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malic acid:see Krebs cycleKrebs cycle,
series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars.
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(also called hydroxysuccinic acid), HOOCCH2CH(OH)COOH, a dibasic hydroxycarboxylic acid. Malic acid takes the form of a colorless, hygroscopic, crystalline compound that is readily soluble in water and ethyl alcohol; it has a melting point of 100°C.
Malic acid was first isolated by K. Scheele, who in 1785 obtained it from unripe apples. The L-form of the acid is found in plants either in the free state or as acid salts; the presence of either the acid or salt makes possible the acid reaction of cell fluid. Fruits rich in malic acid include barberries, raspberries, apples, and the berries of the mountain ash; the vegetative organs of succulents, especially Crassulaceae, also contain considerable amounts of the acid. The tobacco plants Nicotiana rustica and Nicotiana tabacum contain the nicotine salts of the acid.
Malic acid is an intermediate in cell respiration—in the tricarboxylic acid cycle and its variant, the glyoxylate cycle. In plants containing considerable amounts of organic acids (for example, rhubarb, begonia, and dock), free ammonia is rendered harmless by the formation of the ammonium salts of organic acids, including malic acid. Malic acid is used by many microorganisms as an energy substrate or a source of carbon. It is formed as a by-product in various types of fermentation.
Malic acid is used in the production of fruit drinks and candies.
E. P. FEDENKO