Why do aged Rieslings develop a petrol-like character?

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Dear Dr. Vinny,

Why do aged Rieslings develop a petrol-like character?

—John, South Australia

Dear John,

That note of petrol, kerosene, gasoline, diesel or vinyl is an aroma compound identified as TDN (1,1,6-trimethyl-1,2-dihydronaphthalene). I rather like a little bit of petrol character in my Rieslings—it reminds me of that “new car” smell. My favorite is when it is mixed with other Riesling notes like honey or beeswax; then it manifests more as a paraffin accent or wool-sweater note of lanolin. TDN is pretty distinctive to Riesling, but it’s not unusual to see it in other wines; I sometimes get it in Chardonnay or Sauvignon Blanc. It’s a pretty divisive aroma. It’s like the cilantro of wine aromas—people seem to either love it or hate it.

Here’s what’s a little strange about TDN—it is rarely found in grapes or really young wines, but its precursors are. This gets pretty science-y pretty quickly, but wine grapes (and many other plants) have carotenoids, which are a class of pigments—they give color to bananas, carrots, corn and autumn leaves, and they are what’s believed to cause TDN in wine down the road. While the potential for TDN to be noticed in most wines is pretty low, Riesling grapes have more carotenoids than other grapes, thus the highest chance to exhibit those distinctive aromas in the wine.

The evolution in TDN over time is thought to be the result of acid catalyzed hydrolysis of carotenoid derived precursors. I lost you, right? It’s a bit too complicated to go into here, but basically plants have the capacity to taste and smell like stuff, and sometimes that evolves over time. Certainly fermentation can unlock flavors, as does aging. Wine isn’t alone here; black tea and tobacco also develop aromatic compounds due to carotenoids breaking down over time.

There’s been a lot of research on how to mitigate TDN in Rieslings, much of it by the Wine Institute of Australia. Warmer vintages, riper grapes and exposure to sun can intensify TDN. Oxidation, water stress on vines, yeast activity and acidity levels can also affect the presentation of TDN. Closures also play a role, since it’s believed corks can actually absorb some TDN (or possibly the higher levels of oxidation can be obscuring it). Screwcaps can either lead to higher TDN levels, or preserve them better.

—Dr. Vinny

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