Monosodium glutamate, sodium glutamate, flavour enhancer 621 EU food additive code: E621. HS code: 29224220. (IUPAC names: 2-aminopentanedioic acid, 2-aminoglutaric acid, 1-aminopropane-1,3-dicarboxylic acid), commonly known as MSG, Ajinomoto or Vetsin, is a sodium salt of glutamic acid. MSG is a food additive, popularly marketed as a "flavour enhancer". It was discovered and patented in 1909 by Ajinomoto Corporation in Japan. In its pure form, it appears as a white crystalline powder; when dissolved in water (or saliva) it rapidly dissociates into free sodium and glutamate ions (glutamate is the anionic form of glutamic acid, a naturally occurring amino acid).
MSG stimulates specific receptors located in taste buds such as the amino acid receptor T1R1/T1R3 or other glutamate receptors like the metabotropic receptors (mGluR4 and mGluR1) which induce the taste known as umami, one of the five basic tastes (the word umami is a loanword from Japanese; it is also referred to as "savory" or "meaty").
The umami taste may have evolved to help animals ingest food that have high protein content and is of significant importance to the food industry because of its flavor enhancement properties.
Sources of glutamate
Glutamate itself is a widespread amino acid: it is found naturally in human bodies and in protein-containing foods, such as peas, mushrooms, seaweed, tomatoes, fermented soy products, yeast extracts, nuts, legumes, meats and most dairy products. Some of the glutamate in foods is in a "free" form; and only this free form of glutamate can enhance the flavor of foods. Part of the flavour-enhancing effect of tomatoes, certain cheeses, and fermented or hydrolyzed protein products (such as soy sauce and soy bean paste) is therefore due to the presence of free glutamate. Asian cuisine originally used a natural seaweed broth, such as kelp, to bring up the umami taste in soups. Manufacturers, such as Ajinomoto, use selected strains of Micrococcus glutamicus bacteria in a bath of nutrient. The bacteria are selected for their ability to excrete glutamic acid, which is then separated from the nutrient bath, purified, and made into its sodium salt, monosodium glutamate.
Hydrolyzed proteins, or protein hydrolysates, are acid- or enzymatically treated proteins from certain foods. They contain salts of free amino acids, such as glutamate, at levels of 5 to 20 percent. Hydrolyzed proteins are used in the same manner as MSG in many foods, such as canned vegetables, soups, and processed meats. Another source of MSG is fruits, vegetables and nuts that have been sprayed with Auxigro, a growth enhancer that contains 30% glutamic acid.
Glutamate is present in a variety of protein-rich foods, and particularly abundant in aged cheese.
Despite its ubiquity in common food products, the flavour contributions made by glutamate and other amino acids were only scientifically identified early in the twentieth century. In 1907, Japanese researcher Kikunae Ikeda of the Tokyo Imperial University identified brown crystals left behind after the evaporation of a large amount of kombu broth as glutamic acid. These crystals, when tasted, reproduced the ineffable but undeniable flavour he detected in many foods, most especially in seaweed. Professor Ikeda termed this flavour "umami." He then patented a method of mass-producing a crystalline form of glutamic acid, MSG. 
The Ajinomoto (???) company was formed to manufacture and market MSG in Japan; the name 'Ajinomoto' means "essence of taste". It was introduced to the United States in 1947 as Ac'cent flavor enhancer. Modern commercial MSG is produced by fermentation  of starch, sugar beets, sugar cane, or molasses. About 1.5 million metric tons were sold in 2001, with 4% annual growth expected.  MSG is used commercially as a flavour enhancer, and is added as an ingredient to many snack foods, frozen dinners, and instant meals such as the seasoning mixtures for instant noodles.
In 1959, FDA classified MSG as a "generally recognized as safe", or GRAS, substance. This action stemmed from the 1958 Food Additives Amendment to the Federal Food, Drug, and Cosmetic Act, which required premarket approval for new food additives and led FDA to promulgate regulations listing substances, such as MSG, which have a history of safe use or are otherwise GRAS. Since 1970, FDA has sponsored extensive reviews on the safety of MSG, other glutamates and hydrolyzed proteins, as part of an ongoing review of safety data on GRAS substances used in processed foods. One such review was by the Federation of American Societies for Experimental Biology (FASEB) Select Committee on GRAS Substances. In 1980, the committee concluded that MSG was safe at current levels of use but recommended additional evaluation to determine MSG's safety at significantly higher levels of consumption. Additional reports attempted to look at this. In 1986, FDA's Advisory Committee on Hypersensitivity to Food Constituents concluded that MSG poses no threat to the general public but that reactions of brief duration might occur in some people. Other reports gave similar findings:
A 1991 report by the European Community's (EC) Scientific Committee for Foods reaffirmed MSG's safety and classified its "acceptable daily intake" as "not specified", the most favorable designation for a food ingredient. In addition, the EC Committee said, "Infants, including prematures, have been shown to metabolize glutamate as efficiently as adults and therefore do not display any special susceptibility to elevated oral intakes of glutamate." A 1992 report from the Council on Scientific Affairs of the American Medical Association stated that glutamate in any form has not been shown to be a "significant health hazard". Also, the 1987 Joint Expert Committee on Food Additives of the United Nations Food and Agriculture Organization and the World Health Organization have placed MSG in the safest category of food ingredients.
