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lactic acid

lactic acid, CH3CHOHCO2H, a colorless liquid organic acid. It is miscible with water or ethanol. 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 by the breakdown of glucose, and may be used by muscle cells as a source of energy. Calcium lactate, a soluble lactic acid salt, is used as a source of calcium in the diet. Lactic acid is produced commercially for use in pharmaceuticals and foods, in leather tanning and textile dyeing, and in making plastics, solvents, inks, and lacquers. Although it can be prepared by chemical synthesis, production of lactic acid by fermentation of glucose and other substances is a less expensive method. Chemically, lactic acid occurs as two optical isomers, a dextro and a levo form; only the levo form takes part in animal metabolism. The lactic acid of commerce is usually an optically inactive racemic mixture of the two isomers.

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lactic acid

lactic acid The acid produced by the anaerobic fermentation of carbohydrates. Originally discovered in sour milk, it is responsible for the flavour of fermented milk and for the precipitation of the casein curd in cottage cheese. Also produced by fermentation in silage, pickles, sauerkraut, cocoa, and tobacco, its value here is in suppressing the growth of unwanted organisms, and as the product of glucose metabolism in muscle under conditions of maximum exertion.

Used as an acidulant (as well as citric and tartaric acids) in sugar confectionery, soft drinks, pickles, and sauces.

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lactic acid

lactic acid (2-hydroxypropanoic acid) An alpha hydroxy carboxylic acid, CH3CH(OH)COOH, with a sour taste. Lactic acid is produced from pyruvic acid in active muscle tissue when oxygen is limited (see oxygen debt) and subsequently removed for conversion to glucose by the liver. During strenuous exercise it may build up in the muscles, causing cramplike pains. It is also produced by fermentation in certain bacteria and is characteristic of sour milk.

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lactic acid

lactic acid Colourless, organic acid (2-hydroxypropanoic acid, CH3CHOHCOOH) formed from lactose by the action of bacteria. It is also produced in muscles, where it causes muscle fatigue, when anaerobic respiration occurs due to insufficient oxygen. Lactic acid is used in foods and beverages, in tanning, dyeing, and adhesive manufacture. Properties: r.d. 1.206; m.p. 18°C (64.4°F); b.p. 122°C (252°F).

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lactic acid

lactic acid (lak-tik) n. a compound that forms in the cells as the end-product of glucose metabolism in the absence of oxygen (see glycolysis). Lactic acid (owing to its low pH) is an important food preservative.

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lactic acid

lactic acid A 3-carbon hydroxy-acid that is formed as the major metabolic product of certain bacteria (and also from pyruvic acid in animal cells when glycolysis occurs under anaerobic conditions).

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lactic acid

lac·tic ac·id • n. Biochem. a colorless syrupy organic acid, CH3CH(OH)COOH, formed in sour milk, and produced in the muscle tissues during strenuous exercise.

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lactic acid

lactic acid A three-carbon hydroxyacid, formed as the major metabolic product of certain bacteria, and also from pyruvic acid in animal cells when glycolysis occurs under anaerobic conditions.

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Lactic Acid

Lactic Acid

Lactic acid in foods

Lactic acid in human metabolism

Uses of lactic acid

Resources

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 anaerobicrespiration, and is largely responsible for the aches in sore muscles after a vigorous workout.

Lactic acid in foods

Lactic acid is found throughout naturefrom fruits to molasses, although most peoples 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 buildup 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 steadier use of muscles. Eventually the buildup 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 bodys natural pH bufferscombinations of acids, salts, and bases that maintain a constant pH levelcan 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 bodys inability to obtain sufficient oxygen (as in heart attacks or carbon monoxide or cyanide poisoning) or from other diseases such as diabetes.

The bodys ability to metabolize, or break down, lactic acid is decreased significantly by alcohol, which impairs the livers ability to carry out normal metabolic reactions. Thus, alcoholics often have sore muscles from lactic acid buildup that was not caused by exercise. Lactic acid can also lead to a buildup of uric acid crystals in the joints, since lactic acid reduces the elimination of uric acid and related compounds. This buildup 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 in which a sour taste is desired. Lactic acid is used in the dairy industry to make yogurt and cheese, and 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

KEY TERMS

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.

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, p. 47.

Chase, Marilyn. High Lactic Acid Found in Patients with Huntingtons. The Wall Street Journal Oct 27, 1992, p. B12.

Drake, Geoff. The Lactate ShuttleContrary to What Youve Heard, Lactic Acid Is Your Friend. Bicycling vol. 33, August 1992, p. 36.

Roberts, Marjorie. Keeping Yogurt HonestIts Reputation as a Health Food May not Always Be Deserved. U.S. News and World Report vol. 109, November 5, 1990, p. 76.

Some Babies Turn up Their Noses at Breast Milk after Mom Exercises. Environmental Nutrition. Vol. 16, June, 1993, p. 3.

