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 savory flavors and umami taste sensations. Monosodium glutamate (MSG) is used for umami, but has no flavor. Yeast extract, like MSG, often contains free glutamic acid. Yeast extracts in liquid form can be dried to a light paste or a dry powder. Glutamic acid in yeast extracts are produced from an acid-base fermentation cycle, only found in some yeasts, typically ones bred for use in baking.
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), 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 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 flavors and changes the texture.
Hydrolyzed yeast or hydrolyzed yeast extract is another version used as a food additive for flavouring purposes. Exogenous enzymes or acids are used to hydrolyze the proteins.
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