A Matter Of Taste: Petrol In Your Riesling

  • By Jamie Drummond
  • 09 Mar 2021
  • 5 MIN
  • Level 301
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Green, unripe grapes on the vine. - Credit : Deutsches Weininstitut

Although one can expect to find all manner of primary fruit aromatics in Riesling wines (nectarine, pear, apple, lime, Meyer lemon, pineapple, peach et al.), often one will hear tasters speaking of finding an aromatic of petrol, gasoline, or kerosene in their glass. So how on earth does this come to be? 

Like most things olfactory, a petrol/gasoline/kerosene aromatic in wine is something that we all have different individual thresholds for: one for detection, another for recognition, and yet another for rejection.  

TDN (AKA 1, 1, 6, -trimethyl-1,2-dihydronapthalene) is a member of the C13-norisoprenoids family, a minor group of chemical components found in wines produced from many aromatic grape varieties, but none more so that the noble Riesling, and especially some Rieslings with a little bit of age. 

A Rather Polarizing Topic 

At low levels many find TDN to be a pleasant and desirable characteristic in a wine, but at higher concentrations TDN can dominate the wine, and be viewed as an overpowering fault, exerting negative sensory attributes to the wine, often incurring a certain level of consumer rejection. So, in a way it can be viewed as the cilantro of the wine world; people either love it or hate it. 

Winemaker M. Chapoutier once infamously told Decanter magazine that he felt the presence of this compound in Rieslings to be a winemaking transgression and was the “result of decomposition of the veins within the grape… ...and these veins become more fragile as the grape matures.” 

So How Does It Come About? 

It is currently understood that TDN is formed from precursors in the grapes brought about by riper grapes (through low yields and late harvests), higher levels of sunlight exposure, water stress, and higher acid levels, pointing to an argument that it is not actually a fault, but a natural byproduct of the very same conditions that make for an excellent bottle of Riesling. 

Riesling wines typically have both free TDN as well as lots of the bound precursors, and through both fermentation and ageing, these bound reserves are freed. However, some of the free TDN is lost also, meaning that levels are in what amounts to a state of continuous change, until after a few years they reach a stable plateau, before falling off when the bound reserves are finally exhausted.  

In a study published in 2012, Gavin Sacks and his team at Cornell University looked at the thresholds of tasters to TDN and discovered that TDN has a sensory threshold of 2ug/l. In aged Riesling wines one can find quantities as high 50ug/l, but this compound is also found in Chardonnay, Sauvignon Blanc, Pinot Noir, and Cabernet Sauvignon at levels close to its threshold. An exception to this would be Cabernet Franc (6.4ug/l). Further studies by Andrii Tarasov in 2020 went on to show that thresholds for detection, recognition, and rejection were 4, 10 - 12, and 71 - 82 μg/l respectively. 

To put things into a global perspective, the typical TDN level for a German Riesling would be anywhere between 1 and 50 μg/l, whereas as in a young Australian Riesling one could find up levels in excess of 250 μg/l. 

And Now It’s Time For The Nerdy Science Bit 

TDN is currently thought to be a degradation product from β-carotene and lutein. TDN formation is initialised through the photochemical and/or enzymatic degradation of C-40 carotenoid compounds. The aforementioned shifts in TDN levels over time is potentially due to the acid catalyzed hydrolysis of carotenoid derived precursors, such as zeaxanthin (one of the most common carotenoid alcohols found in nature), Riesling acetal (AKA 2,6,10,10-tetramethyl-1-oxaspiro [4.5] dec-6-ene-2,8-diol), and glycosylated precursors. 

At harvest, TDN levels tend to be close to zero, but TDN formation through the hydrolysis and rearrangement of TDN precursors over time can be encouraged by sunlight exposure through leaf plucking, particularly just after veraison. It has been found that defoliation at other times (to increase berry ripeness) has had little impact upon the levels of TDN present in the finished wine. 

TDN formation in the wine has been found to be significantly accelerated through longer storage time, warmer storage temperature, and lower wine ph. It is thought that the lower ph brings about a more acidic environment for the hydrolysis of the glycosylated precursors, and any warmth simply speeds up this process. 

TDN Management Is The New Black 

Closures may also have a small role to play in a tasters perception of TDN. It has been found that natural and synthetic corks either absorb the compound or allow for oxidation that can mask the TDN, whereas screwcap closures tend to preserve levels of TDN. 

With the impact of climate change, the concept of “TDN management” has become a reality for Riesling producers in warmer climates. Saying that, even in Germany, where they have witnessed a spate of warm, dry vintages, for many winemakers excessive TDN formation has become a genuine cause for concern.  

Green, unripe grapes on the vine. - Credit : Deutsches Weininstitut