Some tasting notes leave outsiders baffled. “Flint” is manageable; “wet dog” is already a stretch. But anyone told for the first time that an old Riesling smells wonderfully of petrol could fairly wonder whether someone has been standing at the wrong pump. Yet the petrol note is not a figment of over-imaginative wine lovers. It has a chemical name, an origin and, by now, even a receptor of its own in the human nose.
A molecule with a long name
The culprit is 1,1,6-trimethyl-1,2-dihydronaphthalene, or TDN for short. The compound belongs to the C13-norisoprenoids, which are breakdown products of carotenoids – the yellow and orange pigments also found in carrots and autumn leaves. In the grape, the precursors of TDN are mostly bound and odourless, usually attached to sugars. Only in the acidic environment of wine are they slowly released. That is why a freshly bottled Riesling generally shows little or no petrol, while the same wine ten years on may show it clearly.
Riesling has a marked propensity for this aroma. A study by Gavin Sacks and colleagues, published in 2012 in the Journal of Agricultural and Food Chemistry, found an average of around 6.4 micrograms of TDN per litre in Rieslings one to three years old, against only about 1.3 in wines from other varieties. The note is not entirely exclusive to Riesling, however; traces of it turn up elsewhere too.
Sun, stress and the bottle
What leads to more TDN precursors in the grape has been well researched. The most important factor is light. In 2012 the Australian Wine Research Institute (AWRI) summarised the state of research: according to a 2002 study by Gerdes and colleagues, more than 20 per cent of full sunlight on the grapes from the onset of ripening is already enough to raise TDN levels. One early phase appears to be particularly sensitive. In a Cornell University trial published in 2010 by Kwasniewski and colleagues, TDN potential rose markedly when the fruit zone was leaf-plucked around 33 days after berry set; according to the AWRI, the resulting wines contained more than twice as much TDN. Leaf removal earlier or later showed no such effect. Heat and water stress are also considered drivers, as is high ripeness.
The story continues in the cellar. A project by the DLR Rheinpfalz and the Technical University of Braunschweig, funded by the Forschungskreis der Ernährungsindustrie (FEI, the German food industry research association), showed just how strongly storage temperature matters: at 7°C, 1.2 micrograms of TDN per litre formed over eight months; at 25°C, the figure was 19.3. Anyone who leaves their Riesling to mature in the attic over the summer is, in effect, ordering the petrol note along with it.
The role of the closure is a curious one. In an AWRI trial of five closure types, natural corks and synthetic stoppers absorbed more than half of the TDN in a wine within two years, whereas under screwcap it was retained. Part of the petrol character so often attributed to Australian Riesling may therefore also be connected with the country’s fondness for the screwcap.
When can you smell it?
This is worth looking at closely, because the figures have shifted considerably over the years. For a long time the accepted threshold was 20 micrograms per litre, based on a 1978 paper by R. F. Simpson; according to the Geisenheim study, no details of the tasting panel were given. In 2012 Sacks and colleagues determined a detection threshold of about 2 micrograms in model wine and in a neutral white wine – a tenth of that.
In 2020 Hochschule Geisenheim drew finer distinctions. Andrii Tarasov, Rainer Jung and colleagues established, in young Riesling, a detection threshold of around 4 micrograms per litre, a recognition threshold of 10 to 12 and a rejection threshold of 71 to 82 micrograms. In other words, we notice early on that something is different, identify it as petrol later, and only object to it at considerably higher levels. The FEI project arrived at similar orders of magnitude: consumers only perceived TDN from about 14.7 micrograms, a trained panel at around a sixth of that level. Rejection is the interesting part. For one-year-old Rieslings it lay at 60 micrograms, for eight-year-old wines not until 91. More petrol is tolerated in an older wine, apparently because it fits more naturally into the overall picture.
For comparison: according to the Geisenheim study, European Rieslings typically contain 1 to 50 micrograms of TDN per litre, Australian ones up to 250 and more.
Fault or character?
In Germany the petrol note has a poor reputation. The language of research alone makes that plain: the FEI speaks explicitly of the „unerwünschten Petrol-Fehlnote“ (our translation: “undesirable petrol off-note”) and, when the project concluded, ran the headline that German winegrowers could „aufatmen“ (our translation: “breathe a sigh of relief”). The background is easy to understand. German Riesling trades on an image of cool, precise fruit. A wine that already smells of the filling station when young tends to be read here as a sign of too much sun or too open a canopy. Climate change is making the problem worse; as early as 2015 the FEI was anticipating rising TDN levels. Countermeasures range from the right timing of leaf removal and the choice of clone and rootstock to cool storage.
Elsewhere, attitudes are more relaxed. In Australia TDN is simply part of everyday life: in an AWRI survey of 116 Rieslings from a range of vintages, the median was 74 micrograms per litre in the Clare Valley and 88 in the Eden Valley. Even the young Eden Valley wines of the 2010 vintage averaged 23 micrograms – above the recognition threshold. The tasting panel found no link between perceived TDN and quality scores. And among lovers of mature Mosel or Rheingau Riesling, too, a touch of petrol set amid honey and beeswax is a familiar sign of age. Wine Spectator aptly compared the note to coriander: you either love it or hate it.
Research from the Leibniz Institute offers a cautious explanation, in line with the assessment in the Geisenheim study. Low to moderate TDN levels, it suggests, add to the complexity of the bouquet, while higher levels are often rejected by consumers here. In 2024 the Leibniz Institute for Food Systems Biology at the Technical University of Munich identified OR8H1 as the first human olfactory receptor that responds to TDN at relevant concentrations. Of 766 receptor variants tested, only this one reacted.
Not yet fully decoded
For all the research, a residue of mystery remains. At the International Riesling Symposium at Kloster Eberbach in 2022, Hans R. Schultz, then president of Hochschule Geisenheim, admitted: „Die Petrolnote ist immer noch nicht entschlüsselt.“ (our translation: “The petrol note has still not been decoded.”) It is also puzzling, he said, why it only develops in the bottle, even though the conditions for it are created in the vineyard. The AWRI, too, noted that it remains difficult to predict how strongly TDN will develop in a given wine. Why a wine from one site shows strong petrol while its neighbour shows barely any therefore remains an open question.
For wine drinkers, that may be the best news of all. Whether the note comes across as a fault or as character remains a question of dose, age and taste. Those who like it should store their wine warm and buy screwcaps. Those who do not should drink young, store cool and put their faith in cork.