Scientific knowledge about how the body metabolizes glutamate developed rapidly during the 1980s. Studies showed that glutamate in the body plays an important role in normal functioning of the nervous system. Questions then arose on the role glutamate in food plays in these functions and whether or not glutamate in food contributes to certain neurological diseases.
There have been numerous studies of allergies and/or sensitivities to MSG, attributed to the free glutamic acid component, which has been blamed for causing a wide variety of physical symptoms such as migraines, nausea, digestive upsets, bad dreams, disturbed sleep, drowsiness, heart palpitations, hair loss, asthma, anaphylactic shock, rapidly increasing diabetes, and many other complaints. "Chinese restaurant syndrome" is often used as an example of the symptoms purported to be caused by MSG. Prompted by continuing public interest and a flurry of glutamate-related studies in the late 1980s, FDA contracted with FASEB in 1992 to review the available scientific data. The agency asked FASEB to address 18 questions dealing with: the possible role of MSG in eliciting MSG symptom complex the possible role of dietary glutamates in forming brain lesions and damaging nerve cells in humans underlying conditions that may predispose a person to adverse effects from MSG the amount consumed and other factors that may affect a person's response to MSG the quality of scientific data and previous safety reviews. FASEB held a two-day meeting and convened an expert panel that thoroughly reviewed all the available scientific literature on this issue. FASEB completed the final report, over 350 pages long, and delivered it to FDA on July 31, 1995. While not a new study, the report offers a new safety assessment based on the most comprehensive existing evaluation to date of glutamate safety. Among the report's key findings: An unknown percentage of the population may react to MSG and develop MSG symptom complex, a condition characterized by one or more of the following symptoms: burning sensation in the back of the neck, forearms and chest numbness in the back of the neck, radiating to the arms and back tingling, warmth and weakness in the face, temples, upper back, neck and arms facial pressure or tightness chest pain headache nausea rapid heartbeat weak pulse violent dreams bronchospasm (difficulty breathing) in MSG-intolerant people with asthma drowsiness weakness. In otherwise healthy MSG-intolerant people, the MSG symptom complex tends to occur within one hour after eating 3 grams or more of MSG on an empty stomach or without other food. A typical serving of glutamate-treated food contains less than 0.5 grams of MSG. A reaction is most likely if the MSG is eaten in a large quantity or in a liquid, such as a clear soup. Severe, poorly controlled asthma may be a predisposing medical condition for MSG symptom complex. No evidence suggests that dietary MSG or glutamate contributes to Alzheimer's disease, Huntington's chorea, amyotrophic lateral sclerosis, AIDS dementia complex, or any other long-term or chronic diseases. No evidence suggests that dietary MSG causes brain lesions or damages nerve cells in humans, but there is in infant mice. The level of vitamin B6 in a person's body plays a role in glutamate metabolism, and the possible impact of marginal B6 intake should be considered in future research.
United States Under current FDA regulations, when MSG is added to a food, it must be identified as "monosodium glutamate" in the label's ingredient list. Each ingredient used to make a food must be declared by its name in this list.
While technically MSG is only one of several forms of free glutamate used in foods, consumers frequently use the term MSG to mean all free glutamate. The free glutamic acid component of MSG may also be present in a wide variety of other additives, including hydrolyzed vegetable proteins, hydrolyzed yeast, soy extracts, and "natural flavorings".
For this reason, FDA considers labels such as "No MSG" or "No Added MSG" to be misleading if the food contains ingredients that are sources of free glutamates, such as hydrolyzed protein.
In 1993, FDA proposed adding the phrase "(contains glutamate)" to the common or usual names of certain protein hydrolysates that contain substantial amounts of glutamate. For example, if the proposal were adopted, hydrolyzed soy protein would have to be declared on food labels as "hydrolyzed soy protein (contains glutamate)." However, if FDA issues a new proposal, it would probably supersede this 1993 one. In 1994, FDA received a citizen's petition requesting changes in labeling requirements for foods that contain MSG or related substances. The petition asks for mandatory listing of MSG as an ingredient on labels of manufactured and processed foods that contain manufactured free glutamic acid. It further asks that the amount of free glutamic acid or MSG in such products be stated on the label, along with a warning that MSG may be harmful to certain groups of people. FDA has not yet taken action on the petition.
See Also: 3D Structure of MSG
Jordan Sand, "A Short History of MSG: Good Science, Bad Science, and Taste Cultures", Gastronomica 5:4 (Fall 2005). History of MSG and its marketing in Japan, Taiwan (under the Japanese), China, and the U.S.
Michelle Meadows, "MSG: A Common Flavor Enhancer By Michelle Meadows" ,U.S. Department of Health and Human Services
Federal Register, Dec. 4, 1992 (FR 57467)
Federal Register, Jan. 6, 1993 (FR 2950)
FDA Consumer, December 1993, "Food Allergies: When Eating is Risky."