Monosodium glutamate

  Monosodium glutamate, sodium glutamate, flavour enhancer 621, EU food additive code: E621, HS code: 29224220 (IUPAC name 2-aminopentanedioic acid. Also known as 2-aminoglutaric acid), commonly known as MSG, Ajinomoto, Vetsin, or Accent, is a sodium salt of glutamic acid. MSG is a food additive and it is commonly marketed as a "flavour enhancer".

Although traditional Asian cuisine uses flavour-enhancing ingredients which contain high concentrations of MSG, it was not isolated until 1907. MSG was subsequently patented by the Japanese Ajinomoto Corporation in 1909. In its pure form, it appears as a white crystalline powder; when dissolved in water (or saliva) it rapidly dissociates into sodium cations and glutamate anions (glutamate is the anionic form of glutamic acid, a naturally occurring amino acid).

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Chemical properties

Under Standard conditions for temperature and pressure, MSG is stable, but it reacts with strong oxidizing agents. Two chiral enantiomers exist for monosodium glutamate, but only the naturally-occurring L-glutamate form is used as a flavour enhancer.

Glutamic acid 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 "savoury" or "meaty").

Discovery

Although they occur naturally in many foods, the flavour contributions made by glutamate and other amino acids were only scientifically identified early in the twentieth century. The substance was discovered and identified in the year 1866, by the German chemist Karl Heinrich Leopold Ritthausen. 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.^[1]

Commercialization

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^[2] of starch, sugar beets, sugar cane, or molasses. About 1.5 million metric tons were sold in 2001, with 4% annual growth expected.^[3] MSG is used commercially as a flavour enhancer. Once stereotypically associated with food in Chinese restaurants in America, it is now found in some common food products consumed in the US:^{[citation needed]}

• canned soups of the US food industry
• beef and chicken stocks of the US food industry
• flavored potato chip products of the US food industry
• snack foods
• frozen dinners
• US-originated fast foods
• instant meals such as the seasoning mixtures for instant noodles

Only the L-glutamate enantiomer has flavour-enhancing properties.^[4] Manufactured MSG contains over 99.6% of the naturally-predominant L-glutamate form, which is a higher proportion of L-glutamate than found in the free glutamate ions of naturally-occurring foods. Fermented products like soy sauce, steak sauce, and worcestershire sauce have comparable levels of glutamate as foods with added MSG. However, glutamate in these brewed products may be composed 5% or more of the D-enantiomer.^[4]

Sources of glutamate

Natural Occurrence

Glutamate itself is a widespread amino acid: it is found naturally in human bodies, and is found primarily in the bound form in protein-containing foods, such as mushrooms, seaweed, tomatoes, nuts, legumes, meats, and most dairy products. Only a fraction of the glutamate in foods is in a "free" form, and only free glutamate can enhance the flavour of foods. Part of the flavour-enhancing effect of tomatoes, fermented soy products, yeast extracts, certain sharp cheeses, and fermented or hydrolyzed protein products (such as soy sauce and soy bean paste) is due to the presence of free glutamate ions.

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.^{[citation needed]} The bacteria are selected for their ability to excrete glutamic acid, which is then separated from the nutrient bath and made into its sodium salt, monosodium glutamate.

Other Sources

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.

Approximate Quantities in Food

The following table illustrates the glutamate content of some selected common foods, in milligrams of glutamate per hundred grams of food. Free glutamate is metabolized differently from glutamate bound in protein, so they are listed separately. ^[5]

Health Concerns

In 1959, the 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 the 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 have given the following findings:

• The 1987 Joint Expert Committee on Food Additives of the United Nations Food and Agriculture Organization and the World Health Organization placed MSG in the safest category of food ingredients.
• 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 favourable 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".
• A 1995 FDA-commissioned report acknowledged that "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
  • bronchospasm (difficulty breathing)
  • drowsiness
  • weakness
  • sweating."^[6]
• A 2002 report from researchers at Hirosaki University in Japan found rats fed on diets high in MSG suffered eye damage. Lead researcher Hiroshi Ohguro said the findings might explain why, in eastern Asia, there is a high rate of normal-tension glaucoma.^[7]

Issues surrounding these health implications of MSG consumption are the subject of much debate. A considerable body of anecdotal evidence exists suggesting negative health effects, but this has not yet been supported by recognised research.

