Glutamate flavoring

Glutamic acid versus glutamates

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 or . 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()-()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.

Isomers

Further research into the compound has found that only the L-glutamate enantiomer has flavor-enhancing properties. 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.

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").

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 nucleotides inosine-5'-monophosphate (IMP) or guanosine-5'-monophosphate (GMP) enhances the taste of umami; T1R1 and T1R3 respond primarily to mixtures of glutamate and nucleotides. 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.

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). The metabotropic glutamate receptors such as mGluR4 and mGluR1 can be easily activated at glutamate concentration levels found in food.

Perceptual independence from salty and sweet taste

Since umami taste compounds are frequently sodium salts, the perceptual differentiation of salty and umami tastes has been difficult in taste tests and studies have found as much as 27% of certain populations may be umami "hypotasters".

Furthermore single glutamate(glutamic acid) with no table salt ions(Na+) elicits sour taste and in psychophysical tests, sodium or potassium salt cations seem to be required to produce a perceptible umami taste.

Sweet and umami tastes both utilize the taste receptor subunit T1R3, with salt taste blockers reducing discrimination between monosodium glutamate and sucrose in rodents.

If umami doesn't have perceptual independence, it could be classified with other tastes like fat, carbohydrate, metallic, and calcium, which can be perceived at high concentrations but may not offer a prominent taste experience.

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.

Asia

Japanese cuisine originally used broth made from kombu (kelp) to produce the umami taste in soups.

Rome

In the Roman Empire, glutamic acid was present 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.

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.

Hydrolyzed protein

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 glutamicum bacteria 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.

Risks of oversupply

Medical studies

Glutamate is the most important excitatory neurotransmitter in the central nervous system of vertebrates. Several complex mechanisms in the brain provide a protection against a possible oversupply of glutamate there, because this can cause neurotoxicity even leading to the death of nerve cells.

There are certain groups of persons, however, where these protection mechanisms in the brain against a glutamate oversupply are damaged. This was observed, for example, in cases of post-traumatic stress disorder (PTSD) or depression. Accordingly, it was shown in preclinical trials that the decrease of glutamate in the brain by administration of pyruvate led to a decline of anxiety and depression.

The additional consumption of glutamate, for example monosodium glutamate (MSG), as a flavor enhancer was associated with an increased probability to develop a metabolic syndrome in a large sample of a rural population in Thailand.

Social perceptions

In the late 20th century, the USA experienced a social panic over culinary MSG. Participants believed that MSG caused a syndrome known as Chinese restaurant syndrome or MSG symptom complex (MSC).

The panic began with a joke. In a 1968 letter to the New England Journal of Medicine's editor, Robert Ho Man Kwok noted that 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... Context suggests that Kwok was likely joking about postprandial somnolence, and initial replies assumed the entire letter was a pseudonymous prank, often responding in kind. This satiric context was lost entirely during subsequent republication in the scientific and popular literatures.

Even while still recognized as jokes, the letters also inspired a severely flawed study the next year. 56 subjects were given increasing oral doses of MSG until all had exhibited some subset of "burning, facial pressure, and chest pain." In the popular mind, MSC later extended to include "tightness, flushing, tearing, dizziness, syncope, and facial pressure". Subsequent scientific investigation could not replicate any of these or Kwok's original phenomena consistently and reliably, even when looking for genetic predispositions.

More importantly, MSC became highly racialized in the popular press. Cultural stereotypes had primed non-Chinese Americans to view Chinese food with suspicion, and the "chemical" name of MSG obscured that it was present across a wide variety of savory ingredients. The public thus assumed that MSG was an entirely artificial additive used exclusively in Chinese cuisine.

In 1970, a National Research Council under the US National Academy of Sciences investigated MSG but concluded that MSG was safe for consumption. Despite public backlash, the Food and Drug Administration (FDA) did not remove MSG from the "Generally Recognized as Safe" regulatory category.

Scientific interested in the phenomenon has faded, So, too, has popular belief in MSC. In 2016, Anthony Bourdain said on Parts Unknown: I think [MSG] is good stuff. I don't react to it – nobody does. It's a lie... You know what causes Chinese restaurant syndrome? Racism. 'Ooh, I have a headache, must have been the Chinese guy.' Nevertheless, in 2019, a short-lived NYC restaurant named "Lucky Lee's" became a cause celebre. It claimed to provide "cleaner" and healthier Chinese food by (inter alia) eschewing fat and MSG.

In 2020, Ajinomoto, the leading manufacturer of MSG, and others launched the #RedefineMSC campaign, hoping to highlight the xenophobic prejudice against East Asian cuisine associated with "Chinese restaurant syndrome", and the paucity of scientific evidence for the phenomenon. Following the campaign, Merriam-Webster announced it would review the term. Ultimately, Merriam-Webster retained the definition, but renamed it to "MSG symptom complex".

Regulations

Regulation timeline

In 1959, the U.S. Food and Drug Administration (FDA) classified monosodium glutamate as generally recognized as safe (GRAS). This action stemmed from the 1958 Food Additives Amendment to the Federal Food, Drug, and Cosmetic Act that required premarket approval for new food additives and led the FDA to promulgate regulations listing substances, such as monosodium glutamate, which have a history of safe use or are otherwise GRAS.

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.

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:

• The 1987 Joint Expert Committee on Food Additives of the United Nations Food and Agriculture Organization and the World Health Organization placed monosodium glutamate in the safest category of food ingredients.
• 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 with a maximum level of consumption of 10g/kg expressed as glutamic acid.

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 male, this amount corresponds to 2.1 g per kg of body weight.

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.

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. 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.

, the National Academy of Sciences Committee on Dietary Reference Intakes had not set a NOAEL or LOAEL for glutamate.

Australia and New Zealand

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.

Ingredients

Forms of glutamic acid that can be added to food include:

• Monosodium glutamate
• Glutamic acid (E620), glutamate (E620)
• Monopotassium glutamate (E622)
• Calcium glutamate (E623)
• Monoammonium glutamate (E624)
• Magnesium glutamate (E625)
• Sodium glutamate (E621)

The following are also rich sources of glutamic acid, and may be added for umami flavor:

• Hydrolyzed vegetable protein
• Autolyzed yeast, yeast extract, yeast food, and nutritional yeast
• Cheese products, e.g. parmesan (1200 mg / 100 g)
• Various savory fermented seasonings, including soy sauce and worcestershire sauce
• (See for more examples.)

Notes

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".

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