Malic acid

Malic acid is an organic compound with the formula HO2CCH(OH)CH2CO2H. As a dicarboxylic acid, it is produced by all living organisms, gives fruits their sour taste, and serves as a food additive. It exists in two stereoisomeric forms, L- and D-enantiomers, but only the L-isomer occurs in nature. Its salts and esters are called malates. The malate anion also acts as an intermediate in the citric acid cycle.

The word "malic" comes from the Latin *mālum*, meaning "apple." The related Latin *mālus*, meaning "apple tree," gives its name to the genus *Malus*, which includes apples and crabapples, and appears in taxonomic groups like Maloideae, Malinae, and Maleae.

In biochemistry, L-malic acid is the natural form, while a mix of L- and D-malic acid is made synthetically. Malate is key in C4 carbon fixation, supplying CO₂ for the Calvin cycle. In the citric acid cycle, (S)-malate forms when an -OH group adds to the si face of fumarate. It can also arise from pyruvate through anaplerotic reactions. In plant leaves, malate is made by carboxylating phosphoenolpyruvate in guard cells. There, as a double anion, it often pairs with potassium cations during solute uptake to keep electrical balance. This solute buildup lowers the solute potential, letting water enter the guard cells and open the stomata.

Malic acid was first isolated from apple juice by Carl Wilhelm Scheele in 1785. Antoine Lavoisier proposed the name *acide malique* in 1787, again from the Latin for apple. In German, it is called *Äpfelsäure* (or *Apfelsäure*), while its salts are *Malat(e)*. It is the main acid in many fruits, such as apricots, blackberries, blueberries, cherries, grapes, mirabelles, peaches, pears, plums, and quince, and appears in lower amounts in citrus. It gives unripe apples their sourness, and sour apples have high levels of it. Grapes and most wines contain it, sometimes up to 5 g/L, adding a tart taste that lessens as fruit ripens. Rhubarb’s clear, pure tartness comes from malic acid, and it also gives sumac spice its sour flavor. It is used in artificial vinegar flavors, like in "salt and vinegar" potato chips. Malolactic fermentation turns malic acid into milder lactic acid. Naturally present in all fruits and many vegetables, it is generated in fruit metabolism. As a food additive, it is labeled E296 (or INS 296 in Australia and New Zealand) and is approved in the EU, US, and those countries. It is sometimes used with or instead of less sour citric acid in sour sweets, which may carry a warning about mouth irritation from overconsumption. Malic acid provides 10 kJ (2.39 kilocalories) of energy per gram.

Industrially, racemic malic acid is made by double hydration of maleic anhydride. In 2000, US production capacity was 5,000 tons per year. The enantiomers can be separated by chiral resolution, and S-malic acid is obtained by fermenting fumaric acid. Self-condensation of malic acid with fuming sulfuric acid yields the pyrone coumalic acid, releasing carbon monoxide and water. Malic acid was key in discovering the Walden inversion and Walden cycle: (−)-malic acid converts to (+)-chlorosuccinic acid with phosphorus pentachloride, then wet silver oxide turns that into (+)-malic acid, which reacts with PCl₅ to give (−)-chlorosuccinic acid, and silver oxide completes the cycle back to (−)-malic acid. L-malic acid is used to resolve α-phenylethylamine, a versatile resolving agent.

In plant defense, adding molasses to soil boosts microbial synthesis of malic acid. This is thought to happen naturally as part of soil microbe disease suppression, so molasses soil amendment can be used as a crop treatment in horticulture.