If you haven't had wines from the Ahr be sure to check out the great insights from Dustin Chabert, Jeremy Eubanks, 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).
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...