Few compound names are as descriptive as geosmin: It could be translated as “earth smell” from Ancient Greek and is, unsurprisingly, the main aroma compound in many earthy, musty smells.1The original paper that documents the isolation of geosmin and coined its name: Gerber, N N, and H A Lechevalier. ‘Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes’, n.d., 4 You’d recognise it immediately as the smell that spreads after rain on a warm summer day2which, by the way, is called petrichor, as the flavour of beetroots, or from the distinctly earthy aroma of a mouldy sandwich sealed in a long-forgotten lunchbox tomb.
The aroma of earthy delights
Apart from species of mould, geosmin is produced by bacteria living in soil and in water. The latter are the reason why some ground-dwelling freshwater fish like carp have a “muddy” aroma. In fact, in some regions of Central and Eastern Europe, where carp is traditionally served at Christmas, you might find the occasional bathtub occupied by a carp: Keeping them for a couple of days, changing the water every once in a while washes out the geosmin making the fish more palatable. Moreover, lemon juice is frequently drizzled on the (prepared) fish, and while I’m sure almost nobody who does this is aware of it, geosmin can actually be degraded by acid.3Finding evidence for that was quite a hunt; see the mini literature review I wrote as a bonus.
Another common sight in Eastern European kitchens is beetroot, in which geosmin is the main aroma component.4It’s not entirely clear where the geosmin in beetroot comes from. While it seemed likely that they take it up from the soil they grow in, beets grown in soil- and bacteria-free conditions were shown to contain about as much geosmin as soil-grown ones. So they probably produce it themselves; who knows why. Source: Lu, Guiping, Charles G. Edwards, John K. Fellman, D. Scott Mattinson, and John Navazio. 2003. ‘Biosynthetic Origin of Geosmin in Red Beets ( Beta Vulgaris L.)’. Journal of Agricultural and Food Chemistry 51 (4): 1026–29. https://doi.org/10.1021/jf020905r. While they are popular in Eastern Europe and certainly have a borsch-loving fanbase spread all other the globe, some really can’t stand the earthiness. Might be an acquired taste. Interestingly, whether you like beetroot or not, all humans are unusually sensitive towards their aroma. The odour detection threshold of geosmin lies between 0.006 and 0.01 microgrammes per litre of water. For comparison, vanillin’s more or less average detection threshold in water is at least 2000 times higher, between 20 and 680 microgrammes per litre.5Plotto, Anne, Carlos A. Margaría, Kevin L. Goodner, Renée Goodrich, and Elizabeth A. Baldwin. ‘Odour and Flavour Thresholds for Key Aroma Components in an Orange Juice Matrix: Terpenes and Aldehydes: ODOUR/FLAVOUR THRESHOLDS FOR TERPENES AND ALDEHYDES’. Flavour and Fragrance Journal 19, no. 6 (November 2004): 491–98. https://doi.org/10.1002/ffj.1470. There might be a use for this exceptional sensitivity towards geosmin.
Animals are sensitive towards geosmin too
Indeed, scientists found the same sensitivity for geosmin in animals as different as fruit flies and camels. What do they do with it? Remember that the smell of rain largely comes from geosmin: Soil naturally contains air bubbles, which is where substances like geosmin tend to accumulate. When the soil gets wet, these bubbles fill with water, and geosmin escapes with the air into the atmosphere, hence the characteristic smell. So the presence of geosmin indicates that there’s water touching soil somewhere. It has been suggested that this enables camels to smell the next oasis in the desert from very far away.6https://www.theguardian.com/science/2003/mar/06/science.research
Fruit flies, on the other hand, avoid geosmin.7Stensmyr, Marcus C., Hany K.M. Dweck, Abu Farhan, Irene Ibba, Antonia Strutz, Latha Mukunda, Jeanine Linz, et al. 2012. ‘A Conserved Dedicated Olfactory Circuit for Detecting Harmful Microbes in Drosophila’. Cell 151 (6): 1345–57. https://doi.org/10.1016/j.cell.2012.09.046.; review with more compounds released by microbes altering the behaviour of animals: Weisskopf, L., Schulz, S. & Garbeva, P. Microbial volatile organic compounds in intra-kingdom and inter-kingdom interactions. Nat Rev Microbiol 19, 391–404 (2021). https://doi.org/10.1038/s41579-020-00508-1 This also makes sense: As they feed on overripe fruit, it’s useful for them to know which one is safe and which is best left alone. When there’s geosmin around, the mould that produced it is probably not far away – and so might be other, much more nasty substances. Thinking about that, I noticed that I cannot remember to have ever seen fruit flies on mouldy fruit – it all makes sense now…
Another one tastes the dust?
