Lactic acid

Lactic acid is a colorless, water-soluble liquid that freezes, or solidifies, at 64.4°F (18°C)—just slightly below normal room temperature . It is scientifically known as alpha-hydroxypropanoic acid and has the chemical formula C3H6O3; the structural formula is shown below:

The "hydroxy" portion of the name tells chemists that there is an alcohol (OH) group in the molecule , and the "alpha" part of the name means that the alcohol is attached to the carbon atom adjacent to the acid (COOH) group. The "prop" portion of the name indicates that there are three carbon atoms . Lactic acid can also be called 2-hydroxypropanoic acid. Each of the two isomers rotates polarized light in a different direction: the L-isomer rotates light to the left, and the D-isomer rotates light to the right. Like most acids, lactic acid has a sour taste. It is found in sour milk, molasses, and many fruits . The lactic acid found in milk is usually a mixture of both isomers. It is used commercially in the textile and dairy industries. Lactic acid is the byproduct of anaerobic respiration , and is largely responsible for the aches in sore muscles after a vigorous workout.

Lactic acid in foods

Lactic acid is found throughout nature—from fruits to molasses, although most people's experience with lactic acid is in sour milk. Lactic acid in milk is the product of the fermentation of lactose (milk sugar) by the bacteria Lactobacillus bulgaris and Lactobacillus acidi lacti. In the manufacture of yogurt, this reaction is carefully controlled to ensure the production of yogurt and not spoiled milk. The lactic acid in molasses is the product of the digestion of sugars by other species of bacteria.

Lactic acid in human metabolism

Lactic acid is the product of anaerobic respiration, the burning of stored sugars without sufficient oxygen by cells. Anaerobic respiration is much less efficient than aerobic respiration, for which there is enough oxygen to fully utilize the stored sugar energy . Essentially, anaerobic respiration causes the halving of glucose molecules (C6H12O6) into lactic acid molecules (C3H6O3). The lactic acid builds up in muscles, accounting for the soreness in overworked muscles. This build-up of lactic acid may also lead to cramps. One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and more steady use of muscles. Eventually the build-up of lactic acid is carried away in the bloodstream and the lactic acid is converted to carbon dioxide (CO2) gas and water vapor, both of which are exhaled. If lactic acid levels in the bloodstream rise faster than the body's natural pH buffers—combinations of acids, salts, and bases that maintain a constant pH level—can neutralize them, a state known as lactic acidosis may exist. Lactic acidosis rarely happens in healthy people. It is more likely the result of the body's inability to obtain sufficient oxygen (as in heart attacks or carbon monoxide or cyanide poisoning) or from other diseases such as diabetes.

The ability of the body to metabolize, or break down, lactic acid is decreased significantly by alcohol, which impairs the liver's ability to carry out normal metabolic reactions. Thus, alcoholics often have sore muscles from lactic acid build up that was not caused by exercise . Lactic acid can also lead to a build-up of uric acid crystals in the joints, since lactic acid reduces the elimination of uric acid and related compounds. This build-up can lead to gout, a very painful disease .

Uses of lactic acid

Lactic acid is used as a humectant, or moisturizer, in some cosmetics and as a mordant, a chemical that helps fabrics accept dyes, in textiles . It is also used in making pickles and sauerkraut, foods for which a sour taste is desired. Lactic acid is used in the dairy industry not only in making yogurt but in making cheese as well. It is also used in tanning leather. Lactic acid is important in the pharmaceutical industry as a starting material for other substances and is involved in the manufacturing of lacquers and inks. A related compound that is made from lactic acid is calcium stearoyl-2-lactylate, which is used as a food preservative.

See also Acids and bases; Metabolic disorders; Metabolism.

Resources

periodicals

"Babies Fuss Over Post-Exercise Milk." Science News vol. 142 (July 18, 1992) :47.

Chase, Marilyn. "High Lactic Acid Found in Patients with Huntington's." The Wall Street Journal, October 27, 1992, B12.

Drake, Geoff. "The Lactate Shuttle-Contrary to What You've Heard, Lactic Acid is Your Friend." Bicycling 33 (August 1992): 36.

Roberts, Marjorie. "Keeping Yogurt Honest-Its Reputation as a Health Food May Not Always be Deserved." U.S. News and World Report 109 (November 5, 1990,)" 76.

"Some Babies Turn Up Their Noses at Breast Milk After Mom Exercises." Environmental Nutrition 16 (June, 1993): 3.

Louis Gotlib

KEY TERMS

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Buffers

—Combinations of acids, bases, and salts that neutralize changes in pH.

Humectant

—A moisturizing agent, often used in cosmetics.

Isomers

—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Mordant

—A chemical that helps fabrics accept dyes more readily.

Polarized light

—Light that vibrates in a single plane, as opposed to ordinary light that vibrates in all planes.