The reaction of fats or oils with aqueaous sodium hydroxide to give glycerol and sodium salts of fatty acids (soaps). Also used in reference to the hydrolysis of any ester.
Animal fats have been saponified for over 2300 years in one of the oldest known chemical manufacturing process. An excess of an alkali, traditionally wood ashes, is added to fat or tallow in an open kettle. When the reaction is complete, salt is added to precipitate the soap in thick curds. The watery layer, containing salt, glycerol, and excess alkali, is drawn off, and the soap is purified by boiling with water and reprecipitating with salt repeatedly.
(A noxiously smelling version of this experiment is forced on most junior high school students in Australia, being one of the formative memories of practical science and turning us all off soap for weeks.)
Modern processes hydrolyze large volumes of fat or oil at high temperatures along with a catalyst, usually a zinc salt. Fatty Acids and glycerol are removed as they are formed by distillation, and the acids carefully neutrlalised with a precise volume of alkali to form to soap.