For newcomers to wine, it’s one of the most inexplicable tasting notes: kerosene, or petrol. Even seasoned wine drinkers and winemakers disagree about its desirability. This aroma compound is called TDN (or, more completely, 1,1,6-Trimethyl-1,2-dihydronaphthalene), and climate change has winemakers and viticulturists thinking about it more than ever.
Not all winemakers have this aroma compound on their minds; TDN is peculiarly unique to Riesling. A few other grapes generate miniscule amounts, but for better or for worse, it’s part of Riesling’s identity.
How, exactly, does TDN form in wine, and why do some wines—aged Riesling, for instance—have higher levels of petrol aromas than others? And could rising temperatures around the world lead petrol aromas to become more common than they are now? SevenFifty Daily explores the science behind this intriguing aroma compound.
Picking up on petrol
“In general, we say that the detection threshold is something like two to four micrograms per liter,” says Hans Schulz, Ph.D, the president of Geisenheim University in the Rheingau. “Ten or 15 years ago it was thought to be about 10 times as much. The threshold to actually recognize TDN is about 10 to 12 micrograms, and the rejection threshold—when it becomes so strong that people are repelled by it—is somewhere above 50 or 60.”
Research at Cornell University found young Riesling samples in a range from 2.6 to 10.2 micrograms, whereas all the other varieties they examined showed only 0.3 to 2.1 micrograms. TDN, when present, can also increase in concentration as the wine ages, so older Riesling examples can reach much higher levels—one study measured levels of 50 micrograms or more at 15 years of aging. Generally, kerosene or petrol tasting descriptors are much more common and more accepted in older wines.
Earlier research most likely thought that the detection threshold for TDN was much higher because the compound and its precursors are very difficult to study. “It’s hard to work with,” says Gavin Sacks, Ph.D, a professor of food science in the College of Agriculture and Life Sciences at Cornell University. “It’s very hydrophobic, very oily, so it’s easy to lose the compound if it comes in contact with plastic anywhere in the lab. We have to be very careful to use only glass and Teflon.” Dr. Schulz says the compounds in the berry that are at the root of TDN production are both light sensitive and very oxidative, so grapes need to be handled in the dark and in a chamber flooded with carbon dioxide to get accurate data.
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