How can hydrogenation create trans fatty acids?

During the industrial process of hydrogenation, liquid vegetable oils are converted into semisolid fats by adding hydrogen to unsaturated fatty acids in the presence of a catalyst.12

While the primary goal is to saturate these bonds, a side reaction known as isomerization occurs. Natural unsaturated fatty acids typically feature a cis configuration, where the hydrogen atoms attached to the double bond lie on the same side of the molecular plane. During hydrogenation, the catalytic treatment can cause the double bond to rearrange, forcing the hydrocarbon chains to opposite sides of the plane. This creates a trans configuration, resulting in trans fats.342

Simultaneously, the double bonds may migrate along the carbon chain, further altering the molecular structure. These trans isomers are more linear and have higher melting points than their cis counterparts, which contributes to the desired semisolid texture of products like margarine and shortening.312