Chemistry of trans fats
Where we speak of "fat", chemists use the term "triglyceride" - a molecule containing three "glycerides" or fatty acids, connected through a single molecule of glycerol. The fatty acids consist mainly of long carbon chains with hydrogen atoms attached to their sides, normally two hydrogen atoms on every carbon atom in the chain.
A "saturated" fatty acid is one in which every carbon atom is bound to two hydrogen atoms. An "unsaturated" fatty acid is one in which one or more pairs of the carbon atoms along the chain have only one hydrogen atom each, and the two carbon atoms of the pair are connected by a "double bond" instead of the usual "single bond".
The configuration of the double bond in an unsaturated fatty acid can take two forms (or, to chemists, isomers): the naturally predominant cis form, in which both of the hydrogen atoms are on the same side of the chain; and the uncommon-in-nature trans isomer, in which the hydrogen atoms are on opposite sides. The trans form is (in most cases) best thought of as 'damaged'.
Unsaturated fats tend to be liquid at room temperatures: the double bond is, in the normal cis configuration, asymmetric and so forces a kink or bend into the carbon chain. As a result the unsaturated fatty acids are unable to pack so closely together, or crystallise so readily as straight-chain saturated fatty acids. This is why unsaturated oils are mostly liquid at room temperature, while more saturated fats (such as tallow) are hard.
However a trans double bond does not form a sharp angle, as does a cis double bond. Instead the molecular chain forms a straight line similar to a saturated fat, except with a small kink at the double bond site. Consequently the trans isomer of a given fatty acid will always have a higher melting point than the cis isomer, but one lower than the corresponding saturated fatty acid.
In a "monounsaturated" fatty acid, there is only one such double-bonded pair of carbon atoms. In a "polyunsaturated" fatty acid, there are two or more such pairs.
When an unsaturated oil is hydrogenated, hydrogen is absorbed into the fatty acid molecules. The hydrogen atoms bind to the carbon chain in the vacant double-bonded sites, where only one hydrogen atom is attached to the carbon atoms. In so doing the extra hydrogen removes the double bond, straightens the chain, and makes the molecule more saturated.
If the unsaturated oil is fully hydrogenated, all the double bonds are removed and the result is a fully saturated fat. This very hard product makes great candle wax, but the food industry has little use for it (though it can be used as a feedstock for an interesterification process. Instead it wants partially hydrogenated oil, which is more solid than the original unsaturated oil, but less solid than wax, and has improved keeping qualities as the more fragile polyunsaturates such as alpha linolenic acid (ALA) are preferentially hydrogenated.During the hydrogenation process, conducted at high temperature (typically 260ºC) and pressure, the unsaturated oil is subject to structural transformation. Double bonds in the constituent fatty acids, which are naturally in the cis configuration, can flip to a trans configuration, so creating trans 'isomers'.
In particular the food industry likes elaidic acid - the trans isomer of oleic acid (the main constituent of olive oil) as it has a melting point similar to the temperature of the human mouth. This is far more useful than stearic acid, the corresponding saturated fatty acid, which makes a hard, waxy fat.
[Isomers: Two molecules which have the same chemical formula, but in which the constituent atoms are arranged differently, are 'isomers'. We are concerned here with two types of isomer: 'geometric isomers' (as in trans and cis isomers) and 'positional isomers' (which occur, for example, when double bonds in a fatty acid molecule move from one position to another). ]
This isomerisation can happen purely as a result of the high temperature, but can also result from the hydrogenation process itself: when one double bond site in a polyunsaturated fatty acid molecule is hydrogenated, the energy is imparted to the molecule can 'ripple' down the moelcule and flip the remaining double bond from the cis to the trans configuration. Typically 40-50 percent of the oil will end up in as trans isomers after hydrogenation. A similar process can cause the double bond sites to migrate along the molecular chain, creating positional isomers.
As the Institute of Medicine explains [ Letter Report on Dietary Reference Intakes for Trans Fatty Acids, 2002 ]:
"The trans double bond configuration results in a greater bond angle than the cis configuration. This results in a more extended fatty acid carbon chain more similar to that of saturated fatty acids rather than that of cis unsaturated double bond containing fatty acids. The conformation of the double bond(s) impacts on the physical properties of the fatty acid. Those fatty acids containing a trans double bond have the potential for closer packing or aligning of acyl chains, resulting in decreased mobility; hence fluidity is reduced when compared to fatty acids containing a cis double bond. Partial hydrogenation of polyunsaturated oils causes isomerization of some of the remaining double bonds and migration of others, resulting in an increase in the trans fatty acid content and the hardening of fat."
And to quote the Low Carb Luxury website,
Fatty acids are formed during the process of partial hydrogenation in which liquid vegetable oils are converted to margarine and vegetable shortening. Concern has existed that this process may have adverse consequences because natural essential fatty acids are destroyed and the new artificial isomers are structurally similar to saturated fats, lack the essential metabolic activity of the parent compounds, and inhibit the enzymatic desaturation of linoleic and linolenic acid.
Trans isomers of fatty acids - trans fats - are only formed in significant quantities by industrial processes involving high temperatures such as that of hydrogenation. Many of these will also be positional isomers. Consequently the family of fats known as 'trans fats' will typically comprise a wide range of subtly different positional and structural isomers.
Insufficient research has been carried out into the relative toxicity and specific qualities of different trans isomers. However we do know (see the natural trans fats page) that the trans isomers which occur naturally in meat and dairy produce are essentially harmless in the quantities encountered. It is fair to suppose, therefore, than there is wide range in the effective toxicity of the trans isomers arising from hydrogenation, and that some of these synthetic trans isomers have especially high toxicity.
These are likely to be the trans isomers of fatty acids which play biologically important roles, such as trans isomers of polyunsaturated fatty acids and of essential fatty acids in particular.
The overall proportion of trans fatty acids formed during hydrogenation can by tuned by adjusting the temperature, pressure and the duration of the process, however it is difficult to prevent their formation altogether.