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The Chemistry Behind Weird Wine Labels

Why does some wine smell like cat piss?

Key points

  • The molecular inventory of a wine can be examined with a gas chromatograph.
  • Cat piss aroma in Sauvignon Blancs is said to come from the molecule p-mentha-8-thiol-3-one.
  • Petrol notes in aged Riesling come from the molecule 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN).

Wine reviews often resemble a psychoanalytical character study:

"Its dense purple color is followed by an extraordinary scent of charcoal, roasted espresso, blackberries, blueberries and a touch of wood," wrote the critic Robert Parker of the Château Angélus in 2005.

Other wine snobs are even more direct: a leather aroma, it is said about expensive Bordeaux. Smells like petrol — that's what they say about old Riesling. Smells like cat piss is considered a distinctive characteristic in Sauvignon Blancs of extremely high quality.

The thought of cat piss has not left me for a long time. Why does a wine that costs 50 euros smell like cat urine? This question led me to my current research interest. How can odors be explained? And what is behind such bizarre smell descriptions?

A Smell Experience

Try a good wine with your nose pinched tight. The taste is about as exciting as the English cuisine of the 1970s. Complex aromas such as vanilla, tobacco, or blueberry come from odor molecules that migrate from the mouth through the throat into the nasal cavity — supported by air that comes out of the lungs when you exhale. What feels like a taste in the mouth is actually a smell experience.

Are we similarly misled by our feelings when we think we discover the aroma of cat piss or gasoline in wine? My chemist friend Terry Acree at Cornell University in the United States specializes in wine aroma research. (Unfortunately, I wasn't introduced to this career opportunity at school at the time.) Terry had found out a few years ago why Riesling smells like gasoline. He had chased Riesling through a gas chromatograph and found a relatively tiny inventory of an aroma molecule: 1,1,6-trimethyl-1,2-dihydronaphthalene (chemists are only half as poetic as wine reviewers), or simply TDN.

The Gas Chromatograph

The principle of the gas chromatograph is simple but ingenious. Complex organic substances are separated as vapor in order to find out which components are present in them (and in what quantities). The vapor substance is poured into a gas stream, which then wanders along a tube. The gases are sensibly odorless — for example, helium or nitrogen are used. Volatile molecules are not equally heavy, and so some components arrive at the end of the tube faster than others. There you measure what, when, and how much of it arrives. The result is a precise molecular fingerprint of complex mixtures. And you can smell the individual components in sequence. Using this method, Terry discovered TDN in Riesling in surprisingly small but potent amounts.

So, it is not gasoline in the wine, but a molecule with the aroma, which is associated with the smell of gasoline. Further studies by olfactory chemists show that the situation is similar with cases such as cat piss, which is said to come from the p-mentha-8-thiol-3-one molecule.

So, if you think soon that you smell a note of cat piss in the wine, it is likely that it is a good Sauvignon Blanc. And, unlike wine critics, you don't have to spit the wine out either.

References

Sacks GL, Gates MJ, Ferry FX, Lavin EH, Kurtz AJ, Acree TE. Sensory threshold of 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) and concentrations in young Riesling and non-Riesling wines. Journal of Agricultural and Food Chemistry. 60: 2998–3004. PMID 22397689 DOI: 10.1021/Jf205203B

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