Trans fats are formed when hydrogen attoms incorporated into poly-unsaturated vegetable oils. To improve the physical characeristics of liquid oils, manufacturers add hydrogen into these potential unsaturated spaces and convert them to solid fats.
Why do all of us are so concerned about trans-fats?
During the hydrogenation process, these light vegetable oils converted into solid saturated fats. Additionally, a kind of chemical reaction occurs which results a change in configuration from its natural cis-form to trans-form.
Traditionally, cooking oils are either majority poly-unsaturated (Sunflower oil) or mono-unsaturated (Olive) oils. When oils converted to trans-fats, they behave much like saturated fats inside the body, that is; they elevate "bad cholesterol" or LDL levels and decrease the "good cholesterol" or HDL levels in the blood.
In the natural world, trans fats do not present in the plants. They only present in small amounts in meat and dairy products as vaccenic acid. Actually, most trans-fats consumed today are created industrially through partial hydrogenation of vegetable oils. So formed saturated fats have higher melting points and have a longer shelf-life which makes them attractive for frying and baking. However, high temperature boiling also catalyzes a side-reaction that isomerizes some of the cis-unsaturated fats into trans-unsaturated fats instead of hydrogenating them completely. Trans fats formed from partially hydrogenated oils are, in fact, more dangerous to health than naturally-occurring saturated fats like coconu oil, palm oil etc.
Furthermore, deep frying of food items (especially those rich in starch like corn-flour) results in the production of toxic chemical "acrylamide," a deep brown color soot-like substance appearing on the surface of fried items. Consumption of excess of acrylamide for prolonged periods might leads to liver cell and colon cancers.
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Further reading and References:
U.S. Department of Agriculure-Trans fats