Although petrichor was originally used to describe the oil released from rocks, it has become a portmanteau word to describe the smell that accompanies the first rainfall after a dry spell, however it is produced. French biochemists, Bertholet and André, studied the evocative smell of freshly wetted soil in the late 19th century and succeeded in isolating what they called “l’odeur propre de la terre”. However, it was the American duo, Gerber and Lechevelier, who successfully tracked the smell down to a single compound, which they called geosmin, in their paper published in Applied Microbiology in 1965, a term they derived from the Ancient Greek words for earth and smell.
The organisms responsible for releasing the geosmin into the air are Streptomyces, threadlike bacteria which play an important part in recycling vegetable matter and are found in the soil of woodlands and our gardens. Thriving in wet conditions, they only produce their spores when it is dry. One of their 8,000 genes, painstakingly identified and isolated by a team of scientists at the John Innes Centre in Norwich in 2002, is responsible for secreting geosmin during spore production.
A large supply of spores builds up during a dry spell and when it first rains, the force of the raindrops landing on the ground sends the spores up into the air and the moist air picks up the aroma. Even humans, who are not known for their olfactory acumen, are able to detect geosmin at concentrations of less than 100 parts per trillion.
While our noses may be attuned to its presence and we find its aroma pleasant, at least in small doses, its taste is another matter. Even though it is not toxic, its presence can make liquids seem off, especially in mineral water and wine. Indeed, detecting the presence of a rather musty sensation developing in their younger wines, French wine growers discovered in 2002 that the culprit was geosmin forming on some rotten grapes. It is also a major contributor to the distinctively earthy taste and smell of beetroot, a vegetable which is not everybody’s cup of tea.
Full of anti-inflammatory, antibiotic, and anti-cancer properties, Streptomyces are used extensively in the pharmaceutical industry, but in industrialised concentrations, the smell of geosmin can be overwhelming and unpleasant. The ability now to isolate the gene responsible for geosmin offers the possibility of making the environs of a pharmaceutical factory less pungent.
For camels, though, geosmin is a lifeline. Their sensitive nostrils are able to detect the chemical in the breeze from miles away, allowing them to detect invaluable sources of water. In return for a refreshing drink, they carry away some of the bacteria’s spores. Our sensitivity to the chemicals generated as humidity increases ahead of a rain shower has led anthropologists to conclude that this was an important evolutionary trait, enabling our ancestors to anticipate the arrival of water which would freshen vegetation and attract prey to water sources. In enjoying that fresh smell after a rain shower, we may just be at one with our primal instincts.
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