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Hydrogenation is a chemical process in which hydrogen gas is bubbled through a liquid oil in the presence of a catalyst, often a reactive metal such as platinum or nickel. The resulting reaction forces unsaturated fatty acids to accept additional hydrogen atoms and become at least partially saturated. In practical cooking terms, hydrogenation would convert an unsaturated vegetable oil, the kind often used for deep frying, into a partially solid form like margarine. A fully hydrogenated vegetable oil would be as thick as animal lard, but most food manufacturers do not take the hydrogenation process that far. Hydrogenating vegetable-based oils is generally less expensive than using saturated animal fats, and partial hydrogenation gives processed foods a longer shelf life.
The key to understanding hydrogenation, at least as it pertains to the food industry, is the concept of unsaturated and saturated fatty acids. Unsaturated fatty acids are primarily liquid cooking oils, because they do not contain all of the hydrogen atoms they could possibly hold. In the case of poly-unsaturated vegetable oils, some of the hydrogen atoms they do contain are stuck together in double bonds, leaving holes where hydrogen atoms would ordinarily bond.
It may help to picture a fatty acid chain as a centipede who wears hydrogen boots on each leg. In the case of unsaturated fats, the centipede is missing some hydrogen boots completely and also has two feet in the same hydrogen boot. During hydrogenation, the incoming hydrogen atoms attach themselves to the available legs and also force the double hydrogen bonds to split up. If this process continues until all the centipedes or molecular chains have hydrogen boots, the solid oil can be described as fully saturated.
Saturated fats do serve many purposes in the world of food, but they have a tendency to become rancid quickly as they interact with oxygen. Unsaturated fats work well as cooking oils, but they do not provide much structure for processed foods. The ideal oil for many processed foods is only partially hydrogenated. This means the hydrogenation process is stopped at some point, creating a new form of fat which is more solid than unsaturated oils, but not as solid as fully hydrogenated or saturated fats.
The most common example of a partially hydrogenated oil would be the butter substitute known as margarine. Margarine is solid enough to use in many processed foods, and also has a longer shelf life than a fully saturated fat. This stability and extended shelf life at room temperature is why many food manufacturers prefer to use partially hydrogenated oils in products destined for store shelves.
The problem with partially hydrogenated fatty acids lies in the hydrogenation process. Because the process was halted before all of the molecular chains became completely saturated with hydrogen atoms, a third form of fat was created. These fatty acid chains are neither unsaturated or saturated, but rather in an unstable state of transition. Because these fatty acids are caught between two states of being, they are considered trans fats.
Trans fats can occur naturally, but the human body is not fully equipped to deal with their effects on a large scale. Trans fat molecules are irregularly shaped, for one thing, and cannot be processed in the same way as unsaturated or saturated fats. Trans fats also have negative effects on the body's healthy HDL cholesterol levels while increasing the level of unhealthy LDL cholesterol.
Hydrogenation itself is not considered to be a particularly dangerous or unhealthy process, but it can be expensive because of the need for reactive precious metals such as platinum. Non-precious reactive metals such as nickel can also be used as the catalyst for hydrogenation, but the results are often variable. Hydrogenation is also used to create chemical compounds such as ammonia, which is the result of hydrogen and nitrogen reacting to a catalyst metal. The hydrogenation process is also use in the petroleum industry in order to create more stable hydrocarbon fuels.