Monosodium glutamate
Monosodium glutamate, sodium glutamate, flavour enhancer 621, EU food additive code: E621, HS code: 29224220 (IUPAC name 2-aminopentanedioic acid. Also known as 2-aminoglutaric acid), commonly known as MSG, Ajinomoto, Vetsin, or Accent, is a sodium salt of glutamic acid. MSG is a food additive and it is commonly marketed as a "flavour enhancer".
Although traditional Asian cuisine uses flavour-enhancing ingredients which contain high concentrations of MSG, it was not isolated until 1907. MSG was subsequently patented by the Japanese Ajinomoto Corporation in 1909. In its pure form, it appears as a white crystalline powder; when dissolved in water (or saliva) it rapidly dissociates into sodium cations and glutamate anions (glutamate is the anionic form of glutamic acid, a naturally occurring amino acid).

# Chemical properties
Under standard conditions for temperature and pressure, MSG is stable, but it reacts with strong oxidizing agents. Two chiral enantiomers exist for monosodium glutamate, but only the naturally occurring L-glutamate form is used as a flavour enhancer.

# Commercialization
The Ajinomoto company was formed to manufacture and market MSG in Japan; the name 'Ajinomoto' means "essence of taste". It was introduced to the United States in 1947 as Ac'cent flavor enhancer.
Modern commercial MSG is produced by fermentation of starch, sugar beets, sugar cane, or molasses. About 1.5 million metric tons were sold in 2001, with 4% annual growth expected.
MSG is used commercially as a flavour enhancer. Although once stereotypically associated with foods in Chinese restaurants; it is now found in many common food items, particularly processed foods.  Examples include:
- Canned soups.
- Pre-prepared stocks often known as stock cubes.
- Condiments such as barbecue sauce.
- Frozen dinners.
- Frozen seafood.
- Common snack foods such as flavored potato chips and flavored tortilla chips.
- Most fast food.
- Instant meals such as the seasoning mixtures for instant noodles.
Only the L-glutamate enantiomer has flavour-enhancing properties. Manufactured MSG contains over 99.6% of the naturally predominant L-glutamate form, which is a higher proportion of L-glutamate than found in the free glutamate ions of naturally occurring foods. Fermented products like soy sauce, steak sauce, and Worcestershire sauce have comparable levels of glutamate as foods with added MSG. However, glutamate in these brewed products may be composed 5% or more of the D-enantiomer.

# Health controversy
Monosodium glutamate as a food ingredient is the subject of a health concern controversy.

## United States
Under current FDA regulations, when MSG is added to a food, it must be identified as "monosodium glutamate" in the label's ingredient list. If MSG is part of a spice mix that is purchased from another company, the manufacturer is still required to list the ingredients of that spice mix including MSG. Some companies whether intentionally or unknowingly may simply use the words "flavorings" or "spices" even if other ingredients including MSG are present. This is technically against the regulation and should the company be questioned about it, would be required to update labels.
Also, MSG is only one of several forms of free glutamate used in foods. Free glutamate may also be present in a wide variety of other additives, including: hydrolyzed vegetable proteins, autolyzed yeast, hydrolyzed yeast, yeast extract, soy extracts, protein isolate, "spices" and "natural flavorings." The food additives disodium inosinate and disodium guanylate are useful only in synergy with MSG-containing ingredients, and provide a likely indicator of the presence of MSG in a product.
For this reason, FDA considers labels such as "No MSG" or "No Added MSG" to be misleading if the food contains ingredients that are sources of free glutamate, such as hydrolyzed protein.
In 1993, FDA proposed adding the phrase "(contains glutamate)" to the common or usual names of certain protein hydrolysates that contain substantial amounts of glutamate. For example, if the proposal were adopted, hydrolyzed soy protein would have to be declared on food labels as "hydrolyzed soy protein (contains glutamate)."
In 1994, FDA received a citizen's petition requesting changes in labeling requirements for foods that contain MSG or related substances.  The petition asks for mandatory listing of MSG as an ingredient on labels of manufactured and processed foods that contain manufactured free glutamic acid. It further asks that the amount of free glutamic acid or MSG in such products be stated on the label, along with a warning that MSG may be harmful to certain groups of people (in much the same way products with aspartame have a warning for phenylketonurics). FDA has not yet taken action on the petition.

## Europe
MSG has the E number E621.

## Australia and New Zealand
Standard 1.2.4 of the Australia New Zealand Food Standards Code requires the presence of MSG as a food additive to be labeled. The label must bear the food additive class name (eg. flavour enhancer), followed by either the name of the food additive (eg MSG) or its International Numbering System (INS) number (eg 621).