Glutamate flavoring is the generic name for flavor-enhancing compounds based on glutamic acid and its salts (glutamates). These compounds provide a savory taste to food.
Glutamic acid and glutamates are natural constituents of many fermented or aged foods, including soy sauce, fermented bean paste, and cheese. They can also be found in hydrolyzed proteins such as yeast extract. The sodium salt of glutamic acid, monosodium glutamate (MSG), is manufactured on a large scale and widely used in the food industry.
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 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 the early 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 a 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.Kalapanda M.Appaiah. Monosodium Glutamate in Foods and its Biological Effects. In Ensuring Global Food Safety, 2010, pp. 217–226,
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.See column "Free glutamate" in the Concentration in foods subsection's table
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.
Food Free glutamate (mg/100 g) Protein glutamate (mg/100 g) Monosodium glutamate 87720 Makombu (kelp) 3190 Rausu kombu (kelp) 2286 Rishiri kombu (kelp) 1985 Marmite 1960 Hidaka kombu (kelp) 1344 Nori (seaweed) 1378 Vegemite 1431 Japanese fish sauce 1383 Roquefort cheese 1280 Parmesan cheese 1200 9847 Korean soy sauce 1264 Chinese soy sauce 926 Japanese soy sauce 782 Oyster sauce 900 Green tea 668 Cured ham 337 Sardine 280 Grape juice 258 Clam 208 Peas 200 5583 Scallop 159 Squid 146 Tomatoes 140 238 Oyster 137 Corn 130 1765 Mussel 105 Potatoes 102 Duck 69 3636 Chicken 44 3309 Beef 33 2846 Pork 23 2325 Eggs 23 1583 Human milk 22 229 Salmon 20 2216 Cow milk 2 819
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 thatI 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. Note that Steel was likely lying about writing Kwok's letter, as detailed in
Even while still recognized as jokes, the letters also inspired a severely flawed study the next year., p. 483. Note that Freeman is incorrect about the study size; the largest trial had 56 participants, as (p. 14) recognizes. Nevertheless, Freeman is correct that the 56-participant portion of the study is unblinded and so susceptible to the nocebo effect. 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, MSG symptom complex later extended to include "tightness, flushing, tearing, dizziness, syncope, and facial pressure"., p. 482, summarizing Geha, Beiser, Ren, et al (2000), "Review of alleged reaction to monosodium glutamate and outcome of a multicenter double-blind placebo-controlled study", Journal of Nutrition, 130 (Suppl. 4S), 1058S–1062S. 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, MSG 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.