right, the hydrogen atoms are on opposite sides of the double bond. They cannot rotate to isomerIn chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. Many isomers share similar if not identical properties in most chemical contexts.s, which in fats have been implicated in heart disease ( see trans fatA trans fatty acid (commonly shortened to trans fat is an unsaturated fatty acid molecule that contains a trans double bond between carbon atoms, which makes the molecule kinked. Research suggests a correlation between diets high in trans fats and diseases ). The catalytic hydrogenation process favors the conversion from cis to trans bonds because the trans conformation is lower energy than the natural cis conformation.

Hydrogenation typically uses hydrogen gasFor other meanings see gas (disambiguation). A gas is one of the phases of matter. Gases are, like liquids, fluids: they have the ability to flow and do not resist deformation. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but as a reactantIn chemistry, the reactants are the substances that exist at the start of a chemical reaction. During the course of a reaction, the reactants form one or more products. In a chemical equation, the reactant is placed to the left of the arrow: Reactant(s) & and an undissolved (or "heterogeneous") metalFor alternative meanings see metal (disambiguation). In chemistry, a metal is an element that readily forms cations and has metallic bonds, and it is sometimes said that it is similar to a cation in a cloud of electrons. The metals are one of the three gr catalyst, such as nickel, palladium or platinum. Otherwise, the "homogeneous" rhodium-based catalyst known as Wilkinson's catalyst is often used. The reaction is usually carried out at elevated temperature and pressure. Such reactions belong to organic chemistry.

See also: hydrogenolysis, hydrodeoxygenation .