Geosmin is an organic compound with a distinct earthy flavor and aroma produced by certain bacteria, and is responsible for the earthy taste of beets and a contributor to the strong scent (petrichor) that occurs in the air when rain falls after a dry spell of weather or when soil is disturbed. In chemical terms, it is a bicyclic alcohol with formula C12H22O, a derivative of decalin. Its name is derived from the Greek γεω- "earth" and ὀσμή "smell".

Geosmin is produced by the gram-positive bacteria Streptomyces and various cyanobacteria, and released when these microorganisms die. Communities whose water supplies depend on surface water can periodically experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the local water supply. Under acidic conditions, geosmin decomposes into odorless substances.

In 2006, the biosynthesis of geosmin by a bifunctional Streptomyces coelicolor enzyme was unveiled. A single enzyme, geosmin synthase, converts farnesyl diphosphate to geosmin in a two-step reaction.

Streptomyces coelicolor is the model representative of a group of soil-dwelling bacteria with a complex lifecycle involving mycelial growth and spore formation. Besides the production of volatile geosmin, it also produces many other complex molecules of pharmacological interest; its genome sequence is available at the Sanger Institute.

The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.

Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as carp and catfish. Geosmin combines with 2-methylisoborneol, which concentrates in the fatty skin and dark muscle tissues. Geosmin breaks down in acid conditions; hence, vinegar and other acidic ingredients are used in fish recipes to help reduce the muddy flavor.

This page was last edited on 16 June 2018, at 10:14 (UTC).
Reference: under CC BY-SA license.

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