How we perceive earthy odors
The unmistakable earthy smell of summer rain comes from the fragrance geosmin.
geosmin is a volatile compound of microbial origin with a distinct "earthy" to "musty" odor that can affect the quality of drinking water and food. A research team led by Dietmar Krautwurst from the Leibniz Institute for Food Systems Biology at the Technical University of Munich has now identified and characterized the human odour receptor for geosmin for the first time.
Geosmin is responsible for the typical odor that occurs when rain falls on dry soil. This odorant is produced by microorganisms in the soil and is also found in plants such as cacti and beet.
Many creatures react very sensitively to geosmin, whereby the odorant can have a repellent or attractive effect. For example, it warns fruit flies of spoiled food. Camels, on the other hand, are attracted to water-rich areas. "This shows that geosmin acts as a chemical signaling substance in the animal kingdom and certainly also in humans," explains first author Lena Ball from the Leibniz Institute.
Geosmin can impair food quality
"While the smell of geosmin suits beet, its presence in foods such as fish, beans, cocoa, water, wine or grape juice is problematic. In these, it greatly impairs sensory quality and acceptance," explains Stephanie Frank, food chemist at the Leibniz Institute. Even the lowest concentrations of 4 to 10 ng/L are sufficient for a person to perceive the odor in water. This corresponds to about one teaspoon of geosmin in the water volume of 200 Olympic swimming pools.
Although geosmin has been known since 1965 and is important for food production, it was previously unknown which olfactory receptor humans use to perceive the odorant. The team headed by study leader Dietmar Krautwurst has now carried out a bidirectional receptor screening and identified and functionally characterized the corresponding receptor for the first time.
Only one human odorant receptor for geosmin
Of 616 human olfactory receptor variants tested, only the OR11A1 receptor responded to physiologically relevant concentrations of the odorant. The team also investigated whether the identified receptor reacts to other food-relevant odorants. Of the 177 substances tested, only the earthy-smelling 2-ethylfenchol, which is also of microbial origin, was able to significantly activate the receptor.
"As geosmin is an important signaling substance in the animal kingdom, we also investigated how the odorant receptors of the kangaroo rat, mouse, rhesus monkey, Sumatran orangutan, polar bear and camel, which are genetically most closely related to the human receptor, react to geosmin. We wanted to find out whether the highly selective recognition of geosmin by the same receptor has been preserved over 100 million years of mammalian evolution," reports doctoral student Lena Ball. As the team's comparative studies show, the human receptor, together with the monkey receptors, is one of the less sensitive sensors. In the experiment, the kangaroo rat's olfactory receptor reacted around 100 times more sensitively to geosmin than the human receptor.
"The new findings on the highly sensitive olfactory receptors of some animals once again emphasize the biological relevance of geosmin as a signaling substance. They could also help to develop novel detection systems that can be used to monitor food quality during production and storage or to control the water quality of freshwater reservoirs," concludes Dietmar Krautwurst.
Background information
Human olfactory receptors
Humans possess a total of around 400 different olfactory receptor genes, which in turn encode around 600 different allelic receptor variants in the nasal mucosa. The latter are responsible for the perception and differentiation of various odors. However, there is still a need for research to determine the exact number and function of all receptor variants. At present, it is only known for around 20 percent of human olfactory receptors which odorants they can recognize.
Test system used for screening
According to Dietmar Krautwurst, the cellular test system developed by the Leibniz researchers and used for receptor screening is unique in the world. He and his team have genetically modified the test cells so that they act like small biosensors for odorous substances. The researchers determine exactly which odorant receptor variant the test cells present on their surface. In this way, the researchers can specifically investigate which receptor reacts how strongly to which odorant. The Leibniz Institute has extensive collections of odorants and receptors, which it uses for its research work.
Note: This article has been translated using a computer system without human intervention. LUMITOS offers these automatic translations to present a wider range of current news. Since this article has been translated with automatic translation, it is possible that it contains errors in vocabulary, syntax or grammar. The original article in German can be found here.
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