white crystalline substance, a sodium salt of the amino acid glutamic acid, that is used to intensify the natural flavour of certain foods. MSG was first identified as a flavour enhancer in 1908 by Kikunae Ikeda of Japan, who found that soup stocks made from seaweed contained high levels of the substance. His discovery led to the commercial production of MSG from seaweed; it is now produced using a bacterial fermentation process with starch or molasses as carbon sources and ammonium salts as nitrogen sources. MSG is an important ingredient in the cuisines of China and Japan. The substance is naturally present at high levels in tomatoes and Parmesan cheese.

MSG elicits a unique taste, known as umami, that is different from the four basic tastes (bitter, salty, sour, sweet). MSG does not enhance the basic tastes, but it does enhance the complex flavours of meat, poultry, seafood, and vegetables. It is used commercially in broths, soups, canned and frozen vegetables, flavouring and spice blends, gravies, meats, poultry, sauces, and in other combinations. It is also used to enhance the taste of tobacco and has been used medically to treat hepatic coma.

There have been reports that, when ingested in large amounts, monosodium glutamate may produce such physical reactions as burning sensations, facial tightness or pressure, and a tingling sensation in some individuals. This hypersensitive reaction, first reported in 1968, is commonly called “Chinese restaurant syndrome” because cooks in some Chinese restaurants may use MSG extravagantly. Subsequent studies have shown no conclusive link between the syndrome and the consumption of normal levels of MSG, however.