hydrogenation of double bonds in the unsaturated fatty acids

Catalytic hydrogenation of alkenes is currently a hot topic in food chemistry.  Margarine is produced by partial hydrogenation of double bonds in the unsaturated fatty acids in liquid vegetable oils, usually with a nickel catalyst.  Complete hydrogenation would produce fully saturated fatty acids and lead to a lard-like product that is too hard to spread on toast, so conditions are adjusted to ensure that only some of the double bonds are hydrogenated while others are left in place, resulting in a soft and spreadable product.  This process is called partial hydrogenation.

Recently, however, scientists have become increasingly worried about the presence of unnatural fatty acids found in margarine and other food products made from partially hydrogenized oils. Natural unsaturated fatty acids have mainly cis double bonds.  In the unnatural fatty acids found in margarines, the naturally-occurring cis stereochemistry has been converted to trans.   Trans-fatty acids have been associated with heart disease and some forms of cancer.

It appears that these unnatural trans fatty acid isomers are unintentionally produced by the hydrogenation process. The problem is that there is no control over the regiochemistry or stereochemistry of the reverse (dehydrogenation) reaction.  Because only a limited amount of hydrogen is used in order to achieve partial (rather then complete) hydrogenation, the process is reversible, meaning that double bonds tend to re-form – and when they do, it is often in the lower-energy trans configuration, rather than the natural cis  configuration.  The figure below shows the partial hydrogenation of a linoleic acid hydrocarbon over a nickel catalyst, resulting in oleic acid, which is the desired cis-unsaturated product, as well as elaidic acid, the undesirable trans fat product.

Food producers are increasingly adopting alternative hydrogenation technologies and production strategies in order to offer products that are free of trans fatty acids.