OTHER

PhysOrg. Muscles Burn Lactic Acid as Well as Carbos

<http://www.physorg.com/news64680736.html> (accessed December 2, 2006)

Science Daily. Lactic Acid Bacteria: Zesty Microbes Enliven the Palate, Provide Better Blueprint for Biofuels and Specialty Chemical Production <http://www.sciencedaily.com/releases/2006/10/061017091752.htm> (accessed December 2, 2006)

Louis Gotlib

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Lactic Acid

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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.

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Lactic Acid

Lactic Acid


Lactic acid is an organic compound found in the blood and muscles of animals during extreme exercise. It also is produced in some plants as a result of the fermentation (the process of splitting complex organic compounds into simple substances) of certain carbohydrates. A buildup of lactic acid in the body is toxic and causes muscle fatigue, pain, and cramps.

Athletes who push themselves or exert their muscles and bodies beyond what is a comfortable level of exercise often experience a "burn" in their muscles. If they do not stop exercising, they sometimes will feel muscle cramps, pain, and overall fatigue or exhaustion. This is the direct result of lactate, or lactic acid, building up in the blood and muscles. This occurs naturally when a person's muscles are not given a rest and are made to keep contracting (which is how a muscle works).

During this or any kind of exercise, the body uses up energy that it gets from a process called respiration. This is not the respiration we refer to as breathing, but rather it is the chemical process of breaking food down to release the energy it contains. After we eat and our body's enzymes break down the food we have consumed into glucose (sugar), our bodies store the glucose if we do not need to use it immediately. This depends on our level of activity. A certain amount of glucose will always be needed just to maintain all of our systems and keep everything "running."

When we start to increase our body's energy demands by doing something strenuous like running or exercising, the process of aerobic respiration starts up. Our cells break down the glucose the body has stored by combining it with oxygen. Aerobic respiration releases a large amount of energy. Sometimes we exercise so vigorously or steadily that our muscles start to consume or use up the much-needed oxygen faster than we can take it in, which is why we breathe heavier and faster as we exert ourselves. When this "oxygen debt" occurs and we still do not stop exercising and consuming energy, our cells get the message that they should begin the alternative process called anaerobic respiration.

Anaerobic respiration involves the release of energy without needing to consume any oxygen (anaerobic means no oxygen). This is basically the same process that occurs in fermentation, since fermentation is the breaking down of organic materials without the consumption of oxygen. Although anaerobic respiration provides the muscles with much-needed energy, it also has a toxic or poisonous by-product called lactate or lactic acid. If lactic acid is allowed to buildup in muscles, it causes cramps and pain, makes them work less efficiently, and eventually will simply shut them down.

Lactic acid will only begin to be broken down and removed from the system once the body is able to begin normal, aerobic respiration (with oxygen). Once the body's "oxygen debt" is replenished by enough heavy breathing and an activity slowdown, the unpleasant side effects caused by lactic acid will disappear. Fitness training is thought to increase an athlete's tolerance to lactic acid buildup.

Lactic acid is also found outside the body. Not surprisingly, it is an important acidic component of fermented food products such as yogurt, buttermilk, sauerkraut, green olives, and pickles. The formation of lactic acid in these food products is the result of the activity of lactic acid bacteria. Lactic acid also has industrial uses, as it is used in a variety of processes including tanning (converting animal hide into leather) and wool dyeing.

[See alsoBlood; Carbohydrates; Fermentation ]

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Lactic Acid

Lactic Acid

OVERVIEW

Lactic acid (LAK-tik AS-id) is a colorless, odorless, syrupy liquid that occurs in two isomeric forms, D-lactic acid and L-lactic acid. Isomers are two or more forms of a chemical compound with the same molecular formula, but different structural formulas and different chemical and physical properties. The D form is produced during metabolic reactions that take place in muscle tissue, while the L form is produced by yeast cells. The synthetic production of lactic acid results in a product consisting of equal amounts of the D and L forms, a mixture known as a racemic mixture.

KEY FACTS

OTHER NAMES:

2-hydroxypropanoic acid; ±-hydroxypropanoic acid; milk acid

FORMULA:

CH3CHOHCOOH

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Carboxylic acid (organic)

STATE:

Liquid

MOLECULAR WEIGHT:

90.08 g/mol

MELTING POINT:

16.8°C (62.2°F)

BOILING POINT:

Not applicable; decomposes upon heating

SOLUBILITY:

Very soluble in water and ethanol; slightly soluble in ether

Lactic acid was first discovered in 1780 by the Swedish chemist Karl Wilhelm Scheele (1742–1786), who called his discovery "acid of milk." The two isomeric forms of the acid were first identified in 1863 by the German chemist Johannes Wislicenus (1835–1902), and the compound was first produced commercially in 1881 by American chemist Charles E. Avery. Avery patented his invention in 1885 and constructed a factory for the production of lactic acid in Littleton, Massachusetts.

About 30 million kilograms (72 million pounds) of lactic acid are produced annually in the United States. The most common method of production is the fermentation of glucose by yeast.

