Fumaric acid is an organic compound with the formula HOCCH=CHCOH. A white solid, fumaric acid occurs widely in nature.  It has a fruit-like taste and has been used as a food additive. Its E number is E297. The salts and esters are known as fumarates.  Fumarate can also refer to the  ion (in solution). Fumaric acid is the trans isomer of butenedioic acid, while maleic acid is the cis isomer.