LACTIC ACID, a chemical term, which, though originally invented to designate the particular acid contained in sour milk, has now, through the discovery of other acids isomeric with and very similar to that acid, acquired a generic, in addition to its original specific, meaning.
twenty-four years later Bouillon Lagrange, and, independently of him, also Fourcroy and Vauquelin maintained that Scheele's new acid was nothing but impure acetic. But this notion was combated by Berzelius, and finally refuted (in 1832) by Liebig and Mitscherlieh, who by the elementary analyses of lactates proved the existence of this as a distinct acid.
In the article FERMENTATION (VOL ix. p. 97) it is explained how lactic acid is produced from milk-sugar and from ordinary glucose by "lactic fermentation." The most convenient process for the preparation of the acid is Bensch's. A solution of " invert-sugar " (see vol. ix. p. 96) is produced by dissolving 6 parts of cane-sugar and thth part of tartarie acid in 35 parts of boiling water and allowing to stand for two days. There is then added .-2-Ath part of. foul cheese, 8 parts of sour milk, and 2 parts of carbonate of zinc, and the mixture kept at 40° to 45° C. for eight to ten days. The sugar, C611„0,, ferments into lactic acid, C314,03, which, by the carbonate present, is converted into lactate of zinc. But part of the lactate is invariably lost through "butyric fermentation" with evolution of hydrogen, which latter converts part of the sugar into marmite, C6111406. The fermented liquid is heated to boiling, strained clear, and allowed to cool, when the lactate of zinc separates out in crystalline crusts, which are purified by recrystallization from hot water. The free acid is obtained by decomposing the hot aqueous solution of the salt with sulphuretted hydrogen and filtering off the sulphide of zinc. The filtrate is evaporated on a water bath to a syrup, which is treated with ether. :Marmite and other impurities remain, and the acid passes into the filtrate, from which the ether is easily expelled by distillation,and anbsequent evaporation in an open basin. What ultimately remains is a thick colourless syrup, which, in ordinary chemical parlance, goes as lactic acid, although it is at best only an approximation to the hydrate, C311603.140. The extra ILO is easily enough removed by continued evaporation, but no portion of it can he thus got rid of without the acid C311603 itself suffering dehydration into lactic anhydride, C6H„0„ and lactide, C611,04, by successive subtractions of 1120 from 2C311602.
The behaviour of lactic acid solution to basic reagent; so much lactic is strictly equivalent to one molecule of acetic acid, and as the latter is proved to be CHR.COOII, lactic acid must be assumed to be (C„II,O) C0011. But the radicle C21150 (unlike the CH, of acetic acid) still contains one hydrogen atom, which, although not replaceable by metals, can be replaced by acid radicles such as acetyl C2I1,0 or, conjointly with the oxygen atom, by hydriodic acid the same II conjointly with the 0 of the radicle is replaced by iodine with formation of water. In a word, lactic acid, besides being an acid analogous to, for instance, acetic acid, CH,COOFI, is at the same time an alcohol analogous to ordinary spirit of wine, C2H5.OH, as combine into an ether which is lactic anhydride. Thus : - Lactic anhydride. Water.
The slanting lines show the mode of combination after the reaction. The anhydride, as we see, still contains an could be presumed to be formed from one molecule of lactic acid : II0(C,IL)C001-I - (C,TT4)C00 + H. OH ; but the vapour density determination proves the molecular weight to be in accordance with the larger formula C011504.
+ COOH. The (CH3) and H in the original aldehyde retain their places, so that what we obtain ultimately must be (CH3) - (C1I)<C°01101I and this consequently is the structure of lactic acid. The radicle (CII3XCII) is called ethylidene, to distinguish it from "ethylene," which is (CILACII,).
Parabletic or •.-treolactic Acid. - This acid was disreadily soluble than ordinary lactates, and contain different proportions of crystal water from these. Thus, for instance, we have for the zinc salts Ordinary.
ZukC311,03).,. 31-10 . Zn(C31 . 21120 .
Soluble in 60 parts of cold Soluble in 17 parts of cold awl in 6 parts of boiling seater. 1 water.
The isomerism of the two acids used to be explained by assuming that the sarco-acid contained ethylene in lieu of the ethylidene of the ordinary acil, thus : (01I) - (CII2) - (C14,) - c00B, i.e., that the OH and COOLI were attached to different carbon atoms. But this has been proved by Erlenmeyer to be a mistake. The sareo.acid has precisely the same structure as ordinary lactic acid. It is a case of absolute (i.e., of unexplained) isomerism.
for OH : I. il.,C - Cff, - 0001I (OD). II,C - C11, - 00011, cyanhydrine (01I) - (C,II4) - (CN)• As suggested by the formula, it differs markedly in its reactions from the two more properly so-called lactic acids.
All lactic acids, when heated with hydriodic acid in sealedup tubes, pass ultimately into (the same) propionic acid, (( - ( CIL) - COOH. (W. D.)