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Lactic acid is a colourless liquid organic acid. Lactic acid is a fermentation product of lactose (milk sugar); it is present in sour milk, koumiss, leban, yogurt, and cottage cheese. The protein in milk is coagulated (curdled) by lactic acid. Lactic acid is produced in the muscles during intense activity. Lactic acid also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid (AHA).

In solution, it can lose a proton from the acidic group, producing the lactate ion CH₃CH(OH)COO−. It is miscible with water or ethanol, and is hygroscopic. Lactic acid does not exist as an acid in the body: it exists in another form called “lactate”, and it is this that is actually measured in the blood when “lactic acid” concentration is determined, as is done from time to time. This distinction is important not only for the sake of correctness, but more importantly, because lactate and lactic acid would have different physiological effects. Lactic acid is chiral and has two optical isomers. One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(-)-lactic acid or (R)-lactic acid. L-(+)-Lactic acid is the biologically important isomer.

Chemical formula: C₃H₆O₃
Rel. mol. weight: 90.08
CAS No.: 50-21-5
EINECS No.: 200-018-0

Specific Gravity: 1.2
Boiling Point: 122 °C
Melting Point: 17 °C
Solubility: miscible in water
Appearance: Colorless to slightly yellow, 
syrupy liquid
Odor: Slight acrid odor.