According to Wikipedia, "Yeast extract is the common name for various forms of processed yeast products made by extracting the cell contents (removing the cell walls); they are used as food additives or flavorings, or as nutrients for bacterial culture media. They are often used to create savory flavors and umami taste sensations, and can be found in a large variety of packaged food including frozen meals, crackers, junk food, gravy, stock and more... Yeast extract contains an amount of naturally occurring Glutamic Acid or Monosodium Glutamate." To read the entire article: http://en.wikipedia.