Yanhusuo acid, also known as fumaric acid, violet acid or lichen acid, that is, maleic acid (IUPAC (E)-butene acid), is a colorless, flammable crystals, derived from butene carboxylic acid. Its chemical formula is C4H4O4. Combustion of fumaric acid will release the pungent maleic anhydride smoke. It tastes like fruit and can be found in the genus Corydalis, bovine liver fungus, lichen and Icelandic seaweed. Yanhusuo acid is used in the manufacture of polyester resins and polyols and as a mordant or seasoning for dyes. It is a popular food additive and dietary supplement, and is sometimes used as a substitute for tartaric acid in beverage or baking powder.