Malic acid

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.

History

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#Etymology_2.

Biology

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. 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.

Chemistry

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.

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

See also

• Malate-aspartate shuttle
• Two other dicarboxylic acids have similar names and should not be confused with malic acid:
• Maleic acid
• Malonic acid
• Acids in wine

References