When glutamic acid or any of its salts are dissolved in water, they form a solution of separate negative ions, called glutamates, and positive ions like H 3O+ or Na+ . The result is actually a chemical equilibrium among several ionized forms, including zwitterions, that depends on the pH (acidity) of the solution. Within the common pH range of foods, the prevailing ion can be described as −OOC-C(NH+ 3)-(CH 2)2-COO−, which has an electric charge of −1.
Only the glutamate ion is responsible for the umami flavor, so the effect does not depend significantly on the starting compound. However, some crystalline salts such as monosodium glutamate dissolve much better and faster than crystalline glutamic acid. This has proven to be an important factor in the implementation of substances as flavor enhancers.
Discovery
Although they occur naturally in many foods, glutamic acid and other amino acid flavor contributions were not scientifically identified until early in the twentieth century. In 1866, the German chemist Karl Heinrich Ritthausen discovered and identified the compound. 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 flavor detected in many foods, especially seaweed. Professor Ikeda coined the term umami for this flavor. He then patented a method of mass-producing the crystalline salt of glutamic acid known as monosodium glutamate.[1][2]
Isomers
Further research into the compound has found that only the L-glutamate enantiomer has flavor-enhancing properties.[3] Manufactured monosodium glutamate consists to over 99.6% of the naturally predominant L-glutamate form, which is a higher proportion of L-glutamate than can be found in the free glutamate ions of fermented naturally occurring foods. Fermented products such as soy sauce, steak sauce, and Worcestershire sauce have levels of glutamate similar to those in foods with added monosodium glutamate. However, 5% or more of the glutamate may be the D-enantiomer. Nonfermented naturally occurring foods have lower relative levels of D-glutamate than fermented products do.[3]
Taste perception
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 flavor known as umami. This is classified as one of the five basic tastes (the word "umami" is a loanword from Japanese; it is also referred to as "savory" or "meaty").
Structures of inosine-5'-monophosphate (top) and guanosine-5'-monophosphate (bottom).
The flavoring effect of glutamate comes from its free form, in which it is not bound to other amino acids in protein. Nonetheless, glutamate by itself does not elicit an intense umami taste. The mixing of glutamate with nucleotidesinosine-5'-monophosphate (IMP) or guanosine-5'-monophosphate (GMP) enhances the taste of umami;[4] T1R1 and T1R3 respond primarily to mixtures of glutamate and nucleotides.[5] While research has shown that this synergism occurs in some animal species with other amino acids, studies of human taste receptors show that the same reaction only occurs between glutamate and the selected nucleotides. Moreover, sodium in monosodium glutamate may activate glutamate to produce a stronger umami taste.[6]
Two hypotheses for the explanation of umami taste transduction have been introduced: the first posits that the umami taste is transduced by an N-methyl-D-aspartate (NMDA) type glutamate ion channel receptor; the second posits that the taste is transduced by a metabotropic type glutamate receptor (taste-mGluR4).[7] The metabotropic glutamate receptors such as mGluR4 and mGluR1 can be easily activated at glutamate concentration levels found in food.[8]
Sources
Natural occurrence
Glutamate is ubiquitous in biological life. It is found naturally in all living cells, primarily in the bound form as a constituent of proteins. Only a fraction of the glutamate in foods is in its "free" form, and only free glutamate produces an umami flavor in foods. The savory flavor of tomatoes, fermented soy products, yeast extracts, certain sharp cheeses, and fermented or hydrolyzed protein products (such as soy sauce and fermented bean paste) is partially due to the presence of free glutamate ions.[9][10]
Asia
Japanese cuisine originally used broth made from kombu (kelp) to produce the umami taste in soups.[11]
Rome
In the Roman Empire glutamic acid was found in a sauce called garum, made from fermenting fish in saltwater. The flavor enhancing properties of glutamic acid allowed Romans to reduce the use of expensive salt.[12][13]
Concentration in foods
The following table illustrates the glutamate content of some selected common foods. Free glutamate is the form directly tasted and absorbed whereas glutamate bound in protein is not available until further breakdown by digestion or cooking. In general, vegetables contain more free glutamate but less protein-bound glutamate.[14][15]
Hydrolyzed proteins, or protein hydrolysates, are acid- or enzymatically treated proteins from certain foods. One example is yeast extract. Hydrolyzed protein contains free amino acids, such as glutamate, at levels of 5% to 20%. Hydrolyzed protein is used in the same manner as monosodium glutamate in many foods, such as canned vegetables, soups, and processed meats.
