Tuesday, September 29, 2009

The Smell Of New Rain

Site of the day: http://www.bluehatseo.com/

Do you know the distinctive smell that is discernible when rain starts falling? This happens because of chemical compounds geosmin and petrichor.

Geosmin, which literally translates to "earth smell", is an organic compound with a distinct earthy flavour and aroma, and is responsible for the earthy taste of beets and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (with petrichor). 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 produced by several classes of microbes, including cyanobacteria (blue-green algae) and actinobacteria (especially streptomyces), and released when these microbes die. Communities whose water supply depends 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.

Geosmin is also responsible for the muddy smell in bottom-dwelling freshwater fish such as carp and catfish… Geosmin breaks down in acid conditions; hence vinegar and other acidic ingredients in fish recipes help reduce the muddy flavor.

Petrichor derives from an oil exuded by certain plants during dry periods, whereupon it is adsorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, producing the distinctive scent… the oil retards seed germination and early plant growth.The scent is generally regarded as pleasant and refreshing… In desert regions, the smell is especially strong during the first rain after a long dry spell… The oil yielding the scent can be collected from rocks and concentrated to produce perfume; however, it has yet to be synthesized, perhaps due to its complexity. It is composed of more than fifty distinct chemical substances.

By the way, the smell of newmown grass comes from cis-3-Hexen-1-ol

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