Geosmin is a compound that comes from microbes and has a noticeable ‘earthy’ to ‘musty’ smell, impacting the quality of both food and water. A research group has recently identified and described the human odorant receptor that detects geosmin for the first time.
Geosmin is a volatile substance resulting from microbial activity and has a unique “earthy” to “musty” scent that can influence food and water quality. A team of researchers, led by Dietmar Krautwurst from the Leibniz Institute for Food Systems Biology at the Technical University of Munich, has successfully identified and characterized the human odorant receptor for geosmin for the very first time.
Geosmin is responsible for the characteristic smell that arises when rain hits parched soil. This scent is created by soil microorganisms and can also be found in certain plants, such as cactus flowers and beets.
Numerous animals are highly sensitive to geosmin, with its scent having either an attractive or repellent effect. For instance, it alerts fruit flies to spoiled food, while camels are drawn to areas with abundant water. “This indicates that geosmin serves as a chemical signal in the animal kingdom, and likely in humans as well,” states Lena Ball, the study’s lead author from the Leibniz Institute.
Geosmin can affect food quality
“While the scent of geosmin is appropriate for red beet, its presence in other foods like fish, beans, cocoa, water, wine, or grape juice poses a challenge. In these cases, it significantly diminishes sensory quality and consumer acceptance,” explains Stephanie Frank, a food chemist at the Leibniz Institute. Even low levels of just 4 to 10 ng/L can allow a person to detect the smell in water, equivalent to adding one teaspoon of geosmin into the water volume of 200 Olympic-sized swimming pools.
Although geosmin has been known since 1965 and is significant for food production, it was previously unclear which receptor humans use to sense it. The research team led by Dietmar Krautwurst employed a bidirectional receptor screening method to discover and functionally characterize the relevant receptor for the first time.
Only one human receptor for geosmin
Out of 616 different human olfactory receptors tested, only the OR11A1 receptor reacted to meaningful concentrations of geosmin. The researchers also examined whether the identified receptor responds to other food-related odors. From the 177 substances examined, only the earthy-scented 2-ethylfenchol significantly activated this receptor, which also originates from microbes.
“Since geosmin is an important signaling compound among animals, we also studied the reaction of odorant receptors in animals such as kangaroo rats, mice, rhesus monkeys, Sumatran orangutans, polar bears, and camels. These have genetic similarities to the human receptor. We aimed to determine whether the selective recognition of geosmin by this receptor has remained intact throughout 100 million years of mammalian evolution,” shares doctoral student Lena Ball. The team’s comparative research indicates that the human receptor, along with monkey receptors, is among the less sensitive to geosmin. In their experiments, the kangaroo rat’s odorant receptor displayed sensitivity to geosmin that was about 100 times greater than that of the human receptor.
“These new insights into the highly sensitive odorant receptors found in some animals further highlight the biological significance of geosmin as a signaling compound. They could also assist in developing innovative detection systems for monitoring food quality during production and storage or for managing the water quality of freshwater sources,” concludes Dietmar Krautwurst.
