Olfactive Chemistry: Geosmin- After the Storm

As we come to the end of the first half of 2016 there has been an interesting trend from some of my favorite indie perfumers. There has been more usage of the aromachemical geosmin to different effect. Geosmin is one of the more interesting ingredients on the perfumer’s palette.

Everybody is familiar with the smell of geosmin in Nature. It is that smell in the air which hangs after a heavy rain. It comes about because there is a natural bacteria, Streptomyces, which leaves geosmin behind when it dies. The longer the dry spell the more the chemical is on the surfaces. If a thunderstorm comes along it releases the geosmin into the air. This is that smell also called petrichor. It is earthy and mineralic in turns. The actual chemical structure is below.

decalin geosmin

Geosmin is two six membered carbon rings fused together into a structure called a decalin. Then two methyl (CH3) groups and one alcohol (OH) are what it takes to transform the slightly mentholated odor of decalin into the after the rain smell of geosmin.

The isolation of geosmin is a fascinating study of the ancient and the modern. The ancient way comes from India. Dried out disks of earth which the monsoons have covered and now evaporated are produced. These disks are them placed in primitive distillation apparatus to form what is called mitti attar. This is the earliest isolation of geosmin. There is a great story in The Atlantic from April of 2015 which describes the entire process in detail.

The other way is by mimicking the natural bacteria to make it via biosynthesis. Professor David Cane and his group at Brown University discovered an enzyme from the natural bacteria Streptomyces coelicor. (Journal of the American Chemical Society, vol. 28, pg 8128-8129, 2006) This is the enzyme which transforms the non-cyclic farnesyl diphosphate into geosmin. The study of the transformation of farnesyl diphosphate into natural chemicals has led to the ability to imitate these processes to produce natural products for medicinal as well as olfactory purposes. In the scheme below you can see the process that the enzyme geosmin synthase uses to convert the acyclic to the cyclic. Now geosmin is readily available as a perfume ingredient.

farnesyl to geosmin

The odor profile of geosmin allows it to be used in marine styles of fragrance as perfumer Christi Meshell does in her House of Matriarch Albatross. In that perfume she uses it as the smell of the rocky coast of the Pacific Northwest. Shelley Waddington also is inspired by the same locale and her use of geosmin carries the damp forest milieu in En Voyage Rainmaker. Perhaps my favorite use so far this year comes from Zoologist Bat where perfumer Ellen Covey working under Victor Wong’s creative direction uses geosmin as a key component of the wet cave accord which grounds that fragrance.

If your fragrance carries the smell of after the storm geosmin is probably the reason.

Mark Behnke