Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. This dicarboxylic acid is the active ingredient in many sour or tart foods. The salts and esters of malic acid are known as malates. Malate anion is an intermediate in the citric acid cycle along with fumarate. Ca. 1M kg/y are produced by the hydration of maleic anhydride.
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, malum.
Malate plays an important role in biochemistry. In biological sources, malic acid is homochiral and only exists as the (-)-malic acid enantiomer. 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 dehydrogenase catalyzes the reversible conversion of malate into oxaloacetate using NAD+ as a cofactor, such as the highly-important gluconeogenesis reaction in which oxaloacetate is converted to malate in the mitochondria, then is transported out of the mitochondria into the cytosol, where it is converted back into oxaloacetate. 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 contributes to the sourness of green apples. Malic acid is present in grapes. 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.
As a food additive
Malic acid, when added to food products, is denoted by E number E296. Malic acid is the source of extreme tartness in so-called "extreme candy", for example Mega Warheads or Sour Punch candies. It is also used with or in place of the less sour citric acid in sour sweets such as Jolly Ranchers, SweeTarts and Salt & Vinegar flavor potato chips. These sweets are sometimes labeled with a warning that excessive consumption can cause irritation of the mouth.