|Structural formula of malic acid with no information on stereochemistry|
|Molecular formula||C 4 H 6 O 5|
|External identifiers / databases|
|Molar mass||134.09 g · mol -1|
1.60 g cm −3 (20 ° C)
Decomposition at 140 ° C [ L -malic acid]
|pK s value||
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
Malic acid ( 2-hydroxy succinic acid , more rarely, malic acid ) is a chemical compound from the groups of dicarboxylic acids and hydroxycarboxylic acids serving as dextrorotatory D - and as levorotatory L occurs -malic acid. The L -form is an intermediate product in the citric acid cycle . In nature, L- malic acid is mostly found in unripe fruits such as apples , quinces , grapes , barberry berries , rowan berries and gooseberries . The esters and salts of malic acid are called malates (from Latin malum = apple, not to be confused with maleates , the esters and salts of maleic acid ).
Malic acid has a stereocenter , so it is optically active and there are two enantiomers : L -malic acid [synonym: ( S ) -malic acid] and the mirror- inverted D -malic acid [synonym: ( R ) -malic acid]. A 1: 1 mixture of D - and L -form is used as a racemate or DL referred -malic acid.
|Isomers of malic acid|
|Surname||L - (-) - malic acid||D - (+) - malic acid|
|other names||( S ) -2-hydroxysuccinic acid||( R ) -2-hydroxysuccinic acid|
|ECHA info card||100.002.365||100.010.239|
L- malic acid was first isolated from apple juice and described by Carl Wilhelm Scheele in 1785 . In 1787 Antoine Lavoisier proposed the name acid malique , derived from the Latin word for apple (mālum) . Paul Walden was able to prove the chirality and the reversal of configuration at the carbon atom through the synthesis of L -malic acid and D -malic acid .
The flash point of DL -malic acid is 203 ° C, the decomposition temperature 140 ° C and the ignition temperature 349 ° C. When malic acid is heated quickly to 250 ° C, it splits off 2 molecules of water and turns into maleic anhydride . This results in maleic acid with water .
Like all enantiomers , L -malic acid and D -malic acid have the same physical properties, with the exception of the direction of the rotation value α. In the case of DL -malic acid, the racemate , the physical properties differ, e.g. B. Melting point, but clearly different from those of the pure enantiomers L -malic acid and D -malic acid.
Both the L form and the D form or the racemate may be used as food additive (E 296) . D malic acid can be converted into L malic acid by enzymes in humans. In practice, their use is rather limited due to the relatively high price. Instead, cheaper alternatives such as citric acid (E 330), tartaric acid (E 334) or phosphoric acid (E 338) are usually used. Sodium malate (E 350), potassium malate (E 351) and calcium malate (E 352) are used as additives in potato chips . In medicine, potassium malate is used as an infusion solution for hypokalaemia , if potassium chloride can not be used because of the coexisting hyperchloremia .
The L -malic acid and its salts (Malate), after biotechnological processes, catalyzed by the enzyme fumarate hydratase , of fumaric acid (E 297) or as a metabolite of bacteria and fungi , (z. B. Brevibacterium, Corynebacterium, Escherichia, Microbacterium Proteus, Pichia) won. The enantiospecific addition of water to maleic acid is catalyzed by maleate hydratase and leads to D -malic acid. Racemic malic acid can be split into L -malic acid and D -malic acid by the formation of diastereomeric salts with a suitable enantiomerically pure chiral amine .
L -malic acid is also found in grapes . A low malic acid content is considered a ripening parameter. When the wine is aged, malolactic fermentation can be initiated either specifically or spontaneously . Lactic acid bacteria Oenococcus oeni metabolize the significantly more sour tasting malic acid into the less sour tasting lactic acid , releasing CO 2 and other by-products .
Malic acid is responsible for the sour taste of apples, although many other plants also contain malic acid. As the fruit ripens, its content decreases while the sugar content increases. This effect is of essential importance for the reproduction of the apple tree, as the high content of malic acid in the fruits prevents animals from eating them and thus distributing the still unripe seeds.
In plants with a Crassulacean acid metabolism (CAM), carbon dioxide is absorbed through the respiratory pores of the leaf at night and fixed by the enzyme PEP carboxylase . Another chain of reactions creates malate. Malate is the salt of malic acid and is stored in the vacuoles of CAM plants in the form of the acid at night. During the day, CO 2 is released again from malic acid and fed directly into the Calvin cycle . Due to the temporal separation of the reaction, also known as the diurnal acid rhythm , CAM plants have the advantage of closing their stomata during the day. In this way, evaporation losses can be counteracted.
- Entry on E 296: Malic acid in the European database on food additives, accessed on August 6, 2020.
- entry to MALIC ACID in CosIng database of the European Commission, called on August 6, 2020th
- The Merck Index , 9th edition, 1976, ISBN 0-911910-26-3 , p. 741.
- Entry on CAM plants. In: Römpp Online . Georg Thieme Verlag, accessed on July 25, 2011.
- Entry on malic acid in the ChemIDplus database of the United States National Library of Medicine (NLM), accessed on December 1, 2019.
- William B. Jensen: The Origin of the Names Malic, Maleic, and Malonic Acid , in: J. Chem. Educ. , 2007 , 84 , p. 924 ( doi: 10.1021 / ed084p924 ).
- Beyer-Walter, Textbook of Organic Chemistry, 23rd Edition, p. 356, S. Hirzel Verlag 1998 ISBN 3-7776-0808-4 .
- Zusatzstoffe-online.de: Malic acid
- Albert Gossauer: Structure and Reactivity of Biomolecules , Verlag Helvetica Chimica Acta, 2006, ISBN 3-906390-29-2 , pp. 362-370.
- Chemistry upper level - organic chemistry. 1st edition, Cornelsen-Verlag, Berlin 2010, ISBN 978-3-06-011177-0 .