Here’s the chemistry behind marijuana’s skunky scent

Newly identified sulfur compounds in cannabis flowers give the plant its telltale funky odor

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Scientists have finally sniffed out the molecules behind marijuana’s skunky aroma.

The heady bouquet that wafts off of fresh weed is actually a cocktail of hundreds of fragrant compounds. The most prominent floral, citrusy and piney overtones come from a common class of molecules called terpenes, says analytical chemist Iain Oswald of Abstrax Tech, a private company in Tustin, Calif., that develops terpenes for cannabis products (SN: 4/30/18). But the source of that funky ganja note has been hard to pin down.

Now, an analysis is the first to identify a group of sulfur compounds in cannabis that account for the skunklike scent, researchers report November 12 in ACS Omega.

Oswald and colleagues had a hunch that the culprit may contain sulfur, a stinky element found in hops and skunk spray. So the team started by rating the skunk factor of flowers harvested from more than a dozen varieties of Cannabis sativa on a scale from zero to 10, with 10 being the most pungent. Next, the team created a "chemical fingerprint" of the airborne components that contributed to each cultivar’s unique scent using gas chromatography, mass spectroscopy and a sulfur chemiluminescence detector.

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As suspected, the researchers found small amounts of several fragrant sulfur compounds lurking in the olfactory profiles of the smelliest cultivars. The most dominant was a molecule called prenylthiol, or 3-methyl-2-butene-1-thiol, that gives "skunked beer" its notorious flavor (SN: 11/27/05).

The sulfur compounds have been found in nature, but never before in cannabis, says Amber Wise, an analytical chemist with Medicine Creek Analytics in Fife, Wash., who was not involved in the study.

Oswald was surprised to find that prenylthiol and many of the other sulfurous suspects in cannabis share structural similarities with molecules found in garlic. And like these alliaceous analogs, a little goes a long way.

These compounds "can be in very low concentrations on the flower, but still make a huge impact on the smell," Oswald says. The sulfur molecules are most abundant in cannabis flowers when they reach maturity and during the curing process.

Smell psychologist Avery Gilbert of Headspace Sensory, a startup company in Fort Collins, Colo., that specializes in quantifying the many scents of cannabis, is excited to see the molecules added to marijuana’s chemical repertoire. "The spectrum of cannabis odor is just amazing," he says. "I think it beats the pants off of wine."

The discovery of prenylthiol in marijuana, Gilbert says, is the first step to masking its nuisance odor — or maximizing its perversely pleasant stink.

Prenylthiol has a "polarizing scent," Oswald says. While many people think it reeks, some cannabis users will pay top dollar for skunky grass, which some consider an indicator of quality.

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