Monosodium glutamate is familiar as an additive to Chinese food, but it and its analogues are actually found in a variety of cuisines. Chemically, MSG is a compound of sodium and a chemical called glutamic acid, which is an amino acid that occurs naturally in many food items. Also called glutamate—the difference is that glutamate is naturally occurring and MSG is manufactured—the chemical is responsible for the taste called umami, and it helps enhance and smooth out food flavors.
Glutamate was isolated in 1908 by a Japanese food scientist named Ikeda Kikunae. He recognized in an edible seaweed called kombu a taste distinct from the traditional sweet, sour, bitter, and salty. Ikeda called this flavor umami, a Japanese word often translated as “savory,” and worked to isolate the chemical responsible for it from kombu broth. The flavor actually has a long history, and outside of Japan as well. The 19th-century French chef Auguste Escoffier used the flavor without recognizing the source or realizing it was a distinct flavor. Part of the reason umami was not found until relatively recently is that pure umami taste is hard to enjoy; it needs to be found in a food to have the desired effect.
In the 1970s, MSG picked up a reputation as dangerous, triggered by a 1968 letter to the New England Journal of Medicine. A doctor in Maryland, Robert Ho Man Kwok, noted experiencing certain symptoms after eating northern Chinese cuisine. While Kwok raised the possibility of MSG being the culprit, it was only one of a number of hypotheses he advanced. It appears more likely that simple dehydration from salty food was to blame. In fact, glutamate is found in breast milk, and the lethal dose is about three times that of table salt; a 1995 Food and Drug Administration report found it to be safe to eat.
Indeed, it’s just as well that MSG and glutamate are largely harmless, with no effects of allergy or intolerance reported, because of the variety of common foods in which the compound occurs. Tomatoes may be the most common, but it’s also found in parmesan and Roquefort cheeses, peas, potatoes, several types of shellfish, and grape juice. MSG is sometimes added to Asian food but glutamate occurs naturally in soy sauce, green tea, and of course the kombu Ikeda first got it from.