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The Collaborative International Dictionary of English v.0.48 (gcide)

Lactic \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.
[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]