How are trans fats made from vegetable oils?

Trans fats are produced from vegetable oils through an industrial process known as hydrogenation. During this process, molecular hydrogen is added to the unsaturated fatty acids found in liquid vegetable oils in the presence of a metal catalyst, such as nickel.123

This chemical reaction serves two primary purposes: it increases the stability of the oil, making it more resistant to rancidity and oxidation, and it converts the liquid oil into a more solid or semisolid, pliable consistency suitable for products like margarine and shortening.143

A key side effect of this process is the structural rearrangement of fatty acid molecules. Natural unsaturated fats typically exist in a cis configuration, where hydrocarbon chains lie on the same side of a double bond. Hydrogenation can cause these molecules to twist into a trans configuration, where the chains are positioned on opposite sides of the double bond. These resulting trans isomers have higher melting points than their natural counterparts, contributing to the hardening of the fat.15