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#TIL the organic molecule Geosmin is responsible for the strong earthy/muddy smell of petrichor (the smell of rain on dry earth) and our noses are so sensitive to it that we can detect 5 parts per trillion! It is produced by soil bacteria
A fun fact: when these soil bacteria die in large numbers, their geosmin gets into the ground water, making it taste really muddy. We love its smell in the air and dislike its taste in our water.
And geosmin's presence in freshwater fish (and snails) give them that distinct earthy flavour. And since acids break down geosmin, squeezing lime on freshwater fish is quite common.
At 5 parts per trillion, that's almost as much as the human nose's sensitivity to Ethyl Mercaptan, that is used in LPG gas cylinders to help us detect leaks.
Fascinating story of how this use of Ethyl Mercaptan came to be discovered. Apparently, engineers in California noticed that vultures tended to gather around gas leaks and it was because of the presence of trace amounts of this gas, so they decided to deliberately add it
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