What are trans fats?
Trans fats are fats - triglycerides of fatty acids - which contain trans fatty acids. These are unsaturated fatty acids that contain at least one double bond in the trans configuration, as opposed to the cis configuration ordinarily found in nature.
Some trans fatty acids are also found in the meat and milk of ruminant animals. However there is little evidence that these naturally-occurring trans fatty acids are harmful in their native state, as are the synthetic trans fatty acids which this campaign is concerned with. For more information on this see the natural trans fats page.
The main source of these synthetic trans fatty acids is hydrogenation - an industrial process in which oil is heated to a high temperature (typically 260-270ºC) under pressure and in the presence of a metal catalyst such as nickel, Rayner's nickel (a nickel / aluminium alloy) platinum, palladium or cobalt, then hydrogen is introduced. The catalyst is normally present in the form of a fine powder and one health concern is that a small quantity of it must remain in the oil. The hydrogen is absorbed into the fat molecules, changing its molecular structure and its chemical composition as it converts the unsaturated oil to a more saturated form.
This transforms the oil, naturally liquid at room temperature, into a hard or semi-hard grey-white fatty substance. This "[partially] hydrogenated vegetable oil" is virtually flavourless and has excellent keeping properties, making it an ideal feedstock for the food industry. In particular, unstable components such as the omega-3 fatty acid alpha-linolenic acid (ALA), prone to oxidation and rancidity, are preferentially hydrogenated by the process into more saturated and stable forms.
However hydrogenation, as well as making fats more saturated, also causes geometric isomerisation in unsaturated vegetable oils such as soy, corn or canola / rapeseed oils - that is, the fat molecules to change from one shape to another, such as from the cis to the trans configuration. This is partly the result of high temperature, but also the result of direct molecular excitation during hydrogenation, for example when ALA is hydrogenated to linoleic acid, or when linoleic acid is hydrogenated to oleic / elaidic acid.
Note that trans isomerisation can also occur in the absence of hydrogenation during the high temperature processing of oils, while refining, degumming and deodorising. ALA is especially prone to this trans isomerisation, and this is of particular concern as ALA is an essential fatty acid which plays a key role in human biology: in cell membranes, for example, in prostaglandins and in resolvins.
A bit of history
The hydrogenation process was patented in 1903 in the US by chemist William Normann. Commercial production of hydrogenated oils and fats began soon after, when Procter & Gamble hired chemist E. C. Kayser to develop an industrial process for the hydrogenation of cottonseed oil. Initially the intention was to make a cheap replacement for the animal based tallow used in candles. But in 1911 P&G, noticing that the hydrogenated oil looked and behaved like lard, began to sell it as a lard substitute under the brand name of Crisco.
Hydrogenated fats became more established in the 1930s, due to their lower cost. Their use rapidly increased during World War II when people turned to margarine and vegetable shortenings as alternatives to butter, which was subject to rationing.
They received a further boost in the late 1980s: health concern focused on saturated fats, which were widely vilified as extremely unhealthy. Meanwhile margarines containing hydrogenated vegetable oils were promoted to consumers as a 'healthier' alternative, replacing butter and other animal fats, as well as more saturated tropical vegetable oils such as palm and coconut.
Another factor was the growth of vegetarianism: manufacturers of biscuits, for example, were worried about loss of market share as vegetarians avoided products which contained "animal fat" as shown in the ingredients list.
It has since become clear that even the more saturated vegetable and animal fats are far healthier than hydrogenated fat containing trans fats.
The formation of trans isomers of fatty acids during partial hydrogenation was originally an accidental side effect. However in the 1980s the food industry began to selectively hydrogenate oils to produce a high fraction of elaidic acid, the trans isomer of oleic acid. This is because elaidic acid melts at mouth temperature, giving a luxurious sensation in the mouth. If fully hydrogenated to stearic acid, the result is a hard, waxy fat.
Note: trans fats do not in fact occur in fully hydrogenated oil, only in partially hydrogenated oil. However almost all of what is described as "hydrogenated vegetable oil" is, in fact, partially hydrogenated and does contain trans fat, and this should be assumed unless stated otherwise. Fully hydrogenated oil has the texture and hardness of candle wax (and is used for that purpose)!
Also see the page on the chemistry of trans fats.