tannin

(tannic acid) Any substance capable of precipitating the gelatin of animal hides as an insoluble compound, so changing the hide to leather, resistant to putrefaction. All tannins are obtained from plants and most are polyphenols. Common sources of tannin include tea (Camellia sinensis), sumac (Rhus spp.), and the bark and gallnuts of oak (Quercus spp.).