Malic+Acid

The salts and esters of malic acid are known as malates. The malate anion is an intermediate in the citric acid cycle
 * Malic acid** is an [|organic compound] with the formula HO2CCH2CHOHCO2H. It is a [|dicarboxylic acid] which is made by all living organisms, contributes to the pleasantly 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 [|esters] of malic acid are known as **malates**. The malate anion is an intermediate in the [|citric acid cycle].

L -Malic acid is the naturally occurring form, whereas a mixture of L - and D -malic acid is produced synthetically. L -Malic acid D -Malic acid Malate plays an important role in [|biochemistry]. In the [|C4 carbon fixation] process, malate is a source of [|CO2] in the [|Calvin cycle]. In the [|citric acid cycle], (//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] 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.

Malic acid in food
Malic acid was first isolated from [|apple juice] by [|Carl Wilhelm Scheele] in 1785. [|Antoine Lavoisier] in 1787 proposed the name // [|acide malique] //which is derived from the [|Latin] word for apple, // [|mālum] //. [|[3]] Malic acid contributes to the sourness of green [|apples]. It is present in [|grapes] and in most wines with concentrations sometimes as high as 5 g/l. [|[4]] It confers a tart taste to [|wine], although the amount decreases with increasing fruit [|ripeness]. 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. [|[5]] Malic acid, when added to food products, is denoted by [|E number] E296. Malic acid is the source of extreme tartness in [|USA] -produced confectionery, the so-called //extreme candy//. It is also 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, [|[6]] USA [|[7]] and Australia and New Zealand [|[8]] (where it is listed by its INS number 296).

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Production and main reactions
Malic acid is produced industrially by the double hydration of [|maleic anhydride]. [|[9]] [|Self-condensation] of malic acid with [|fuming sulfuric acid] gives the [|pyrone] **coumalic acid**: [|[10]] 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.