A colourless, syrupy, sour liquid, which is produced by the action of bacteria upon lactose, the sugar found in milk. The growth of this organism and consequent formation of lactic acid cause the souring of milk, and the same change takes place to a limited extent when food is retained for a long time in the stomach.

Lactic acid (is produced in the body during muscular activity, from the breakdown of GLYCOGEN. Muscle fatigue is associated with an accumulation of lactic acid in the muscle. Recovery follows when enough oxygen reaches the muscle, part of the lactic acid being oxidised and most of it then being built up once more into glycogen.