Petrol / Kerosene (TDN) — 1,1,6-Trimethyl-1,2-Dihydronaphthalene

TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) belongs to the C13-norisoprenoid family, a group of 13-carbon compounds derived from the breakdown of C40 carotenoids. In Riesling, carotenoids including beta-carotene, lutein, violaxanthin, and neoxanthin degrade during grape ripening and wine aging, forming non-volatile glycosidically bound precursors that subsequently release TDN through acid-catalyzed hydrolysis in the bottle. Research published in 2022 identified mono- and di-glycosylated TDN precursors and confirmed that lutein is also a likely source of TDN-generating intermediates. The compound was first associated with the bottle-aged bouquet of wine by Simpson in 1978, and decades of subsequent research by groups at Cornell University, Geisenheim University, the Australian Wine Research Institute, and German institutions have built a detailed picture of its formation pathways.

• Chemical family: C13-norisoprenoid, derived from C40 carotenoid precursors (beta-carotene, lutein, violaxanthin, neoxanthin) via acid-catalyzed hydrolysis
• Formation pathway: glycosidically bound carotenoid breakdown products release TDN over time in the bottle; quantity near zero at harvest, increasing with aging
• Related compound: vitispirane, a biosynthetically related C13-norisoprenoid that co-occurs with TDN in aged Riesling
• First description: R.F. Simpson, 1978, identified TDN as an important contributor to the bottle-aged bouquet of wine

Sunlight exposure is the single most important viticultural factor controlling TDN potential. More sunlight drives greater carotenoid production in grape skins, which in turn produces more TDN precursors. This explains why Australian Rieslings from the Clare and Eden Valleys historically show higher TDN levels than cool-climate German Rieslings: the intense Australian sun, longer daily sunlight hours, and wider rows with less inter-row shading all increase carotenoid formation. Research at Cornell University showed that timing of leaf removal matters: removal around 33 days past berry set significantly elevated total TDN compared to controls, while early leaf removal right after flowering appeared to limit formation. Water stress and nitrogen deficiency also contribute to TDN precursor development by reducing canopy growth and allowing more sunlight to reach fruit. With ongoing climate change, TDN management is an increasing concern for Riesling producers worldwide.

• Sunlight is the primary driver: more sun exposure increases carotenoid production and thus TDN precursor levels in the grape
• Timing of leaf removal matters: defoliation around 33 days past berry set produced significantly higher total TDN (195 µg/L vs. 54–87 µg/L in controls, Kwasniewski et al., 2010)
• Water stress and nitrogen deficiency promote TDN precursor development by reducing canopy cover and increasing fruit sun exposure
• Climate change concern: warmer growing seasons and higher sun exposure are expected to elevate TDN levels in European Riesling, potentially shifting a prized complexity marker toward an off-flavor

TDN is a context-dependent aroma compound: desirable and complex at low concentrations, potentially unpleasant and dominant at high ones. A 2020 study by Tarasov et al. (Hochschule Geisenheim University and collaborators) established three thresholds in Riesling wine: a detection threshold of about 4 µg/L, a recognition threshold of 10–12 µg/L, and a rejection threshold of 71–82 µg/L. Earlier research had estimated the detection threshold to be far higher, but methodological improvements in handling this highly hydrophobic compound corrected the figures substantially. The same study found that elevated free SO2 in wine can mask TDN perception, and that serving the wine at a cooler temperature (around 11 degrees C) actually facilitates identification of the kerosene or petrol note. Consumer attitudes toward TDN vary considerably by region: it is often prized in aged Australian and export-market Rieslings but viewed more skeptically by some German producers and consumers who prefer fruitier, lower-TDN styles.

• Detection threshold: approximately 4 µg/L; recognition threshold: 10–12 µg/L; rejection threshold: 71–82 µg/L (Tarasov et al., 2020, in young Riesling wine)
• Elevated free SO2 (around 40 mg/L) can partially mask TDN aroma perception in the glass
• Cooler serving temperature (around 11 degrees C) facilitates TDN identification compared to room temperature
• Cultural divide: TDN is generally prized in aged Australian Riesling and by export markets, but viewed with more ambivalence by some German producers focused on fresh, fruity styles

Scientific understanding of TDN has advanced substantially over several decades through contributions from multiple international research groups. R.F. Simpson first identified the compound as a contributor to bottle-aged wine aroma in 1978. Sacks et al. (2012, Cornell University) established that young Riesling wines average 6.4 µg/L of TDN, far exceeding other varieties at 1.3 µg/L, and revised the sensory detection threshold downward from earlier estimates. Daniel et al. (2009, Australian Journal of Grape and Wine Research) confirmed that Riesling acetal is a precursor to TDN at wine pH. A 2024 study published in the Journal of Agricultural and Food Chemistry identified human odorant receptor OR8H1 as the TDN-selective receptor, explaining at the molecular level why individuals differ in their perception of petrol aromas. Research into closure effects has demonstrated that screw caps retain significantly more TDN than cork closures, because cork absorbs a portion of the compound from the wine.

• Simpson (1978): first identified TDN as an important contributor to bottle-aged wine bouquet
• Sacks et al. (2012, Cornell): established young Riesling TDN averages (6.4 µg/L) vs. other varietals (1.3 µg/L) and revised the detection threshold
• Daniel et al. (2009): confirmed Riesling acetal as a TDN precursor via acid-catalyzed hydrolysis at wine pH
• Haag et al. (2024): identified human odorant receptor OR8H1 as selectively activated by TDN, explaining individual variation in petrol aroma sensitivity

How a wine is sealed and stored has a dramatic effect on the TDN levels a drinker will eventually encounter. Research by Josh Hixson at the Australian Wine Research Institute found that a 15-year-old Riesling sealed under screw cap contained over 200 µg/L of TDN, compared to approximately 50 µg/L in a cork-sealed bottle of the same wine. Cork gradually absorbs TDN from the wine, reducing the perceived intensity, whereas screw caps retain it in full. This partly explains why Australian Clare Valley Riesling, a region that adopted screw caps early and widely, became associated with pronounced petrol character in aged examples. Storage temperature also plays a role: higher ambient temperatures accelerate the acid-catalyzed hydrolysis reactions that release free TDN. For producers seeking to minimize petrol character, managing canopy and sun exposure in the vineyard is the most effective intervention, as options to control TDN after grapes arrive at the winery are limited.

• Screw cap vs. cork: a 15-year-old Riesling under screw cap measured over 200 µg/L TDN vs. around 50 µg/L under cork (Hixson, AWRI)
• