What Is Geosmin?
Geosmin is an organic compound responsible for the earthy scent often perceived after rain or soil disturbance. Its name, coined in 1965 by scientists Nancy N. Gerber and Hubert A. Lechevalier, combines the Greek words “geō-” (earth) and “osmḗ” (smell). This molecule, a bicyclic alcohol with the chemical formula C₁₂H₂₂O, is the primary component of petrichor.
Where Geosmin Comes From
Geosmin is primarily produced by microorganisms found in soil and water. Soil-dwelling bacteria, particularly Streptomyces, are significant producers. They release geosmin when they die or when conditions are unfavorable, prompting spore dispersal.
Cyanobacteria (blue-green algae) also produce geosmin in aquatic environments. It is released when these microbial populations decline or die off. Geosmin is also produced by some plants, such as beetroots, contributing to their earthy flavor.
Why We Smell Geosmin So Easily
The human sense of smell possesses extraordinary sensitivity to geosmin, detecting it at remarkably low concentrations. People can perceive geosmin in water at levels as low as 5 to 15 parts per trillion (nanograms per liter). Even a minuscule amount, equivalent to a teaspoon diluted across hundreds of Olympic-sized swimming pools, can be detected.
This extreme sensitivity is thought to have an evolutionary basis, potentially aiding early humans in locating scarce water sources or identifying healthy soil environments. Animals, including insects like springtails and mosquitoes, are also highly attuned to geosmin, using it as a signal for finding microbial prey or as an attractant. Heating water increases geosmin’s volatility, making its smell more noticeable, such as during a hot shower or when preparing hot beverages.
Geosmin in Daily Life
Geosmin is commonly experienced in daily life, beyond just the smell of rain. The familiar “earthy” smell after rain, known as petrichor, occurs when raindrops hit the ground, trapping geosmin in air pockets. These pockets burst, releasing geosmin-containing aerosols into the air.
In drinking water, geosmin can cause an “earthy” or “musty” taste and odor, even if the water is safe. This taste and odor is not contamination, but a result of geosmin-producing microorganisms in water sources. Geosmin also contributes to the earthy flavor of foods like beetroots and mushrooms. It causes the “muddy” taste in freshwater fish like carp, catfish, and tilapia, as they absorb geosmin from algae through their gills.
Addressing Geosmin’s Presence
Water treatment facilities employ various methods to manage geosmin’s presence, particularly in drinking water, although its presence is primarily an aesthetic concern and not a health risk. Conventional water treatment processes often do not effectively remove geosmin, especially at higher concentrations. Advanced treatment technologies are utilized to mitigate its taste and odor.
Activated carbon (granular and powdered) is a widely used and effective method for adsorbing geosmin from water due to its porous structure. Oxidation processes, such as treatment with ozone or chlorine dioxide, can also break down geosmin molecules, making them odorless and tasteless. Some facilities may also increase chloride and copper sulfate to control algae, which in turn helps reduce geosmin levels.
For consumers, chilling water, adding ice, or a slice of lemon can help reduce the perception of geosmin’s taste and odor. In food, the “muddy” taste in fish can be reduced by keeping fish in clean water for a few days before consumption, allowing them to purge the compound. Cooking methods, such as using acidic ingredients like vinegar, can break down geosmin into odorless substances.