Malic acid

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Common name	malic acid
Systematic name	hydroxybutanedioic acid
Chemical formula	C₄H₆O₅
Molecular mass	134.09 g/mol
Melting point	-25C
CAS number	617-48-1
Density	1.609 g/cm³
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Malic acid is a tart-tasting organic dicarboxylic acid that plays a role in many sour or tart foods. In its ionised form it is malate, an intermediate of the TCA cycle along with fumarate. It can also be formed from pyruvate as one of the anaplerotic reactions.

Apples contain malic acid, which contributes to the sourness of a green apple. Malic acid can make a wine taste tart, although the amount decreases with increasing fruit ripeness. The process of malolactic fermentation converts malic acid to much milder lactic acid. Malic acid, when added to food products, is denoted by E number E296.

In biological sources malic acid is homochiral and only exists as the (-)-malic acid enantiomer. Malic acid stars 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 takes 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.

Self-condensation of malic acid with fuming sulfuric acid gives the pyrone coumalic acid ^[1]:

Malic acid is the source of extreme tartness in so called "extreme candy", for example Mega Warheads. It is also used with or in place of the less sour citric acid in sour candies such as Jolly Ranchers and SweeTarts. These candies are sometimes labeled with a warning that excessive consumption can cause irritation of the mouth.