Malic acid is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a

food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives, such as vinegar ( pickling), salt ( salting), smoke ( smoking) and sugar ( crystallization), have been used f ...

. Malic acid has two stereoisomeric forms (L- and D-enantiomers), though only the L-isomer exists naturally. The salts and

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

s of malic acid are known as malates. The malate anion is a metabolic intermediate in the

citric acid cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of chemical reaction, biochemical reactions that release the energy stored in nutrients through acetyl-Co ...

Etymology

The word 'malic' is derived from Latin , meaning 'apple'. The related Latin word , meaning 'apple tree', is used as the name of the genus '' Malus'', which includes all apples and crabapples; and is the origin of other taxonomic classifications such as Maloideae, Malinae, and Maleae.

Biochemistry

L-Malic acid is the naturally occurring form, whereas a mixture of L- and D-malic acid is produced synthetically. File:L-Äpfelsäure.svg, L-Malic acid (''S'') File:D-Äpfelsäure.svg, D-Malic acid (''R'') Malate plays an important role in

biochemistry Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...

. In the C4 carbon fixation process, malate is a source of CO₂ in the Calvin cycle. In the

, (''S'')-malate is an intermediate, formed by the addition of an -OH group on the ''si'' face of fumarate. It can also be formed from pyruvate via anaplerotic reactions. Malate is also synthesized by the carboxylation of

phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the carboxylic acid derived from the enol of pyruvate and a phosphate anion. It exists as an anion. PEP is an important intermediate in biochemistry. It has the high-energy phosphate, highest-e ...

in the guard cells of plant leaves. Malate, as a double anion, often accompanies potassium cations during the uptake of solutes into the guard cells in order to maintain electrical balance in the cell. The accumulation of these solutes within the guard cell decreases the solute potential, allowing water to enter the cell and promote aperture of the stomata.

In food

Malic acid was first isolated from apple juice by

Carl Wilhelm Scheele Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish Pomerania, German-Swedish pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified the elements molybd ...

in 1785.

Antoine Lavoisier Antoine-Laurent de Lavoisier ( ; ; 26 August 17438 May 1794), When reduced without charcoal, it gave off an air which supported respiration and combustion in an enhanced way. He concluded that this was just a pure form of common air and that i ...

in 1787 proposed the name '' acide malique'', which is derived from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

word for apple, '' mālum''—as is its

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

name '' Malus''. In German it is named '' Äpfelsäure'' (or ''Apfelsäure'') after plural or singular of a sour thing from the apple fruit, but the salt(s) are called ''Malat(e)''. Malic acid is the main acid in many fruits, including apricots, blackberries, blueberries,

cherries A cherry is the fruit of many plants of the genus ''Prunus'', and is a fleshy drupe (stone fruit). Commercial cherries are obtained from cultivars of several species, such as the sweet ''Prunus avium'' and the sour ''Prunus cerasus''. The name ...

grape A grape is a fruit, botanically a berry, of the deciduous woody vines of the flowering plant genus ''Vitis''. Grapes are a non- climacteric type of fruit, generally occurring in clusters. The cultivation of grapes began approximately 8,0 ...

s, mirabelles,

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and Agriculture, cultivated in China. It bears edible juicy fruits with various characteristics, most called peaches and the glossy-skinned, non-fuzzy varieties called necta ...

es,

pear Pears are fruits produced and consumed around the world, growing on a tree and harvested in late summer into mid-autumn. The pear tree and shrub are a species of genus ''Pyrus'' , in the Family (biology), family Rosaceae, bearing the Pome, po ...

plum A plum is a fruit of some species in Prunus subg. Prunus, ''Prunus'' subg. ''Prunus'.'' Dried plums are often called prunes, though in the United States they may be labeled as 'dried plums', especially during the 21st century. Plums are ...

s, and quince, and is present in lower concentrations in other fruits, such as citrus. It contributes to the sourness of unripe apples. Sour apples contain high proportions of the acid. It is present in

s and in most wines with concentrations sometimes as high as 5 g/L. It confers a tart taste to

wine Wine is an alcoholic drink made from Fermentation in winemaking, fermented fruit. Yeast in winemaking, Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made f ...

; the amount decreases with increasing fruit ripeness. The taste of malic acid is very clear and pure in rhubarb, a plant for which it is the primary flavor. It is also the compound responsible for the tart flavor of sumac spice. It is also a component of some artificial vinegar flavors, such as "salt and vinegar" flavored potato chips. The process of malolactic fermentation converts malic acid to much milder lactic acid. Malic acid occurs naturally in all fruits and many vegetables, and is generated in fruit metabolism. Malic acid, when added to food products, is denoted by

E number E numbers, short for Europe numbers, are codes for substances used as food additives, including those found naturally in many foods, such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...

E296. It is sometimes used with or in place of the less sour citric acid in sour sweets. These sweets are sometimes labeled with a warning stating that excessive consumption can cause irritation of the mouth. It is approved for use as a

in the EU, US and Australia and New Zealand (where it is listed by its INS number 296). Malic acid contains 10 kJ (2.39 kilocalories) of energy per gram.

Production and main reactions

Racemic malic acid is produced industrially by the double hydration of maleic anhydride. In 2000, American production capacity was 5,000 tons per year. The

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s may be separated by chiral resolution of the racemic mixture. ''S''-Malic acid is obtained by fermentation of fumaric acid. Self-condensation of malic acid in the presence of fuming sulfuric acid gives the pyrone coumalic acid: :

Carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

and water are liberated during this reaction. Malic acid was important in the discovery of the Walden inversion and the Walden cycle, in which (−)-malic acid first is converted into (+)-chlorosuccinic acid by action of phosphorus pentachloride. Wet silver oxide then converts the chlorine compound to (+)-malic acid, which then reacts with PCl₅ to the (−)-chlorosuccinic acid. The cycle is completed when silver oxide takes this compound back to (−)-malic acid. -malic acid is used to resolve α-phenylethylamine, a versatile resolving agent in its own right.

Plant defense

Soil supplementation with molasses increases microbial synthesis of malic acid. This is thought to occur naturally as part of soil microbe suppression of disease, so soil amendment with molasses can be used as a

crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. In other words, a crop is a plant or plant product that is grown for a specific purpose such as food, Fiber, fibre, or fuel. When plants of the same spe ...

treatment in horticulture.

Interactive pathway map

References

External links

Calculator: Water and solute activities in aqueous malic acid
{{DEFAULTSORT:Malic Acid Cellular respiration Citric acid cycle compounds Cosmetics chemicals Dicarboxylic acids Food acidity regulators Alpha hydroxy acids Acids in wine Chelating agents E-number additives Metabolic intermediates