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Not so long ago, trans unsaturated fats (often called trans fats) were some of the most common components of our food. They are still very common in many countries, despite the attempts to eliminate them from our diet. Trans unsaturated fats have got plenty of negative publicity, and rightly so. This type of fat is almost non-existing in natural foods, and the evidence of its negative effects on human health is growing.
What are the trans fats?
The term “trans fats” refers to the type of organic molecules from the class of unsaturated fatty acids. Fatty acids belong to the group of carboxylic acids which contain long linear chains. They are major components of all fats of plant and animal origin. Unsaturated fatty acids contain one or more double bond inside this chain. Geometrically, double bond may have either trans or cis configuration. As the name suggests, trans unsaturated fatty acids have at least one double bond which is in trans configuration.
Fatty acids can have different length and contain several double bonds. Depending on the structure, they play different role in cellular biochemistry.
From the point of view of human biochemistry and toxicology, trans fatty acids are important primarily because they are not normally present in natural sources of food. Unfortunately, modern food contains lots of modified components, and trans fat is one of them. Trans fat is present in small amounts in normal dairy and meats, but nowadays humans are exposed to the amounts of trans fats that are significantly exceeding this norm.
Why modern food contains lots of trans fats?
Trans fatty acids were introduced to the food industry with the invention of partial hydrogenation process in the beginning of 20thcentury. The process helps to convert unsaturated fatty acids of plant origin to saturated fat.
The term “saturated” refers to the absence of double bonds in the molecule of fatty acid. Saturation is achieved by hydrogenation, addition of hydrogen molecule to the unsaturated double bond.
Unsaturated fats are prone to randicity, the decomposition of fats due to oxidation and action of microorganisms which leads to the bad odor and decrease the quality of fat-containing food. Hydrogenation provides additional chemical stability to the fats. This increases their shelf life and reduces the need in refrigeration.
Partial hydrogenation of fats leads to the formation of trans fats as undesirable side products
Hydrogenation changes the double bond of unsaturated fatty acids into single bong. Full hydrogenation of unsaturated fatty acids leads to saturated acids that have no isomers. However, the purpose of hydrogenation is to harden the fat (saturated fatty acids have higher melting point than their unsaturated analogues) and reduce the amount of double bonds there, not to achieve the full saturation of double bonds as such. As a result, full hydrogenation is almost never done. Instead, a process of partial hydrogenation is widely employed. The use of only partial hydrogenation leads to the problem of formation of undesirable fatty acids as side product. It deals with the mechanism of hydrogenation. In the course of catalytic hydrogenation the double bond should get two additional hydrogen atoms. They never get added simultaneously. Attachment of first hydrogen leads to the intermediate without double bond, but still without the second hydrogen. Due to the absence of double bond, this intermediate is conformationally flexible. The first step of reaction is reversible. This means that the intermediate may lose the hydrogen and return to the original unsaturated state. However, this return step may lead to the production of both cis and trans isomers without any preferences. As a result of partial hydrogenation, some molecules of cis fatty acids get isomerized into trans fats.
Production of hydrogenated fats quickly grew in popularity in the first half of 20th century. As a result, now lots of fats in our food contain these artificially modified fats with significantly higher content of trans fatty acids.