(α-hydroxypropionic acid, CH3CH(OH)-COOH), a monobasic hydroxycarboxylic acid; an important intermediate metabolite in animals, plants, and microorganisms.
Lactic acid occurs as colorless water-soluble crystals. It exists in two optically active forms, (−) and (+), with a melting point of 25°–26°C, and in an inactive racemic form, (±), with a melting point of 18°C. The (−) and (+) lactic acids undergo racemization at 130°–150°C and produce salts (lactates) and esters similar to carboxylic acids.
Lactic acid is formed during the fermentation of sugary substances by various species of lactic acid bacteria (for example, during the souring of milk, the pickling of cabbage and vegetables, the aging of cheese, and ensilage). Either a racemate or one of the optically active forms of lactic acid is produced, depending on the species of bacteria and the type of sugar present. The industrial fermentation of cane sugar, dextrose, and maltose by Bacillus Delbrucki yields (±) lactic acid, which is used to delime leather in tanning and for chromate reduction in mordanting chrome. Lactic acid salts are used as mordants in dyeing and printing, and lactic acid esters as varnish thinners.
In animal tissue (primarily muscle), the formation of (+) lactic acid is the final stage in the anaerobic enzymic breakdown of carbohydrates, or glycolysis. In the absence of O2, the amount of lactic acid produced in muscle tissue equals the amount of glycogen lost. (Muscle fatigue is associated with the depletion of glycogen reserves and the accumulation of lactic acid.) In the presence of O2, approximately 20 percent of the lactic acid is oxidized to CO2 and H2O, the energy released is used to convert the remaining lactic acid to glycogen, and the onset of fatigue is more gradual. The reversible reduction of pyruvic acid (pyruvate) to lactic acid (lactate) occurs during the final stage of glycolysis upon the action of lactate dehydrogenase. Plants and microorganisms produce lactic acid by pyruvate reduction.
V. N. FROSIN