1911 Encyclopædia Britannica/Lactic Acid

LACTIC ACID (hydroxypropionic acid), C₃H₆O₃. Two lactic acids are known, differing from each other in the position occupied by the hydroxyl group in the molecule; they are known respectively as α-hydroxypropionic acid (fermentation or inactive lactic acid), CH₃·CH(OH)·CO₂H, and β-hydroxypropionic acid (hydracrylic acid), (q.v.), CH₂(OH)·CH₂·CO₂H. Although on structural grounds there should be only two hydroxypropionic acids, as a matter of fact four lactic acids are known. The third isomer (sarcolactic acid) is found in meat extract (J. v. Liebig), and may be prepared by the action of Penicillium glaucum on a solution of ordinary ammonium lactate. It is identical with α-hydroxypropionic acid in almost every respect, except with regard to its physical properties. The fourth isomer, formed by the action of Bacillus laevo-lacti on cane-sugar, resembles sarcolactic acid in every respect, except in its action on polarized light (see Stereoisomerism).

Fermentation, or ethylidene lactic acid, was isolated by K. W. Scheele (Trans. Stockholm Acad. 1780) from sour milk (Lat. lac, lactis, milk, whence the name). About twenty-four years later Bouillon Lagrange, and independently A. F. de Fourcroy and L. N. Vauquelin, maintained that Scheele’s new acid was nothing but impure acetic acid. This notion was combated by J. Berzelius, and finally refuted (in 1832) by J. v. Liebig and E. Mitscherlich, who, by the elementary analyses of lactates, proved the existence of this acid as a distinct compound. It may be prepared by the lactic fermentation of starches, sugars, gums, &c., the sugar being dissolved in water and acidified by a small quantity of tartaric acid and then fermented by the addition of sour milk, with a little putrid cheese. Zinc carbonate is added to the mixture (to neutralize the acid formed), which is kept warm for some days and well stirred. On boiling and filtering the product, zinc lactate crystallizes out of the solution. The acid may also be synthesized by the decomposition of alanine (α-aminopropionic acid) by nitrous acid (K. Strecker, Ann., 1850, 75, p. 27); by the oxidation of propylene glycol (A. Wurtz); by boiling α-chlorpropionic acid with caustic alkalis, or with silver oxide and water; by the reduction of pyruvic acid with sodium amalgam; or from acetaldehyde by the cyanhydrin reaction (J. Wislicenus, Ann., 1863, 128, p. 13)

CH₃·CHO → CH₃·CH(OH)·CN → CH₃·CH(OH)·CO₂H.

It forms a colourless syrup, of specific gravity 1.2485 (15°/4°), and decomposes on distillation under ordinary atmospheric pressure; but at very low pressures (about 1 mm.) it distils at about 85° C., and then sets to a crystalline solid, which melts at about 18° C. It possesses the properties both of an acid and of an alcohol. When heated with dilute sulphuric acid to 130° C., under pressure, it is resolved into formic acid and acetaldehyde. Chromic acid oxidizes it to acetic acid and carbon dioxide; potassium permanganate oxidizes it to pyruvic acid; nitric acid to oxalic acid, and a mixture of manganese dioxide and sulphuric acid to acetaldehyde and carbon dioxide. Hydrobromic acid converts it into α-brompropionic acid, and hydriodic acid into propionic acid.

Lactide, OCH(CH₃·CO
CH·CH(CH₃)O, a crystalline solid, of melting-point 124° C., is one of the products obtained by the distillation of lactic acid.