Malic acid is an organic compound 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. Malic acid has two stereoisomeric forms (L- and D-enantiomers), though only the L-isomer exists naturally. The salts and of malic acid are known as malates. The malate anion is a metabolic intermediate in the citric acid cycle.
Etymology
The word 'malic' is derived from Latin mālum, meaning 'apple'. The related Latin word mālus, 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. In the C4 carbon fixation process, malate is a source of
carbon dioxide in the
Calvin cycle. In the citric acid cycle, (
S)-malate is an intermediate, formed by the addition of an
Hydroxy group 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 in 1785.
[Carl Wilhelm Scheele (1785) "Om Frukt- och Bår-syran" (On fruit and berry acid), Kongliga Vetenskaps Academiens Nya Handlingar (New Proceedings of the Royal Academy of Science), 6 : 17-27. From page 21: " ... vil jag hådanefter kalla den Åple-syran ." ( ... I will henceforth call it apple acid.)] Antoine Lavoisier in 1787 proposed the name
, which is derived from the Latin word for apple, —as is its
genus name
Malus.
[de Morveau, Lavoisier, Bertholet, and de Fourcroy, Méthode de Nomenclature Chimique (Paris, France: Cuchet, 1787), p. 108.]
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
,
blackberries,
blueberries,
cherries,
,
,
,
,
, and
quince,
[ Tabelle I of ] and is present in lower concentrations in other fruits, such as
Citric acid It contributes to the sourness of unripe apples. Sour apples contain high proportions of the acid. It is present in
and in most wines with concentrations sometimes as high as 5 g/L.
It confers a tart taste to
wine; 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 , Bartek Ingredients (retrieved 2 February 2012)]
Malic acid, when added to food products, is denoted by E number 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 food additive in the EU,[UK Food Standards Agency: ] US[Australia New Zealand Food Standards Code] (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
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 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 treatment in horticulture.
Interactive pathway map
See also
-
Acids in wine
-
Citrate–malate shuttle
-
Crassulacean acid metabolism
-
Malate–aspartate shuttle
-
Maleic acid, resulting from malic acid dehydration
External links