Pure salts
Manufacturers, such as Ajinomoto, use selected strains of Corynebacterium glutamicumbacteria in a nutrient-rich medium. The bacteria are selected for their ability to excrete glutamic acid, which is then separated from the nutrient medium and processed into its sodium salt, monosodium glutamate.[11]
Safety as a flavor enhancer
Monosodium glutamate sold as an umami flavor enhancer
Studies
Monosodium glutamate (MSG) is regarded as safe for consumption.[17][18] An association between MSG consumption and a constellation of symptoms has not been demonstrated under rigorously controlled conditions.[19][20][21] Techniques used to adequately control for experimental bias include a placebo-controlleddouble-blinded experimental design and the use of capsules to deliver the compound to mask the strong and unique after-taste of glutamates.[19] Even though there are also reports of MSG sensitivity among a subset of the population, this has not been demonstrated in placebo‐controlled trials.[20]
Chinese restaurant syndrome
Origin
The controversy surrounding the safety of MSG started on 4 April 1968, when Dr. Robert Ho Man Kwok wrote a correspondence letter to the New England Journal of Medicine, coining the term "Chinese restaurant syndrome".[22][23] In his letter, Kwok suggested several possible causes before he nominated MSG for his symptoms.[24][20] This letter was initially met with insider satirical responses, often using race as prop for humorous effect, within the medical community.[22] Some claimed that during the discursive uptake in media, the conversations were recontextualized as legitimate while the supposed race-based motivations of the humor were not parsed.[22]
In January 2018, Dr. Howard Steel claimed that it was actually a prank submission by him under a pseudonym.[23][25] However, it turned out that there was a Dr. Robert Ho Man Kwok who worked at the National Biomedical Research Foundation, both names Steel claimed to have invented.[25] Kwok's children, his colleague at the research foundation, and the son of his boss there confirmed that Dr. Robert Ho Man Kwok, who had died in 2014, wrote this letter.[25] After hearing about Kwok's family, Steel's daughter Anna came to believe that the admission that the letter was a prank was itself one of the last pranks by her late father.[25]
The claims of 'Chinese restaurant syndrome' have the same symptoms as hypernatremia, so it may actually be salt poisoning.[26]
Reactions
Some authors and activists[27][28][29] have attributed the negative perceptions around MSG to xenophobic or racist biases towards East Asians and East Asian cuisine, criticising the classification of alleged MSG-related symptoms as "Chinese Restaurant Syndrome".[30][31][32][33][34][35]
In 2020, Ajinomoto, the leading manufacturer of MSG, launched the #RedefineCRS campaign to combat what it said was the myth that MSG is harmful to people's health.[36] Following the #RedefineCRS campaign, Merriam-Webster announced it will be "reviewing" the term,[37] which was added to the dictionary in 1993.[38][39]
Since 1970, FDA has sponsored extensive reviews on the safety of monosodium glutamate, 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 monosodium glutamate was safe at current levels of use but recommended additional evaluation to determine monosodium glutamate's safety at significantly higher levels of consumption. Additional reports attempted to look at this.[citation needed]
In 1986, FDA's Advisory Committee on Hypersensitivity to Food Constituents concluded that monosodium glutamate 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:
A 1991 report by the European Community's (EC) Scientific Committee for Foods reaffirmed monosodium glutamate'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." Legislation in effect since June 2013 classifies glutamic acid and glutamates as salt substitutes, seasonings, and condiments[41] with a maximum level of consumption of 10g/kg expressed as glutamic acid.[42]
European Union
Following the compulsory EU-food labeling law the use of glutamic acid and its salts has to be declared, and the name or E number of the salt has to be listed. Glutamic acid and its salts as food additives have the following E numbers: glutamic acid: E620, monosodium glutamate: E621, monopotassium glutamate: E622, calcium diglutamate: E623, monoammonium glutamate: E624, and magnesium diglutamate: E625. In the European Union, these substances are regarded as "flavor enhancers" and are not allowed to be added to milk, emulsified fat and oil, pasta, cocoa/chocolate products and fruit juice. The EU has not yet published an official NOAEL (no observable adverse effect level) for glutamate, but a 2006 consensus statement of a group of German experts drawing from animal studies was that a daily intake of glutamic acid of 6 grams per kilogram of body weight (6 g/kg/day) is safe. From human studies, the experts noted that doses as high as 147g/day produced no adverse effects in males when given for 30 days; in a 70 kg (150 lb) male, this amount corresponds to 2.1 g per kg of body weight.[43]
United States
In 1959, the Food and Drug Administration classified MSG as a "generally recognized as safe" (GRAS) food ingredient under the Federal Food, Drug, and Cosmetic Act. In 1986, FDA's Advisory Committee on Hypersensitivity to Food Constituents also found that MSG was generally safe, but that short-term reactions may occur in some people. To further investigate this matter, in 1992 the FDA contracted the Federation of American Societies for Experimental Biology (FASEB) to produce a detailed report, which was published in 1995. The FASEB report reaffirmed the safety of MSG when it is consumed at usual levels by the general population, and found no evidence of any connection between MSG and any serious long-term reactions.[44]
Under 2003 U.S. Food and Drug Administration regulations, when monosodium glutamate is added to a food, it must be identified as "monosodium glutamate" in the label's ingredient list. Because glutamate is commonly found in food, primarily from protein sources, the FDA does not require foods and ingredients that contain glutamate as an inherent component to list it on the label. Examples include tomatoes, cheeses, meats, hydrolyzed protein products such as soy sauce, and autolyzed yeast extracts. These ingredients are to be declared on the label by their common or usual names.[44] The term 'natural flavor' is now used by the food industry when using glutamic acid. Because of lack of regulation, it is impossible to determine what percentage of 'natural flavor' is actually glutamic acid.
