Trans fat is the common name for unsaturated fat with ''trans''-isomer fatty acid(s). Trans fats may be monounsaturated or polyunsaturated but never saturated.
Unsaturated fat is a fat molecule containing one or more double bonds between the carbon atoms. Since the carbons are double-bonded to each other, there are fewer bonds connected to hydrogen, so there are fewer hydrogen atoms, hence "unsaturated". ''Cis'' and ''trans'' are terms that refer to the arrangement of chains of carbon atoms across the double bond. In the ''cis'' arrangement, the chains are on the same side of the double bond, resulting in a kink. In the ''trans'' arrangement, the chains are on opposite sides of the double bond, and the chain is straight.
Each fat molecule has three hydrocarbon chains. The process of hydrogenation adds hydrogen atoms to ''cis''-unsaturated fats, eliminating double bonds and making them into partially or completely saturated fats. These more-completely saturated fats have a higher melting point, which makes them more attractive for baking, and the saturation extends their shelf-life. However, partial hydrogenation converts a part of ''cis''-isomers into ''trans''-unsaturated fats instead of hydrogenating them completely. Complete hydrogenation converts the fat into a saturated "hard" fat.
''Trans'' fats occur also naturally, although to a limited extent: vaccenyl and conjugated linoleyl (CLA) containing ''trans'' fats occur naturally in trace amounts in meat and dairy products from ruminants, although the latter also constitutes a ''cis'' fat.
Unlike other dietary fats, trans fats are not essential, and they do not promote good health. The consumption of trans fats increases the risk of coronary heart disease by raising levels of "bad" LDL cholesterol and lowering levels of "good" HDL cholesterol.
Health authorities worldwide recommend that consumption of trans fat be reduced to trace amounts. Trans fats from partially hydrogenated oils are more harmful than naturally occurring oils.
Alternatively, hard fats can be softened by cutting with ''cis'' fats such as vegetable oil, or by transesterification with ''cis'' fats into fats with ''cis'' unsaturated and saturated hydrocarbon chains.
Further Reading
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