If you haven't had wines from the Ahr be sure to check out the great insights from Dustin Chabert, guildsomm user, Blake Leja, Alex Ring, Mark Guillaudeu and inderpal singh last week.
This week: TDN in Riesling
What does the acronym stand for? How does it manifest in the glass? What causes it? Are there particular regions that display more than others?
Trimethyldihydronapthalene. Smells like diesel fuel. It happens in the bottle over time as beta-carotene (that stuff found in colorful red and orange vegetables that makes them good for our eyes or something) degrades.
It's is a marker for Reisling, but occurs in most grapes near the sensory threshold (why does this 1990 Corton smell like petrol?).
Causes of higher levels of TDN are mostly due to sun exposure on the grapes themselves. Shaded grapes will develop less. Germans seem to have become hip to this, but the new world regions of Australia and Washington haven't so much. Part of the problem lies in spring rains bringing disease pressure. If the farmer has to decide between pulling leaves to help things dry out and prevent mildew vs an increased risk of TDN, which direction do you think they're going to choose?
Another reason why we see it more in Aussie wines is their commitment to screwcaps. Corks absorb TDN. Screwcaps can preserve it as more is created.
Why is it so much more prevelant in Aussie versions than Washington, though? Probably because the pH is much lower in those Aussie wines. Low pH will help with the degradation (as do higher storage temps- ever seen a 3 year old bottle of Pewsey Vale sitting on a grocery store shelf...which then gets bought and brought to tasting group?).
Since TDN is caused by degradation of the beta-carotene, it will increase over time. High Quality Germans Rieslings in particular seem to age at a snail's pace that make it difficult sometimes to put them in the correct age bracket. Intensity of TDN can be a clue (but the cork could be taking it in and saving you from it in the glass, so be careful in using that as a crutch).
Tastes/smells like: Gasoline, Petrol, Kerosene, Lamp Oil, Plastic.
Common causes: Excess sun exposure, High acid levels, Water stress.
More common when yields are low.
Seems as though the regions that make wines showing high amounts of TDN deal with at least one if not many of the causes listed above.
For example: Germany, Alsace, Hunter Valley, and Clare Valley. The most interesting time I have come across it was in High quality Chardonnay from Argentina. Which was surprising at first, yet after considering the conditions in which the fruit was grown, began to make a lot of sense.
Different sources disagree on its ultimate cause, but in riesling at least it seems that the aromatic precursors to it require a certain amount of sun exposure in the grapes. That said, recent studies out of Adelaide have shown that TDN is present as a byproduct of yeast in virtually all white wine, but only rises above detection threshold in certain varieties and regions (Semillon, Germany, etc.).
Then you also have the rebels like Marc Kreydenweiss who insist that TDN is in fact a bacterial flaw that consumers have been educated to accept (like the skunking of Stella) rather than winemakers trained to eradicate. In spite of it being considered by many the signature aroma of the riesling grape there still seems to be surprisingly little funding and ongoing research toward a fuller understanding of its ultimate causes.
AWRI droppin some serious science on dat gosolene smell here https://www.awri.com.au/wp-content/uploads/Sept-Oct-2012-AWRI-Report.pdf #smellslikevictory or alternatively.....#smellslikeLimeMarmalade
Actually quite a few Ozzie producers including Jeffrey Grosset employ a quite sprawly version of VSP with lots of vigour and consequent shading of bunches. I feel the screw cap vs flavor scalping effects of natural cork closure could weigh in very significantly as a preserver of those devisive TDN aromas...
Interesting article, but it doesn't really deal with the fact that TDN builds up in the skins. How does pressing and skin contact affect TDN development? I don't have any hard evidence here, but there's a good contingent of winemakers out there who don't want to be too aggressive in pressing to ensure that they don't have too much TDN in their wine.
TDN isn't in the skins of the grapes - eat a riesling grape at maturity and it doesn't taste like petrol - but the *precursors* of TDN do reside in the skins, hence the gentle pressing to avoid a TDN-y wine by avoiding too many of the compounds that yeasts assemble into TDN during fermentation.
That said, what I associate as the more bitter and extracted styles of riesling (like Austrian grosse lagen) while having more phenolic content that I would associate with more rigorous pressing regimes also somehow seem to have less finished TDN character. The mystery deepens!
Correct, but I typically associate that type of extraction with ripeness as well, which you would think would lead to precursors, but not if they're able to get ripeness without overexposure to the sun. It would be interesting to see a study where they test different press regimes over a single parcel of grapes and measure TDN development.