HOW IT IS MADE

In muscle cells, lactic acid is the product of anaerobic respiration, the process by which glucose is oxidized in the absence of oxygen to produce energy required by cells. Although some lactic acid is always produced in muscle cells in very low concentrations, it tends to accumulate during exercise, when cells do not receive adequate amounts of oxygen to metabolize oxygen by normal pathways. Lactic acid produced during exercise remains in the body for only short periods of time, sometimes in less than thirty minutes. It is metabolized in the muscle cells where it was produced, resulting in the production of energy, carbon dioxide, water, and other products.

Lactic acid is also produced by yeast during the process of fermentation. Fermentation is the process by which yeast cells convert glucose to an alcohol and carbon dioxide. Yeast cells use almost precisely the same enzyme in fermentation that muscle cells use in anaerobic respiration. The muscle cell enzyme and the yeast enzyme differ only in the orientation of one group of atoms, resulting in the production of the D isomer in one case and the L isomer in the other.

A synthetic process for the production of lactic acid was first introduced in 1963. That process begins with the addition of hydrogen cyanide (HCN) to acetaldehyde (ethanal; CH3CHO), resulting in the formation of lactonitrile (CH3CH2OCN). The lactonitrile is then hydrolyzed, using a strong acid, such as sulfuric acid, as a catalyst, to make lactic acid.

Interesting Facts

  • For the better part of a century, athletes and physiologists have considered lactic acid a primary cause of fatigue during high-intensity exercise. However, scientists have learned that lactic acid actually helps to prevent muscle fatigue. Muscle soreness once thought to be caused by lactic acid is, instead, more likely to be a result of damaged muscle cells caused by excess use.
  • Lactic acid present on the skin attracts mosquitoes.
  • Lactic acid in the body exists in its ionic form, known as lactate.

COMMON USES AND POTENTIAL HAZARDS

The primary use for lactic acid in the United States is as a food additive, where it acts as an acidulant and a flavor additive. An acidulant is a compound that provides an acidic environment for foods, as is the case with yogurt, buttermilk, sauerkraut, green olives, pickles, and other acidic foods. As a flavor additive, it adds a tart or tangy flavor to foods and beverages, as well as acting as a preservative to keep them from spoiling. Lactic acid also has a number of important industrial uses, the most important of which is the production of other organic chemicals, especially ethyl lactate, acrylic acid, propylene glycol, and the polymer known as polyactide. Polyactide is used in the manufacture of plastic film, fiber, packaging material, and filling materials. Other commercial and industrial applications of lactic acid include:

  • As a mordant in dyeing;
  • As a solvent for dyes that are not soluble in water;
  • For the treatment of animal hides in the preparation of leather products;
  • As a catalyst in the production of certain types of plastics; and
  • As an additive in electroplating baths.

Words to Know

CATALYST
A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.
ELECTROPLATING
Adding a layer of nickel, silver, or gold, on another type of metal using an electric current.
FERMENTATION
The process by which yeast convert glucose to an alcohol and carbon dioxide.
HYDROLYSIS
The process by which a compound reacts with water to form two new compounds.
ISOMER
One of two or more forms of a chemical compound with the same molecular formula, but different structural formulas and different chemical and physical properties.
METABOLISM
A process that includes all of the chemical reactions that occur in cells by which fats, carbohydrates, and other compounds are broken down to produce energy and the compounds needed to build new cells and tissues.
MORDANT
A substance used in dyeing and printing that reacts chemically with both a dye and the material being dyed to help hold the dye permanently to the material.
SYNTHESIS
A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.

Lactic acid in normal concentrations poses no safety or health hazards to humans or other animals. One health consequence related to lactic acid, however, is a condition known as gout, a type of arthritis that causes severe pain in the joints. Gout is caused by an accumulation of uric acid in the blood. Since lactic acid blocks the elimination of uric acid from the body, individuals with excess lactic acid buildup, usually caused by high alcohol consumption, may develop an excess of uric acid crystals in the blood and joints, leading to gout.

FOR FURTHER INFORMATION

"Cell Respiration." SparkNotes. http://www.sparknotes.com/testprep/books/sat2/biology/chapter6section1.rhtml (accessed on October 14, 2005).

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

Friel, Joel. "All Athletes: Lactic Acid's Bad Rap." Ultrafit's e-Tips For Endurance Athletes. October 2004, Vol. 7, No. 10. http://www.ultrafit.com/newsletter/october04.html#Joe (accessed on October 14, 2005).

"Lactic Acid." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/l0522.htm (accessed on October 14, 2005).

Rogers, Palmer, Jiann-Shin Chen, and Mary Jo Zidwick. Organic Acid and Solvent Production, Part I: Acetic, Lactic, Gluconic, Succinic, and Polyhydroxyalkanoic Acids. Section 2: Lactic Acid. Available online at http://141.150.157.117:8080/prokPUB/chaphtm/306/04_00.htm (accessed on October 14, 2005).

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