\Lac"tic\, 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.
{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}.