LACTIC ACID (C.H.O.). Scheele, in 1780, was the first to describe the acid present in sour milk. In 1832 Liebig and Mitscherlich showed it to be a distinct acid. Lactic acid is widely distributed in nature, occurring in the sap of several plant families, in sour milk, hi the saliva, blood and urine, and the brain and gastric juice of animals, Lactic acid is a result of the. fermentation of the various sugars, and of dextrin and mannitol. There are many modes of preparing it artificially; and its iso meric varieties, of which four have been de scribed, have excited much attention. Its two principal kinds are fermentation lactic acid and paralactic or sarcolactic acid. The former is got from concentrated sour whey by remov ing the curd, adding lime, filtering, diluting with water, removing the lime with oxalic acid, evaporating and extracting the lactic acid with alcohol. It is more usual, however, to get it by what is called the lactic fermentation, from spgar or saccharine substances. The sugar is dissolved in water; to the solution is added sour milk or decaying cheese and a quantity of fine, well-washed prepared chalk, and the ture is kept for about four weeks at between 86° and 95° F. Fermentation ensiles and much lactic acid is formed, which combines with the chalk and forms lactate of calcium. This salt is then decomposed by sulphuric acid, filtered, and the fluid is boiled with carbonate of zinc.' Lactate of zinc is formed, which is collected and decomposed by sulphureted hydrogen. ' The fluid filtered from the sulphide of zinc is evaporated, and the syrupy fluid which remains contains the lactic acid. Lactic acid of com merce is a syrup (specific gravity, 1215) which contains a small' percentage of water. re mains liquid even at vary low temperathires, deliquesces in moist air, dissolves in all propor tions in alcohol and water, has no odor and has a purely sour, taste. It cannot be distilled, or even heated, without undergoing decompo sition, lactic anhydride or lactolactic acid being formed; at a higher temperature carbonic oxide is evolved, and a variety of products distil over, and charcoal is left in the retort. By oxidizing agents, such as bleaching-powder and nitric acid, it is converted into oxalic acid; by oxide of manganese into aldehyde.
The paralactic or sarcolactic acid was ob served in flesh by Berzelius in 1806, and he con sidered it the same as that derived from milk. Liebig showed that they were not absolutely identical, but the nature of their differences is at present unknown. This acid is readily got from the cold aqueous extract of meat by add ing a solution of baryta, coagulating and re moving albumen, concentrating the solution, precipitating the baryta, filtering and evaporat ing. The syrupy residue contains the acid. Paralactic acid can be distinguished from acid by its property of rotating the plane of polarization to the right, lactic acid being inactive. The calcium salt of the fermentation acid contains more water of crystallization; when heated it retains it for a shorter time; and it is more soluble than the sarcolactate. Again, the zymolactate of zinc contains more water, loses it more quickly on heating, but itself endures a much higher temperature with out decomposition than the sarcolactate. The zymolactate is much less soluble in water and in alcohol than the other; the crystalline forms of the two salts also are different. The other salts of lactic acid are for the most part crystalline, and soluble in water. Lactic acid forms compound ethers and substitution acids.
In the arts, lactic acid is used in the dyeing of wool when mordanting with potassium dichromate, supplanting tartaric and oxalic acids. It is less corrosive, and imparts a finer feel and lustre. With an equal part of potas sium lactate it forms lactolin, regarded as su perior to the plain acid. Lactic acid is a solvent for several dyestuffs which are in soluble in water, and is of special use in the dyeing and printing of cotton textiles. It is also used in the tanning industry, in coloring, bating and plumping the skins, to which it imparts a fine grain. In the distilling industry it is used to check the development of delete rious bacteria.
In medicine, lactic acid is used in laryngeal tuberculosis, and diluted with six parts of water, it is applied locally in diphtheria and croup to dissolve the false membrane. It is also used as a local application in tuberculous ulcers, lupus and epithelioma.