saponin

Any of a class of bitter-tasting *glycosides in which the aglycone portion (the sapogenin) is a steroid alcohol. Saponins are soluble in water, characteristically producing a foam, hence their name. Many are toxic to animals and have been used as fish poisons. Some saponins are important in the commercial production of steroid hormones, notably diosgen, obtained from various yam (Dioscorea) species. The function of saponins in the plant is obscure though they may serve to deter predators.