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How does hydrogenation make unsaturated oils more solid and resistant to rancidity?

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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.

Hydrogenation is a chemical process used to convert liquid unsaturated oils into more solid, stable fats by adding hydrogen atoms across the double bonds of fatty acid chains in the presence of a catalyst.123

Increasing Solidity

Natural oils are liquid at room temperature because they contain unsaturated fatty acids with double bonds. By adding hydrogen, these double bonds are reduced, converting the fatty acids into saturated forms, which have higher melting points and are solid at room temperature. Additionally, the process can cause isomerization, where the molecular structure rearranges into trans configurations. These trans isomers have higher melting points than the natural cis form, further contributing to the hardening of the product.134

Resistance to Rancidity

Rancidity is primarily caused by the aerial oxidation of unsaturated fatty acids, which creates volatile compounds with unpleasant odors and flavors. Because saturated fats are inherently more stable and resistant to oxidation than unsaturated ones, the hydrogenation process protects the oil by eliminating the reactive double bonds that are susceptible to this breakdown.53