LACTIC ACID

Lactic acid was first identified as an acidic constituent of foods nearly 200 years ago. I t has since been shown to be widespread in nature and has been made industrially b y fermentation since 1881 (Fieser & Fieser, 1950). Englehard, in 1848 (Filachione, 1952), showed tha t the lactic acids isolated from muscle by Berzeleus, in 1807 (Filachione, 1952), and by Scheele from sour milk (1780), had the same composition. Wislicenus, in 1873 (Filachione, 1952), proved that they had the same structure, bu t different physical properties. He attributed the differences to differences in the spatial arrangement of atoms within the molecules. The acid occurs in two stereoisomeric forms, and in a so-called racemic mixture of these isomers. In biological systems, the true racemic mixture, or equal molarity of the isomers, is of rare occurrence. Usually, one or the other isomer predominates. Chemosynthetic lactic acid derived from lactonitrile is truly racemic. The stereoisomers have been variously named dand 1-lactic acid and D(-) and L( +) lactic acid. The lower case letters, dand 1, were used to indicate the clockwise or counterclockwise direction of rotation of a plane of polarized light by their solutions, and give no indication of molecular configuration. In the more modern terminology, D( ) and L ( + ) lactic acid, the symbols ( ) and (+) indicate the direction of rotation of a plane of polarized light, the ( ) being equivalent of the lower case 1and the (+) being equivalent to the lower case dof the older nomenclature. The capital letters D and L preceding the directional symbols refer to the molecular structural relationship with the isomers of glyceric acid. Additional names, sarcolactic acid and paralactic acid, have been applied to the L ( + ) isomer, but are no longer widely used.