Malic acid is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

with the

molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

. 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 have been used for centuries as part of an effort to preserve food, for example vinegar (pickling), salt (sal ...

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

salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively ...

and

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

s of malic acid are known as malates. The malate

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

is an intermediate in the

citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and prote ...

Etymology

The word 'malic' is derived from Latin ' mālum', meaning 'apple'. The related Latin word , meaning 'apple tree', is used as the name of the genus ''

Malus ''Malus'' ( or ) is a genus of about 30–55 species of small deciduous trees or shrubs in the family Rosaceae, including the domesticated orchard apple, crab apples, wild apples, and rainberries. The genus is native to the temperate zo ...

'', which includes all apples and crabapples; and 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 File:D-Äpfelsäure.svg, D-Malic acid 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 ...

. 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 In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy g ...

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 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 German pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified molybdenum, tungsten, barium, hydro ...

in 1785.

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS ( acide malique'', which is derived from the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

word for apple, '' mālum''—as is its

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

name ''

''. 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

apricot An apricot (, ) is a fruit, or the tree that bears the fruit, of several species in the genus '' Prunus''. Usually, an apricot is from the species '' P. armeniaca'', but the fruits of the other species in ''Prunus'' sect. ''Armeniaca'' are al ...

s, blackberries, blueberries, cherries,

grape A grape is a fruit, botanically a berry (botany), berry, of the deciduous woody vines of the flowering plant genus ''Vitis''. Grapes are a non-Climacteric (botany), climacteric type of fruit, generally occurring in clusters. The cultivation of ...

s, mirabelles,

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and cultivated in Zhejiang province of Eastern China. It bears edible juicy fruits with various characteristics, most called peaches and others (the glossy-skinned, non- ...

es, pears,

plum A plum is a fruit of some species in ''Prunus'' subg. ''Prunus'.'' Dried plums are called prunes. History Plums may have been one of the first fruits domesticated by humans. Three of the most abundantly cultivated species are not found ...

s, and

quince The quince (; ''Cydonia oblonga'') is the sole member of the genus ''Cydonia'' in the Malinae subtribe (which also contains apples and pears, among other fruits) of the Rosaceae family. It is a deciduous tree that bears hard, aromatic brigh ...

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 typically made from fermented grapes. Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different varieties of grapes and strains of yeasts are ...

; the amount decreases with increasing fruit ripeness. The taste of malic acid is very clear and pure in

rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of '' Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick r ...

, 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 Vinegar is an aqueous solution of acetic acid and trace compounds that may include flavorings. Vinegar typically contains 5–8% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting simple sugars to ...

flavors, such as "salt and vinegar" flavored potato chips. In citrus, fruits produced in organic farming contain higher levels of malic acid than fruits produced in conventional agriculture. The process of malolactic fermentation converts malic acid to much milder

lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natur ...

. 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 ("E" stands for "Europe") 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 Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...

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 The International Numbering System for Food Additives (INS) is a European-based naming system for food additives, aimed at providing a short designation of what may be a lengthy actual name."Class Names and the International Numbering System for Fo ...

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 ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

s may be separated by

chiral resolution Chiral resolution, or enantiomeric resolution, is a process in stereochemistry for the separation of racemic compounds into their enantiomers. It is an important tool in the production of optically active compounds, including drugs. Another term ...

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 Coumalic acid is an organic compound with the molecular formula C6H4O4. Its melting point is around 210 °C. In laboratory coumalic acid may be obtained by self-condensation of malic acid in fuming sulfuric acid Fuming may refer to; *Ammonia fum ...

: : Note that this scheme is incorrect. 4 H₂O and 2 CO (carbon monoxide, not carbon dioxide) are liberated during the condensation. Malic acid was important in the discovery of the Walden inversion and the

Walden cycle Walden inversion is the inversion of a stereogenic center in a chiral molecule in a chemical reaction. Since a molecule can form two enantiomers around a stereogenic center, the Walden inversion converts the configuration of the molecule from ...

, 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.

Uses

-malic acid is used to

resolve Resolve may refer to: * ''Resolve'' (Lagwagon album) * ''Resolve'' (Last Tuesday album) * "Resolve" (song), by the Foo Fighters *'' The Resolve'', a 1915 American silent short drama film * "Resolve" (''One Tree Hill'' episode) *''Resolve'', a Brit ...

α-phenylethylamine, a versatile resolving agent in its own right.

Plant defense

Soil supplementation with

molasses Molasses () is a viscous substance resulting from refining sugarcane or sugar beets into sugar. Molasses varies in the amount of sugar, method of extraction and age of the plant. Sugarcane molasses is primarily used to sweeten and flavour foods ...

increases microbial synthesis of MA. 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. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropo ...

treatment in horticulture.

Interactive pathway map

References

External links

Malic acid MS SpectrumCalculator: 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