Fumaric acid

Fumaric acid is the chemical compound with the formula HO2CCH=CHCO2H. This colorless crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid wherein the carboxylic acid groups are cis. It has a fruit-like taste. The salts and esters of fumaric acid are known as fumarates.

Fumaric acid, when added to food products, is denoted by E number E297.

Biology
Fumaric acid is found in fumitory (Fumaria officinalis), bolete mushrooms (specifically Boletus fomentarius var. pseudo-igniarius), lichen, and Iceland moss.

Fumarate is an intermediate in the citric acid cycle used by cells to produce energy in the form of adenosine triphosphate (ATP) from food. It is formed by the oxidation of succinate by the enzyme succinate dehydrogenase. Fumarate is then converted by the enzyme fumarase to malate. Human skin naturally produces fumaric acid when exposed to sunlight.

Fumarate is also a byproduct of the urea cycle.

Medicine
Fumaric acid esters are sometimes used to treat psoriasis, as it has been suggested that the condition is caused by an impairment of fumaric acid production in the skin. A starting dose is 60-105 mg daily, which may be gradually increased to as much as 1,290 mg per day. Side-effects include kidney or gastrointestinal disorders, as well as skin flushing; these are mainly caused by excess intake. Decreased white blood cell counts have been reported with prolonged use.

Food
Fumaric acid is a food acidulent used since 1946 because it is non-toxic. It is generally used in beverages and baking powders for which requirements are placed on purity. It is generally used as a substitute for tartaric acid and occasionally in place of citric acid, at a rate of 1.36 g of citric acid to every 0.91 grams of fumaric acid for the same taste. It is also used in candy to add sourness, similar to the way malic acid is used.

Chemistry
Fumaric acid is inexpensive, so it is typically purchased rather than prepared. It was first prepared from succinic acid. A traditional synthesis involves oxidation of furfural (from the processing of maize) using sodium chlorate in the presence of a vanadium-based catalyst.

The chemical properties of fumaric acid can be anticipated from its component functional groups. This weak acid forms a diester, it undergoes additions across the double bond, and it is an excellent dienophile.

Other uses
Fumaric acid is used in the manufacture of polyester resins and polyhydric alcohols and as a mordant for dyes.

Safety
Fumaric acid converts to the irritant maleic anhydride, upon partial combustion.