The process of converting an ester into soap by heating with sodium hydroxide is called

Fats and oils are esters of long-chain fatty acids with glycerol. Heating them with NaOH gives sodium salts of the fatty acids, which are soap, and glycerol; this is saponification. Esterification is the reverse type reaction of an acid with an alcohol, and hydrogenation converts unsaturated oils into saturated fats.