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How are trans fats formed during hydrogenation?

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A composite diagram shows the structural formulas, space-filling models, and simplified depictions of lipid molecules including stearic acid and phosphatidylcholine, and how these assemble into a micelle and lipid bilayer, with labels identifying hydrophilic heads and hydrophobic tails. — Lipid Structure and properties of two representative lipids. Both stearic acid (a fatty acid) and phosphatidylcholine (a phospholipid) are composed of chemical groups that form polar “heads” and nonpolar “tails.” The polar heads are hydrophilic, or soluble in water, whereas the nonpolar tails are hydrophobic, or insoluble in water. Lipid molecules of this composition spontaneously form aggregate structures such as micelles and lipid bilayers, with their hydrophilic ends oriented toward the watery medium and their hydrophobic ends shielded from the water.

During the industrial process of hydrogenation, liquid vegetable oils containing unsaturated fatty acids are treated with molecular hydrogen in the presence of a catalyst to create semisolid fats.12

While the primary goal is to saturate double bonds, a side reaction known as isomerization occurs. Natural unsaturated fatty acids typically exist in a cis configuration, where the hydrogen atoms attached to the carbons of a double bond lie on the same side of the molecule. During catalytic treatment, some of these double bonds undergo a structural rearrangement where the configuration shifts to the trans form, placing the hydrocarbon chains on opposite sides of the double bond.341

Simultaneously, the double bonds may migrate to different positions along the carbon chain. This conversion results in a more linear molecular structure compared to the "v" shape of the natural cis form, which increases the melting point of the fat and creates the semisolid consistency characteristic of trans fats.31