malic acid

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A colorless, crystalline compound, COOH·CH₂·CHOH·COOH, that occurs naturally in a wide variety of unripe fruit, including apples, cherries, and tomatoes, and is used as a flavoring and in the aging of wine.

Wiktionary

1. n. organic chemistry A colourless crystalline dicarboxylic acid, hydroxy-malonic acid, found in wine, apples and other fruit; it is converted to lactic acid by malo-lactic fermentation.

GNU Webster's 1913

1. adj. (Chem.) a hydroxy acid (HO.CO.CH2.CH(OH).CO.OH) obtained from unripe fruit (such as green apples, currants, tomatoes or cherries) as a substance which is sirupy or crystallized with difficulty, and has a strong but pleasant sour taste. It is levorotatory or dextrorotatory according to the temperature and concentration; the natural form is of L- conformation. A synthetic variety is a derivative of succinic acid, but as with most simple synthetic compounds, is a racemic mixture of isomers and thus has no rotatory action on polarized light.