Lactic Acid
fermentation, milk, acids, inactive and sour
LACTIC ACID, four acids of the same chemical structure, the commonest being an acid produced in sour milk (Lat. lac, lactis, whence the name) during the bacterial fermentation of milk sugar or lactose; they have the formula HO(C2H4)CO2H and are hydroxypropionic acids. Other sugars, such as glucose and sucrose, and allied carbohydrates, such as starch and certain gums, also undergo this biochemical change, which is induced by certain species of bacteria known as lactic bacilli. The fermentation pro ceeds most favourably at 34-35° C and in a nearly neutral solu tion. Fermentation lactic acid as set free from crude zinc lactate (see below) by sulphuretted hydrogen is a viscid hygroscopic non crystallising syrup (sp. gr. 1.2485 at 15° C) miscible with water, alcohol or ether; it decomposes on distillation under atmospheric pressure, but under reduced pressure (I mm.) it distils at about 85° C and then sets to a crystalline solid melting at 18° C. This acid was first isolated from sour milk by K. W. Scheele in 1780, and its composition was established by J. von Liebig and E. Mitscherlich in 1832.
In carrying out the lactic fermentation, sugar is dissolved in water, acidified with tartaric acid and fermented by the addition of sour milk with some putrid cheese. Zinc carbonate is added to neutralise excess of acid and the mixture is kept warmed and stirred.
Ethylidenelactic Acids,
(OH) containing one asymmetric carbon atom, fermentation lactic acid is optically inactive but has been resolved into laevo- and dextro modifications by fractional crystallisation of the strychnine salts.(See STEREOCHEMISTRY.) Moreover, when Penicillium Glaucunz is cultivated in a solution of inactive ammonium lactate, the laevo-acid is assimilated more rapidly so that the solution acquires excess of dextro-acid and becomes optically active. The dextro lactic acid, formerly termed sarcolactic acid, occurs in meat juice and is accordingly found in Liebig's extract. The laevo-lactic acid is obtained from the fermentation of sucrose (cane sugar) by means of Bacillus laevo-lacti. Its salts have the same composition, solubilities, and chemical properties as those of the dextro-acid but both series differ from the corresponding salts of the inactive or racemoid variety of lactic acid. This inactive acid has been synthesised by several methods of which the following two are the simplest : hydrolysis of the cyanohydrin of acetaldehyde, (2) digestion of a-chloropropionic acid, with caustic alkali or moist silver oxide.
