Lactic acid (ethyl ester) is a colorless liquid with a mild odor. It is classified as moderately toxic, flammable, combustible liquid when exposed to heat and flame. It is used in food as flavoring agent.
Lactic acid is widespread in nature. A clue to its ubiquity is obtained by comparing its molecular formula, C3H6O3, with that of glucose, C6H12O6. A lactic acid molecule is, in fact, half a glucose molecule. Indeed, a widespread source of lactic acid is the anaerobic fermentation of sugars and the action of enzymes on glucose supplies. Fresh milk rapidly becomes populated with bacteria that act on the milk sugar lactose, break it up for energy, and excrete lactic acid. The acid causes the fatty droplets to coalesce, and the milk curdles. This process is encouraged in a controlled way in the production of yogurt, which depends on lactic acid production by a mixed culture of the bacteria Lactobacillus bulgaricus and Streptococcus thermophilus. Fermented pickles also owe their tartness to lactic acid. Sauerkrout is produced by steeping fresh cabbage in brine, which suppresses the growth of some bacteria and gives others a chance to thrive. As they do, they consume glucose and excrete lactic acid.
Lactic acid is also produced from glucose by enzymes in our sweat glands, which accounts for the acid taste of perspiration, and by the action of bacteria on the lining of the vagina, which is a rich store of glucose. It is also produced, as a last resort, in muscles that have exhausted their immediate oxygen supply and cannot metabolize glucose aerobically. Thus, a sprinter may unconsciously resort to the anaerobic energy supply of his or her ancestors, and make do with the energy released by slicing glucose molecules into lactic acid halves. Unfortunately, this builds up the acid concentration in the muscles, interfering with their operation so that they feel heavy and weak, and may produce cramping.
Bodily lactic acid becomes involved indirectly with heavy drinking. Since the metabolizing capacity of the liver may be impaired by the demands of the alcohol, lactic acid is not removed so efficiently. It may then build up in the bloodstream, raise the acidity of the muscles, and lead to the kind of fatigue experienced by an athlete. Lactic acid can also influence the deposition of solid compounds, specifically salts of uric acid. These are normally excreted in the urine, but their excretion is inhibited by lactic acid, and they may, instead, deposit in the joints causing the painful condition known as gout.
- Peter William Atkins. Molecules
- Richard J. Lewis, Sr. Food Additives Handbook