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.
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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.
Those compounds are called carotenoids, and play a key role in most plants. “The carotenoids act like little antennas to help broaden the range of light that the chlorophyll can access,” Dr. Sacks says. “They actually help the chlorophyll do what it’s supposed to do, which is capture sunlight.” Carotenoids also protect the chlorophyll from excessive sunlight. They are not unique to grapes, and late in the growing cycle, when chlorophyll breaks down, the carotenoids might show, for example, through the beautiful red and orange leaves of a New England autumn.
The carotenoids also degrade, and in Riesling two types can form a set of non-volatile TDN precursors called C13-norisoprenoids. These precursors eventually form TDN. “It’s a long pathway of chemical changes,” says Josh Hixson, Ph.D, a senior research scientist at the Australian Wine Research Institute, “which is why it typically shows after a number of years.” Acid plays a key role in these changes, so higher acid Rieslings tend to develop TDN more quickly.
Managing TDN Levels in Riesling
Much of the effort in managing TDN levels starts in the vineyard, where those initial carotenoids form; more sunlight means higher carotenoid production, so managing sun exposure is a major factor. This explains why historically TDN has been much more common in young Australian Rieslings than young German ones. Dr. Hixson says the more intense Australian sun, longer hours of sun each day, and wider rows with little or no inter-row shading create more opportunities for carotenoid formation and breakdown throughout the growing season compared to the Rheingau.
But even the Rheingau is becoming increasingly concerned about TDN formation. “One of the things, when we’re working with Germany, was whether the impact of a higher-temperature world was going to push higher levels of TDN in younger wines,” Hixson says. While Australian vineyards prefer to shade the grapes, defoliation is common in the wetter German vineyards to limit disease pressure—that, however, exposes the grapes to more sunlight.
Schulz says that the traditional timing for defoliation, about 30 days after flowering, actually leads to the highest amounts of TDN formation. Research suggests that defoliating right after flowering actually limits formation; the exact mechanism is unknown, but one hypothesis is that the grapes are able to adapt to the sunlight exposure early in their development rather than experiencing it as a shock later.
Water stress and, to a lesser degree, nitrogen deficiencies in the soil also contribute to TDN-precursor development, at least in part because they both lead to less canopy growth and greater sunlight penetration. Variation amongst different Riesling clones also seems to be minimal.
Once the grapes arrive at the winery, there is little that can be done. “TDN is something that happens regardless of fermentation,” says Sacks. “Some yeast strains can convert the TDN precursors into something else, so yeast choice can increase or mitigate TDN, but it’s not a huge difference.” C13-norisoprenoids also break down into TDN more quickly when a wine is stored at higher storage temperatures.
TDN formation continues even in bottled wine, and closures can make a huge difference in TDN expression; Hixson says it ranked as the number one factor in their research. “TDN is hydrophobic, so it doesn’t want to be in water,” he says. “So in a bottled wine it disappears into the headspace and gets absorbed by the cork.”
A screw-cap closure, on the other hand, doesn’t absorb the TDN, so it remains in the wine. One of Hixson’s studies found a 15-year-old Riesling under screw cap with over 200 micrograms of TDN, compared to 50 micrograms for cork-sealed wines of the same age. “Clare Valley Riesling, that’s sort of where the screw cap became popular here in Australia,” says Hixson, “so for a long time, we’ve been used to wines that have higher levels of TDN because of that choice of closure.”
Hixson’s point touches on the most debated points about TDN: is it good? Famous names like Rhône winemaker Michel Chapoutier have described it as the result of winemaking errors. While personal tastes will vary widely, it is generally considered to be less acceptable in young Riesling but often expected and even prized in older bottles. Hixson contrasts TDN development with the slow evolution of the monoterpenes that give young Rieslings their fruity, floral character.
“Once you sort of cut through all the noise, it’s essentially this: young Riesling character is very monoterpene-driven, and those slowly die off, and then you slowly get things like TDN ramping up,” he says. “It’s almost a straight transition from monoterpenes at one end to TDN-like compounds at the other end.”
Dispatch
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Jim Clarke writes about wine, beer, and spirits for trade and consumer publications, including Beverage Media, Fortune, and World of Fine Wine. He is a sommelier and the U.S. marketing manager for Wines of South Africa.
