Sure, here is how MSG acts: The tongue has five kinds of basic taste-sensing receptors: sweet, sour, bitter, salty and the most recently discovered one, umami. Table salt (NaCl) activates—no surprise here—the salty sensors. MSG, the sodium salt of a naturally-occurring amino acid, hits up the umami sensors.

This taste is most often described as brothy, savory, or meaty. Glutamates are found in some fruits and vegetables (like tomatoes and mushrooms), but most often in fermented foods, like soy sauce or hard cheeses, and give them an added flavor boost. Hydrolyzed proteins and yeasts also yield glutamates.

When used in small amounts (much less than table salt), MSG adds this additional dimension to the food’s flavor profile. Overuse of MSG, often as a substitute for more expensive ingredients, can result in flavors that many people find unpleasant.

Though an extremely small portion of the population (probably less than 1%) exhibit an intolerance to MSG as individuals can to any food, it is a safe food ingredient in use for more than a century.

I hope that clears the mechanism of action of MSG.
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