Rating: Certain people should avoid

Flavoring: ready made meals, snack foods, meat products, gravies and sauces, soups, broths, and soup mixes.
Autolyzed yeast extract is a flavoring agent made from yeast, usually the same kind used to make bread rise or ferment beer.  Generally, the yeast is heated or otherwise killed in a way that allows enzymes inside the cells break down the yeast, including the proteins.  (Other types of yeast extracts are made by adding enzymes, rather than using the enzymes already present inside the yeast cell.)

Some people who have allergic reactions to inhaling molds also react to ingesting yeast or yeast extracts. 

All proteins are made up of amino acids, and one amino acid of interest—glutamic acid—is present in autolyzed yeast extract, as well as in many other foods and in our bodies.  Glutamate is a form of glutamic acid and is responsible for “umami,” the savory taste associated with foods like meat and mushrooms.  The sodium salt of glutamate is called sodium glutamate, better known as MONOSODIUM GLUTAMATE or MSG.  A small number of people experience headache, numbness, flushing, tingling, or other short-term symptoms when consuming large amounts of MSG.  Autolyzed yeast extract is sometimes used to substitute for MSG, but has much lower levels of glutamate so adverse reactions are unlikely. 

Foods such as Parmesan cheese, seaweed, dried shitake mushrooms, and dried tomatoes naturally contain relatively high levels of glutamate, and so could also potentially be a problem for individuals sensitive to MSG, although that does not seem to be the case.  FDA does not allow foods that contain autolyzed yeast extract (or yeast extract, hydrolyzed yeast, soy extracts, hydrolyzed vegetable protein, or protein isolate) to say “No MSG” or “No added MSG” on their packaging.  Although autolyzed yeast extract affects the flavor of foods, FDA requires that it be identified on the label; it cannot be hidden under the term “natural or artificial flavoring.”