Why does cannabis smell skunky?

If you've ever taken a whiff of buds that have not been burned, and thought you smelled something different, you might wonder why the two smell the same.
Why does cannabis smell like a spray of skunk spray? The stinky culprits in both substances belong to a family of prenylated volatile sulfur compounds (VSCs), a subset of volatile organic compounds (VOCs), according to a study published in the journal ACS Omega.

Iain Oswald, an analytical chemist at Abstrax Tech, told Live Science that they suspected sulfur-based compounds were the origin of the smell. The team suspected that cannabis had the same properties as skunks because they had a bunch of VOCs in their aerosol spray.
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The sulfurous origins of the plant's pungency were expected, but the novelty of some of the compounds that the team discovered were not. The scientific director and an analytical chemist at Medicine Creek Analytics, a cannabis-testing lab in Fife, Washington, was not involved in the study.

"If you want to call it 'fart smell', you should call it sulfur compounds, like that's known to chemists," Wise said. There's sort of unique molecule to cannabis.
Oswald said that the compounds VSC6 and VSC7 have not been found in any plants before.

Cannabis has a complex scent profile, with more than 200 secondary metabolites, which are small molecule plants that help defend themselves against insects or other animals. The authors wrote that terpenes are responsible for up to 50% of the variation in odor between different cannabis strains. The cannabis strain called OG Kush has a signature gasoline-like smell from the terpenes -myrcene and -caryophyllene, while higher concentrations of terpinolene and D-limonene make it smell like wood.

Oswald and his colleagues wanted to home in on the compounds that give all cannabis strains their skunky odor, because the role of terpenes in individual profiles has been well established.

An image of marijuana. Yaman Mutart is credited with the image.

VSCs are difficult to analyze due to their complex structures. The concentrations of VSCs in Cannabis plants are minute compared to those in hops and durian.

Oswald said that the amount in the plant is very low, even though it has a strong effect on the smell. It doesn't take a lot to go a long way because the human nose is very sensitive to these types of compounds.

Oswald and his colleagues used a custom-built two-dimensional gas spectrometry system to understand the structures of the sulfurous compounds. They were able to identify the seven structures of the VSC. The scientists noted that five of the seven contained a functional prenyl group, which is found in garlic.

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A panel of four scientists were trained to rank the pungency of the VSCs and a collection of flowers on a scale of zero to 10. The panel noted that VSC3 is the primary compound in "skunked" beer that has been ruined by exposure.

Oswald said that VSC3 smells like cannabis. The terpenes are somewhat reminiscent.

The panel noted that the smell of the Bacio Gelato cannabis strain was not as strong as it could have been because of the exclusion of VSC3 from the mix. VSC3 doesn't do all of the heavy lifting on its own.

Oswald said there's a synergistic effect. The flavor panel said, "This is cannabis this; this is definitely it."

The paper highlighted the contribution of VSC3 to the cannabis' funky odor, but the compounds VSC4 and VSC5 can also play an important aromatic role.

Oswald said that VSC3 was the most important and correlated with the cultivars. If we were to ever discover a cannabis cultivator that didn't have VSC3 at all but had VSC4 and VSC5 it would smell like cannabis.

Live Science published the original article.