a. [ L. lac, lactis, milk: cf. F. lactique. See Lacteal, and cf. Galactic. ] (Physiol. Chem.) Of or pertaining to milk; procured from sour milk or whey; as, lactic acid; lactic fermentation, etc. [ 1913 Webster ]

Lactic acid (Physiol. Chem.), a sirupy, colorless fluid, soluble in water, with an intensely sour taste and strong acid reaction. There is one center of optical activity, and this results in the observation of three isomeric modifications all having the formula C3H6O3; one is dextrorotatory (L-lactic acid), the other levorotatory (D-lactic acid), and the third an optically inactive mixture of the first two (DL-lactic acid); chemically it is 2-hydroxypropanoic acid. Sarcolactic acid or paralactic acid occurs chiefly in dead muscle tissue, while ordinary lactic acid (DL-lactic acid) results from fermentation, such as the fermentation of milk by lactic acid bacteria. The two acids are alike in having the same constitution (expressed by the name ethylidene lactic acid), but the latter is optically inactive, while sarcolactic acid rotates the plane of polarization to the right. The third acid, ethylene lactic acid, accompanies sarcolactic acid in the juice of flesh, and is optically inactive. --
Lactic ferment, an organized ferment (Bacterium lacticum or Bacterium lactis), which produces lactic fermentation, decomposing the sugar of milk into carbonic and lactic acids, the latter, of which renders the milk sour, and precipitates the casein, thus giving rise to the so-called spontaneous coagulation of milk. --
Lactic fermentation. See under Fermentation. [ 1913 Webster +PJC ]