In April 1968, Dr Ho Man Kwok wrote an article for the New England Journal of Medicine where he said, "I have experienced a strange syndrome whenever I have eaten out in a Chinese restaurant, especially one that served northern Chinese food. The syndrome, which usually begins 15 to 20 minutes after I have eaten the first dish, lasts for about two hours, without hangover effect. The most prominent symptoms are numbness at the back of the neck, gradually radiating to both arms and the back, general weakness and palpitations...". This comment began a global health scare about MSG and "Chinese restaurant syndrome" was born. However, research has failed to prove that MSG affects a large percentage of the population, and Chinese restaurant syndrome is largely resigned to urban legend status. However, the damage was done, and MSG is still thought of as suspect by a large proportion of the general public, and many foods continue to be labeled "MSG free". ^[8]

Excitotoxicity

Because MSG is absorbed very quickly in the gastrointestinal tract (unlike glutamic acid-containing proteins in foods), MSG could spike blood plasma levels of glutamate.^[9][10][11] Glutamic acid is in a class of chemicals known as excitotoxins, high levels of which have been shown in animal studies to cause damage to areas of the brain unprotected by the blood-brain barrier and that a variety of chronic diseases can arise out of this neurotoxicity.^[12][13] The debate among scientists on the significance of these findings has been raging since the early 1970s, when Dr. John Olney found that high levels of glutamic acid caused damage to the brains of infant mice.^[14] The debate is complex and has focused on several areas:

• Whether the increase in plasma glutamate levels from typical ingestion levels of MSG is enough to cause neurotoxicity in one dose or over time.
• Whether humans are susceptible to the neurotoxicity from glutamic acid seen in some animal experiments. It is known that the glutamate ion is important in memory retrieval in humans.
• Whether neurotoxicity from excitotoxins is caused by the combined effect of glutamic acid and other excitotoxins such as aspartic acid from aspartame.

At a meeting of the Society for Neuroscience, the delegates had a split opinion on the issues related to neurotoxic effects from excitotoxic amino acids found in some additives such as MSG.^[15]

Some scientists believe that humans and other primates are not as susceptible to excitotoxins as rodents and therefore there is little concern with glutamic acid from MSG.^[16][17] While they agree that the combined effects of all food-based excitotoxins should be considered,^[18] their measurements of the blood plasma levels of glutamic acid after ingestion of monosodium glutamate and aspartame demonstrate that there is not a cause for concern.^[19] Other scientists feel that primates are susceptible to excitotoxic damage^[20] and that humans concentrate excitotoxins in the blood more than other animals.^[21] Based on these findings, they feel that humans are approximately 5-6 times more susceptible to the effects of excitotoxins than rodents are.^[22] While they agree that typical use of MSG does not spike glutamic acid to extremely high levels in adults, they are particularly concerned with potential effects in infants and young children^[23] and the potential long-term neurodegenerative effects of small-to-moderate spikes on plasma excitotoxin levels.^[24]

Obesity

Monosodium glutamate has been shown to indirectly cause obesity in lab rats by downregulating hypothalamic appetite suppression and, thus, increasing the amount of food the lab rats consumed. Animal research compels some researchers to theorize that MSG has a role in the occurrence of obesity in humans.^[25]

However, a similar effect has not yet been observed in humans. An epidemiological survey of 4938 ethnically Japanese men drawn from the Honolulu heart program in Hawaii found that self-reported dietary MSG consumption was not statistically linked with obesity.^[26] Researchers furthermore found that MSG consumption at every meal was not statistically correlated to the participant's incidence of heart disease or stroke. Frequent MSG consumption also did not significantly affect blood sugar or serum cholesterol levels among the participants.

Ingredient listing

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. If MSG is part of a spice mix that is purchased from another company, the manufacturer is still required to list the ingredients of that spice mix including MSG. Some companies whether intentional or unknowingly may simply use the words "flavorings" or "spices" even if other ingredients including MSG are present. This is technically against the regulation and should the company be questioned about it, would be required to update labels.

Also, MSG is only one of several forms of free glutamate used in foods. Free glutamate may also be present in a wide variety of other additives, including: hydrolyzed vegetable proteins, autolyzed yeast, hydrolyzed yeast, yeast extract, soy extracts, protein isolate, "spices" and "natural flavorings." The food additives Disodium inosinate and Disodium guanylate are useful only in synergy with MSG-containing ingredients, and provide a likely indicator of the presence of MSG in a product.

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 glutamate, such as hydrolyzed protein.^[6]

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.^[6] For example, if the proposal were adopted, hydrolyzed soy protein would have to be declared on food labels as "hydrolyzed soy protein (contains glutamate)."

In 1994, FDA received a citizen's petition requesting changes in labeling requirements for foods that contain MSG or related substances.^[6] 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.^{[citation needed]}

Europe

MSG has the E number E621.

Australia and New Zealand

Standard 1.2.4 of the Australia New Zealand Food Standards Code requires the presence of MSG as a food additive to be labeled. The label must bear the food additive class name (eg. flavour enhancer), followed by either the name of the food additive (eg MSG) or its International Numbering System (INS) number (eg 621)

See also

• Ajinomoto
• Excitotoxicity
• Flavour enhancer
• Disodium glutamate
• List of food additives
• Yeast extract

References

• 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.
• Federal Register, Dec. 4, 1992 (FR 57467)
• Federal Register, Jan. 6, 1993 (FR 2950)
• FDA Consumer, December 1993, "Food Allergies: When Eating is Risky."

Notes