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, snack foods, gravy, stock and more. Yeast extracts in liquid form can be dried to a light paste or a dry powder.
Yeast extracts and fermented foods contain glutamic acid, an amino acid found in meat, cheese, fungi and vegetables, including mushrooms, broccoli, and tomatoes. A number of consumer groups have claimed that certain food ingredients, such as autolyzed yeast and hydrolyzed protein, are MSG in disguise  but food scientists and large food retailers such as Whole Foods dispute these claims.
Yeast extract consists of concentrations of yeast cells that are allowed to die and break up, so that the yeasts’ endogenous digestive enzymes break their proteins down into simpler compounds (amino acids and peptides).
Yeast autolysates are used in AussieMite, Mightymite, Vegemite (Australia), Marmite, Promite, Oxo (Ireland, New Zealand, Canada, South Africa and the United Kingdom), Cenovis (Switzerland), Vitam-R (Germany) and Maggi sauce. Bovril (Ireland and the United Kingdom) switched from beef extract to yeast extract for 2005 and most of 2006, but later switched back.
The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is to add heat to a suspension of yeast. Yeast extract results from natural breakdown of yeast cells when they are heated. The natural enzymes found in the yeast cell disintegrates the cell wall so the extract dissolves out. Removing the cell walls concentrates the flavors and changes the texture.
Yeast extract is used as a flavor enhancer in processed foods of all kinds. Skincare companies like Orved use yeast extract in its products.
Autolyzed yeast contains the cell walls. Hydrolyzed yeast or hydrolyzed yeast extract is another version used as a flavor enhancer. Exogenous enzymes or acids are used to hydrolyze the proteins. It mimics MSG when combined with sodium.