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 H3O+ or Na+. 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(NH+3)-(CH2)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. 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.
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