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 flavourings, or as nutrients for bacterial culture media. They are often used to create savoury flavours 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 extracts in liquid form can be dried to a light paste or a dry powder.
Autolyzed yeast 
Autolyzed yeast (containing the cell walls) or autolyzed 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 Vegemite (Australia), AussieMite (Australia & United Kingdom) Marmite, Promite, Oxo (New Zealand, South Africa, United Kingdom, and Republic of Ireland), Cenovis (Switzerland) and Vitam-R (Germany). Bovril (United Kingdom and Republic of Ireland) 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 sodium chloride (salt) to a suspension of yeast, making the solution hypertonic, which leads to the cells shrivelling up; this triggers autolysis, in which the yeast self-destructs. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls) are separated. Removing the cell walls concentrates the flavours and changes the texture.
Yeast extract is commonly used as a flavour enhancer in processed foods of all kinds.
Hydrolyzed yeast 
Hydrolyzed yeast or hydrolyzed yeast extract is another version used as a flavour enhancer. Exogenous enzymes or acids are used to hydrolyze the proteins.
Yeast extract contains naturally occurring glutamic acid or monosodium glutamate; this is produced from an acid-base fermentation cycle, and is only found in some yeasts, typically ones bred for use in baking.
Many food producers have replaced monosodium glutamate with yeast extract, which requires no E-number labeling, and allows food producers to claim their product is 'all natural' or 'with natural flavorings'. Health proponents claim this is a subversive move by the industry to hide an additive since the MSG contained within the yeast extract does not require explicit labeling. Food industry bodies  state:
- Yeast extract is often mistaken with monosodium glutamate (MSG) – a common flavor enhancer – despite the fact that these ingredients differ strongly, both in composition and function. While yeast extract is made up of a rich mix of proteins, vitamins and amino acids, MSG is composed exclusively of glutamate salt. As a result, monosodium glutamate does not have a taste of its own and is only used to make existing flavors stronger.
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (June 2012)|
- Herbst, Sharon (2001). Food Lover's Companion. Hauppauge, New York: Barron's Educational Series, Inc.
- A salty tale of yeast spreads
- yeastextract.info: Homepage of Eurasyp (European Association of Specialty Yeast Products)
- What is Autolyzed Yeast Extract?, Live Strong
- Edens, NK; Reaves, LA; Bergana, MS; Reyzer, IL; O'Mara, P; Baxter, JH; Snowden, MK (2002). "Yeast extract stimulates glucose metabolism and inhibits lipolysis in rat adipocytes in vitro". The Journal of nutrition 132 (6): 1141–8. PMID 12042423.