Geosmin is a chemical you’ve never heard of but your nose recognizes it whenever it rains. It is the chemical that produces that unmistakable aroma of freshly turned soil or rain mixing with dry earth. The smell is so distinctive it even has a name, petrichor, which loosely translates as earth odor. This chemical impacts our lives in many interesting ways.
Where Does Geosmin Come From?
Geosmin is produced from the breakdown of organic matter by a variety of microbes such as cyanobacteria, actinomycetes & streptomycetes that are found in soil. It finds its way into various fruits, vegetables & even fish. In some cases it enhances the flavor of the food, in others it can be a nuisance. (1)
Does Geosmin Affect Our Food?
Geosmin is largely responsible for the earthy taste of beets which are used in borscht (beet soup) and produces sucrose, the same sugar as sugarcane & a red pigment used a natural dye in ice cream, drinks & various dairy products. Geosmin can also be found in spinach, lettuce & mushrooms. (2)
What About Geosmin and Water?
Geosmin finds its way into multiple water sources such as irrigation ponds, drinking water reservoirs & runoff catch basins. Its distinctive odor is undesirable so municipal authorities treat your water before it finds its way into your home & out your taps. The chemical is not toxic at naturally occurring levels. In fact some wines are enhanced by it while others are not. (3)
Geosmin and Evolution
Our noses are exceptionally sensitive to geosmin such that we can detect it in low parts per trillion (ppt) concentrations. For dogs this ability applies to a vast range of chemicals but for humans, this is very unusual. It has been suggested that sensitivity for this smell has been strongly selected for throughout our evolutionary history. It enabled us to find water even in arid environments. Of course, in this regard we are not unique. Camels are thought to be able to find oases many kilometres distant even before we can. (1).
Geosmin The Molecular Mobile
If you’ve ever taken organic chemistry, there is a certain familiarity to geosmin. It is basically 2 cyclohexane rings with 2 methyl & 1 hydroxyl group attached. As a mobile, it looks a bit like a space station you might see in old science fiction TV episodes & movies. You can build your own by entering geosmin in our 3D Molecular Model Builder.
Special thanks to Sue Murphy, Water Quality Specialist at sidwater.org in Vacaville, CA, USA for providing us with the references used in the article.