The food additives disodium inosinate and disodium guanylate are usually used in synergy with monosodium glutamate-containing ingredients, and provide a likely indicator of the addition of glutamate to a product.
Standard 1.2.4 of the Australia New Zealand Food Standards Code requires the presence of monosodium glutamate as a food additive to be labeled. The label must bear the food additive class name (such as "flavor enhancer"), followed by either the name of the food additive (such as "MSG") or its International Numbering System (INS) number (e.g., "621").
Canada
The Canada Food Inspection Agency considers claims of "no MSG" or "MSG free" to be misleading and deceptive when other sources of free glutamates are present.[46]
Ingredients
Forms of glutamic acid that can be added to food include:
^Kalapanda M.Appaiah. Monosodium Glutamate in Foods and its Biological Effects. In Ensuring Global Food Safety, 2010, pp. 217–226, doi:10.1016/B978-0-12-374845-4.00013-8
^Chaudhari N., Landin A. M., Roper S. D. (2000). "A metabotropic glutamate receptor variant functions as a taste receptor". Nat. Neurosci. 3 (2): 113–119. doi:10.1038/72053. PMID10649565. S2CID16650588.((cite journal)): CS1 maint: multiple names: authors list (link)
^ abTarasoff L.; Kelly M.F. (1993). "Monosodium L-glutamate: a double-blind study and review". Food Chem. Toxicol. 31 (12): 1019–1035. doi:10.1016/0278-6915(93)90012-N. PMID8282275.
^ abcdSullivan, Lilly (February 15, 2019). "668: The Long Fuse". This American Life. Retrieved January 7, 2020.
^Wellmann, K. F. (1969). "The Chinese restaurant syndrome (acute sodium-L-glutamate poisoning)". Deutsche Medizinische Wochenschrift. 94 (24): 1297–1298. PMID5771078.
^LeMesurier, Jennifer L. (February 8, 2017). "Uptaking Race: Genre, MSG, and Chinese Dinner". Poroi. 12 (2): 1–23. doi:10.13008/2151-2957.1253. Introduction: 'Chinese Restaurant Syndrome' as Rhetorical [...] Finally, I trace how the journalistic uptakes of this discussion, in only taking up certain medical phrases and terms, reproduce the tacit racism of this boundary policing while avowing the neutrality of medical authority.
^Barry-Jester, Anna Maria (January 8, 2016). "How MSG Got A Bad Rap: Flawed Science And Xenophobia". FiveThirtyEight. That MSG causes health problems may have thrived on racially charged biases from the outset. Ian Mosby, a food historian, wrote in a 2009 paper titled "'That Won-Ton Soup Headache': The Chinese Restaurant Syndrome, MSG and the Making of American Food, 1968-1980" that fear of MSG in Chinese food is part of the U.S.'s long history of viewing the "exotic" cuisine of Asia as dangerous or dirty.
^Blythman, Joanna (May 21, 2018). "Chinese restaurant syndrome: has MSG been unfairly demonised?". The Guardian. Although Chang doesn't use MSG in his kitchens, he has defended its use, telling a high-level meeting of top chefs that Chinese restaurant syndrome is nothing more than a "cultural construct". That is a polite way of saying that avoidance of MSG is an expression of Western ignorance, or worse, racism, drawing on stereotypes of East Asian countries as dangerous or dirty.
^Geiling, Natasha (November 8, 2013). "It's the Umami, Stupid. Why the Truth About MSG is So Easy to Swallow". Smithsonian. Everyone knows this connection, and probably associates MSG use in America most heavily with Chinese restaurants – thanks in large part to the absurdly racist name for MSG sensitivity "Chinese Restaurant Syndrome."
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".
External links
"The Long Fuse". This American Life (Radio broadcast). Episode 668. February 15, 2019. Public Radio Exchange (PRX). WBEZ. Retrieved February 21, 2019. See the segments "Prologue" and "Humor Is Not the Best Medicine" for the story behind "Chinese restaurant syndrome".
