<p>Ever wondered where the distinct smell of mud, just before it rains, comes from? It is caused by the release of Geosmin, an organic compound found in micro-organisms, says K S Someswara.<br /><br /></p>.<p>Before a rainfall in the spring, we feel a particular fresh and sweet smell which seeps into the air a few minutes before the first drop of rain falls. If one is familiar with this smell, they can easily gauge that it is about to rain.<br /><br />When it begins to rain after a long period of humid weather, we can smell the earth. This pleasant odour makes us feel cool and fresh. This aroma is because of an organic chemical called ‘Geosmin’.<br /><br />There are many folk tales about this smell. To quote a few, a long time ago, people believed that clouds picked up the sweet smell from heaven and rain brought this angelic odour to Earth. A more refined story says that the sweet smell comes from the ozone carried from the upper atmosphere by the falling rain. <br /><br />However, the truth is that this fresh smell is rising from the ground beneath our own feet. As the spring rain approaches, the humidity level at the ground tends to increase. Moist air is much better than dry air to transmit this smell. As humidity rises, moist air carries the fresh smell up to us. This also carries the odour of ground-dwelling bacteria and fungi.<br /><br />Released by microbes<br /><br />Geosmin is an organic compound with a distinct earthy flavour and aroma. This is responsible for the earthy taste of beets and a contributor to the strong scent (petrichor) that hangs in the air. This can be felt when it rains after a dry spell of weather or when soil is disturbed.<br /><br /> This smell is produced by a variety of micro-organisms. Chemically, it is a bicyclic alcohol with the formula C12H22O, a derivative of decatin. The name is derived from a Greek word.<br /><br />A number of enzymes are involved in making Geosmin. One key enzyme is germacradienol synthase, which was recently sequenced and found. It is a protein with over 700 amino acids of which little is known except that it is made up of two homologous domains. Of the two, only one is necessary for Geosmin synthesis.<br /><br />According to microbial geneticist Keith Chater of England, the traces of Geosmin are due to a common bacterium Streptomyces coelicolor. He is also said to have found out exactly which gene in the little critter causes them to produce Geosmin. A bacterium is actually producing the chemical that gives such a pleasant smell.<br /><br />Many unconfirmed reports say that animals are attracted to fresh dirt in the desert as they go in search of water. As they nose around the dirt, they pick up plenty of Streptomyces coelicolor which then hitch a ride from one oasis to another. Geosmin is produced by several classes of microbes including cyanobacteria (blue-green algae) and actino bacteria (esp. streptomyces) and released when the microbes die.<br /><br />The community which depends on water on the surface can periodically experience the unpleasant taste of water. This happens when a sharp drop in the population of this bacterium releases Geosmin into the local water supply. Under acidic conditions, Geosmin decomposes into odourless substances.<br /><br />Geosmin is responsible for the muddy smell in bottom-dwelling freshwater fish like carp and catfish. Cyanobacteria produces Geosmin and 2-methylisoborneol which concentrate in their skin and dark muscles tissue. As noted earlier, Geosmin breaks down in acidic conditions. Hence vinegar and other acidic ingredients are used in fish recipes which reduce the muddy flavour.<br /><br />Multiple purposes<br /><br />The role of Geosmin is not just limited to the taste or smell of wine or water. Some think it produces a pang and predators clear away from their prey. This particular fragrance occurs when humidity is involved. Camels in the desert may smell an oasis by sniffing air from the tract of fragrant metabolic activity of bacteria.<br /><br />While they quench their thirst, hordes of streptomyces spores will be supped with water or find their way to stick on to animal hide. In this way, spores are carried for many miles. Similarly, some cacti flowers may also use Geosmin scent to fool insects in a sort of fragrant mimicry. Insects are attracted to plants in the hope of little refreshment and in their quest for water, they actually serve as pollinators.<br /><br />Geosmin can also seep into the flesh of vegetables and fruits, making them quite distasteful for consumption. Pharma plants where streptomyces are used in the production of antibiotics, and units which produce pulp, create the characteristic smell of Geosmin in warm conditions. Edible mushrooms and fungi also possess the earthy smell similar to certain types of cheese like stilton.</p>
