Lactic acid (Physiol. Chem.), a sirupy, colorless fluid, soluble in water, with an intensely sour taste and strong acid reaction. There are at least three isomeric modifications all having the formula C₃H₆O₃. Sarcolactic or paralactic acid occurs chiefly in dead muscle tissue, while ordinary lactic acid results from fermentation. 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 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.