Malic acid

Malic acid is an organic compound with the formula HO₂CCH₂CHOHCO₂H. This carboxylic diacid is the active ingredient in many sour or tart foods. Malic acid is found mostly in unripe fruits. 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.

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 produced from starch in guard cells of plant leaves. A build-up of malate leads to a low water potential. Water then flows into the guard cells causing the stoma to open. However, this process does not always induce the opening of stomata.

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. Malic acid contributes to the sourness of green apples. Malic acid is present in grapes. It confers a tart taste towine, 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. Malic acid is the source of extreme tartness in USA-produced confectionary, the so-called extreme candy. It is also used with or in place of the less sour citric acid in sour sweets and Salt & Vinegar-flavor potato chips. These sweets are sometimes labeled with a warning stating that excessive consumption can cause irritation of the mouth.

Malic acid is produced industrially by the double hydration of maleic anhydride.

Self-condensation of malic acid with fuming sulfuric acid gives the pyrone coumalic acid

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.