<p>Ever wondered where the distinct smell of mud, just before it rains, comes from? It is caused by the release of Geosmin, an organic compound found in micro-organisms, says K S Someswara.<br /><br /></p>.<p>Before a rainfall in the spring, we feel a particular fresh and sweet smell which seeps into the air a few minutes before the first drop of rain falls. If one is familiar with this smell, they can easily gauge that it is about to rain.<br /><br />When it begins to rain after a long period of humid weather, we can smell the earth. This pleasant odour makes us feel cool and fresh. This aroma is because of an organic chemical called ‘Geosmin’.<br /><br />There are many folk tales about this smell. To quote a few, a long time ago, people believed that clouds picked up the sweet smell from heaven and rain brought this angelic odour to Earth. A more refined story says that the sweet smell comes from the ozone carried from the upper atmosphere by the falling rain. <br /><br />However, the truth is that this fresh smell is rising from the ground beneath our own feet. As the spring rain approaches, the humidity level at the ground tends to increase. Moist air is much better than dry air to transmit this smell. As humidity rises, moist air carries the fresh smell up to us. This also carries the odour of ground-dwelling bacteria and fungi.<br /><br />Released by microbes<br /><br />Geosmin is an organic compound with a distinct earthy flavour and aroma. This is responsible for the earthy taste of beets and a contributor to the strong scent (petrichor) that hangs in the air. This can be felt when it rains after a dry spell of weather or when soil is disturbed.<br /><br /> This smell is produced by a variety of micro-organisms. Chemically, it is a bicyclic alcohol with the formula C12H22O, a derivative of decatin. The name is derived from a Greek word.<br /><br />A number of enzymes are involved in making Geosmin. One key enzyme is germacradienol synthase, which was recently sequenced and found. It is a protein with over 700 amino acids of which little is known except that it is made up of two homologous domains. Of the two, only one is necessary for Geosmin synthesis.<br /><br />According to microbial geneticist Keith Chater of England, the traces of Geosmin are due to a common bacterium Streptomyces coelicolor. He is also said to have found out exactly which gene in the little critter causes them to produce Geosmin. A bacterium is actually producing the chemical that gives such a pleasant smell.<br /><br />Many unconfirmed reports say that animals are attracted to fresh dirt in the desert as they go in search of water. As they nose around the dirt, they pick up plenty of Streptomyces coelicolor which then hitch a ride from one oasis to another. Geosmin is produced by several classes of microbes including cyanobacteria (blue-green algae) and actino bacteria (esp. streptomyces) and released when the microbes die.<br /><br />The community which depends on water on the surface can periodically experience the unpleasant taste of water. This happens when a sharp drop in the population of this bacterium releases Geosmin into the local water supply. Under acidic conditions, Geosmin decomposes into odourless substances.<br /><br />Geosmin is responsible for the muddy smell in bottom-dwelling freshwater fish like carp and catfish. Cyanobacteria produces Geosmin and 2-methylisoborneol which concentrate in their skin and dark muscles tissue. As noted earlier, Geosmin breaks down in acidic conditions. Hence vinegar and other acidic ingredients are used in fish recipes which reduce the muddy flavour.<br /><br />Multiple purposes<br /><br />The role of Geosmin is not just limited to the taste or smell of wine or water. Some think it produces a pang and predators clear away from their prey. This particular fragrance occurs when humidity is involved. Camels in the desert may smell an oasis by sniffing air from the tract of fragrant metabolic activity of bacteria.<br /><br />While they quench their thirst, hordes of streptomyces spores will be supped with water or find their way to stick on to animal hide. In this way, spores are carried for many miles. Similarly, some cacti flowers may also use Geosmin scent to fool insects in a sort of fragrant mimicry. Insects are attracted to plants in the hope of little refreshment and in their quest for water, they actually serve as pollinators.<br /><br />Geosmin can also seep into the flesh of vegetables and fruits, making them quite distasteful for consumption. Pharma plants where streptomyces are used in the production of antibiotics, and units which produce pulp, create the characteristic smell of Geosmin in warm conditions. Edible mushrooms and fungi also possess the earthy smell similar to certain types of cheese like stilton.</p>
