Compounds · Alcohols · Microbial origin
Geosmin
Also known as trans-1,10-Dimethyl-trans-9-decalol — an analysis may use any of these names
First rain, earth after rain, beetroot. The human nose detects it at a few parts per trillion — one of the lowest thresholds ever measured in humans.
- CAS
- 16423-19-1
- Formula
- C12H22O
- Molar mass
- 182.30
- Odour threshold
- 0.004 ppbin water
- Boiling point
- ≈ 270 °C at atmospheric pressure
The two axes are 63 points apart. Volatility tells you whether the compound leaves a surface early or late; the threshold tells you how much of it the nose needs before it notices. When the two sit this far apart, they explain things the percentages on an analysis cannot explain on their own.
About the CAS number and optical isomers
The CAS number above does not specify the optical isomer. The geosmin secreted by bacteria is the (−) form, and the human nose is many times more sensitive to it than to the other form — one more example of olfactory receptors recognising three-dimensional shape.
Where it comes from
Biosynthetic pathway: Microbial origin — made by bacteria and yeasts, not by plants.
About the threshold figure
People detect geosmin at a few parts per trillion. That is why the smell of first rain arrives instantly even though the molecule is fairly heavy.
Published thresholds differ by several orders of magnitude, because of differences in measuring method, sample purity and panel. The figure here is an order of magnitude for relative comparison, not a measurement. We give it only for compounds where the literature broadly agrees — 52 of the 106 compounds in the library. The rest are left blank rather than guessed.
Why it sits where it does
A bicyclic twelve-carbon alcohol — a large molecule with a hydroxyl group, so it boils high and stays on every surface for a long time.
Where else you meet it
Earth after rain, beetroot, freshwater fish, and tap water during algal blooms.
Dictionary entries that mention this compound
About the boiling-point figure
Cross-checked in August 2026: 270–271 °C at atmospheric pressure, consistent across several sources. Why this detail matters