Why does hydrogenation make oils more resistant to rancidity?

Hydrogenation makes oils more resistant to rancidity by reducing their degree of unsaturation. Rancidity is primarily caused by the oxidation of unsaturated fatty acids when exposed to air, a process that proceeds through a highly reactive free-radical chain reaction.123

During the hydrogenation process, hydrogen molecules are added to the double bonds (points of unsaturation) within the fatty acid chains in the presence of a catalyst. By converting these unstable double bonds into saturated bonds, the oil becomes chemically more stable. Because saturated fats are inherently resistant to oxidation, the resulting hydrogenated product is significantly less prone to developing the unpleasant odors and flavors associated with rancidity.4152