any of the complex organic nonnitrogenous compounds of the plant glycoside group. Upon acid or enzymatic hydrolysis, saponins are split into monosaccharides (one or several molecules) and a noncarbohydrate part called the aglycon (sapogenin). Depending on the chemical structure of the aglycon, a distinction is made between triterpenoid saponins, in which the aglycons are triterpenoids, and steroid saponins, in which the aglycons are steroids. Uronic acids may also be components of saponins.
Saponins are found mainly in plants (in the Rosaceae, Caryo-phyllaceae, and Sapindaceae families) and in certain marine animals (starfish and holothurians). Saponins are characterized by the capacity to give, like soaps, collodial solutions that readily form foams. With phenols and higher alcohols, for example, sterols, saponins form stable molecular compounds that are used for the separation, purification, and quantitative determination of saponins themselves and such sterols as cholesterol.
Saponins have a bitter, sharp taste. Upon intravenous injection, they are highly toxic; extremely low concentrations lead to the destruction of erythrocytes (hemolysis). However, saponins are not toxic upon ingestion because they either are not absorbed or else are destroyed in the intestines.
Steroid saponins are used as an inexpensive raw material for the production of steroid hormones. As foaming agents, saponins are used in charging foam fire extinguishers and in the production of soft drinks and beer. Saponins are contained in many medicinal plants (soapbark, licorice, jalap, senega root), which are used as expectorants and diuretics. Espin and other saponins of the horse chestnut, as well as aralosides from the Japanese angelica tree, have a cardiotonic effect.