Geosmin

Geosmin is an organic compound with a distinct earthy flavor and aroma produced by certain bacteria, and is responsible for the earthy taste of beetroots 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. Geosmin is an organic compound with a distinct earthy flavor and aroma produced by certain bacteria, and is responsible for the earthy taste of beetroots 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 reduce the muddy flavor. Taste and odor compounds including geosmin lead to an unpleasant taste of drinking water which is perceived by consumers as an indication of poor water quality. Geosmin is also an issue for saltwater fish grown in recirculating aquaculture systems, such as Atlantic salmon. These systems rely on biological filtration using cultured microbial communities to process the nitrogenous waste from the fish (ammonia) into less harmful compounds (nitrite and nitrate) that can be tolerated at higher concentrations. However geosmin-producing bacteria can also grow in these systems, and often require fish to be transferred to an additional 'finishing' or 'purge' system where they are not fed for several days prior to harvest to remove off-flavor compounds and empty the intestinal tract.