If so distantly related species such as fruit flies, camels and humans share such a distinct trait, it may be that the sensitivity for geosmin developed very early on in the animal kingdom and that most animals have it. The question remains in what way such sensitivity would be useful for them. I wouldn’t be surprised to find that there are more uses for it than detecting water or mould. What about humans, then? The taste for the earthiness of beetroot that some of us have might be much older than human consumption of beetroot: Geophagy, the practice of eating soil, has historically been documented for communities all over the world and is still practised in some places (mainly in parts of Subsaharan Africa and the South of the US); indulging in soil is widespread among animals too, even when food is abundant.8Woywodt, Alexander, and Akos Kiss Fcs. 2002. ‘Geophagia: The History of Earth-Eating’ 95: 4 Some human geophages mention hunger as a reason for eating soil but for many, that is not an issue. Rather, lots of regular soil eaters report a craving for the consistency and smell of soil, especially types of clay. I have read an article citing the story of a girl that got such strong cravings for dirt when it rained that her mother had to close the windows to keep the smell out so that her daughter wouldn’t run outside with a spoon.
Cleaning with dirt
As you might have guessed, geophagy is stigmatised as dirty and uncivilised and is even considered a psychological disorder. I was careful not to use “dirt” as a synonym for soil here in order to avoid the connotation of literal dirtiness. I’m not going to discuss psychology and the notion of civilisation here; still, I think it’s interesting to ask whether there’s an actual benefit to eating soil unrelated to filling your stomach.
The physiological benefits are not completely clear yet, but they might get some micronutrients, namely minerals and vitamin B12, out of the soil, and it has been shown that some types of soil have a detoxifying and even antimicrobial activity. In German, there’s a paradoxical saying that’s commonly cited when someone eats something that was dropped on the floor: “Dirt cleans the stomach.” – I’ll continue to roll my eyes at this one, but I acknowledge that it’s got some minor scientific validation after all.
Regardless of whether geosmin plays a role in some people’s taste for soil, I think it’s interesting to consider that stigmatised behaviour like this might play a role in eating preferences. That way, you could see eating beetroot as a culturally acceptable alternative to eating dirt – which might get you into psychological treatment.
When it comes to geosmin, are you team fruit fly or team camel? Let me in the comments; I’d love to read some! I personally can’t stand beetroot on its own, but I like to add small amounts to dishes like hummus for sort of a hearty, earthy background.
Footnotes
- 1The original paper that documents the isolation of geosmin and coined its name: Gerber, N N, and H A Lechevalier. ‘Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes’, n.d., 4
- 2which, by the way, is called petrichor
- 3Finding evidence for that was quite a hunt; see the mini literature review I wrote as a bonus.
- 4It’s not entirely clear where the geosmin in beetroot comes from. While it seemed likely that they take it up from the soil they grow in, beets grown in soil- and bacteria-free conditions were shown to contain about as much geosmin as soil-grown ones. So they probably produce it themselves; who knows why. Source: Lu, Guiping, Charles G. Edwards, John K. Fellman, D. Scott Mattinson, and John Navazio. 2003. ‘Biosynthetic Origin of Geosmin in Red Beets ( Beta Vulgaris L.)’. Journal of Agricultural and Food Chemistry 51 (4): 1026–29. https://doi.org/10.1021/jf020905r.
- 5Plotto, Anne, Carlos A. Margaría, Kevin L. Goodner, Renée Goodrich, and Elizabeth A. Baldwin. ‘Odour and Flavour Thresholds for Key Aroma Components in an Orange Juice Matrix: Terpenes and Aldehydes: ODOUR/FLAVOUR THRESHOLDS FOR TERPENES AND ALDEHYDES’. Flavour and Fragrance Journal 19, no. 6 (November 2004): 491–98. https://doi.org/10.1002/ffj.1470.
- 6
- 7Stensmyr, Marcus C., Hany K.M. Dweck, Abu Farhan, Irene Ibba, Antonia Strutz, Latha Mukunda, Jeanine Linz, et al. 2012. ‘A Conserved Dedicated Olfactory Circuit for Detecting Harmful Microbes in Drosophila’. Cell 151 (6): 1345–57. https://doi.org/10.1016/j.cell.2012.09.046.; review with more compounds released by microbes altering the behaviour of animals: Weisskopf, L., Schulz, S. & Garbeva, P. Microbial volatile organic compounds in intra-kingdom and inter-kingdom interactions. Nat Rev Microbiol 19, 391–404 (2021). https://doi.org/10.1038/s41579-020-00508-1
- 8Woywodt, Alexander, and Akos Kiss Fcs. 2002. ‘Geophagia: The History of Earth-Eating’ 95: 4
