Riesling is like Rosie the Riveter: it’s got some gentle features, a sweet personality, yet a perfume that smells distinctly like gasoline.
This gasoline odor comes from a big molecule called TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) which belongs to the norisoprenoid family of aromas.
Here are some common norisoprenoids and their corresponding aromas in wine:
- TDN: petroleum
- ß-Damascenone: sweet and fruity, also an aroma enhancer (think of it like the MSG of wine aromas. A little bit will enhance other smells in the wine!)
- Vitispirane: Floral, fruity, woody (similar to eucalyptus oil)
Norisoprenoids are formed from the breakdown of carotenoids, the yellow-ish color pigments that develop in berries in the vineyard. When it gets really sunny, the grapes need a way to protect themselves from UV light — just like humans. So they build up carotenoids to act as a natural sunscreen!
As berries reach maturity, these carotenoids break down into norisoprenoids. At this point, the norisoprenoids are bound to sugar molecules. If you’ve followed my other blogs on terpenes and thiols, this will sound familiar. When an aroma molecule is bound to a sugar molecule, it is “tied-up” and odorless. We say these aromas are actually “aroma precursors” because they will one day be cut free from the sugar molecule and give off an odor. TDN has a very low odor threshold, meaning that even a tiny tiny amount of it (~ 2 μg/L) is detectable by humans.
ß-Damascenone generally becomesodorous during fermentation when the yeasts enzymatically cut the link between the sugar molecule and the ß-Damascenone molecule. But TDN and Vitispirane don’t become odorous until the aging process when acid hydrolysis cuts the link between the norisoprenoid and the sugar molecule. This usually happens after extended aging in the bottle, and the effect is higher when storage temperatures are higher. One theory on why TDN is more pronounced in older wines is that in younger wines, other aromas like floral monoterpene aromas may mask TDN’s presence. As time goes on the fresh, floral, or fruity aromas may give way to deeper, less fresh aromas like gasoline.
TDN can also be higher in bottles that have a screwcap vs. bottles that have a cork. This is likely due to “flavor scalping” whereby a packaging material absorbs some of the volatile aromas of its contents.
- The common gasoline odor in Riesling wines comes from TDN, which is in the norisoprenoid family of aromas.
- Norisoprenoids form from the breakdown of carotenoids, which are like a natural sunscreen that builds up in berries to protect them from UV light.
- The norisoprenoid ß-Damascenone (fruity, sweet smell) is released during fermentation by enzyme hydrolysis. TDN (gasoline) and Vitispirane (similar to eucalyptus oil) become odorous during wine aging in the bottle.
- Screwcap closures and warmer bottle storage temperatures can lead to more pronounced TDN aromas.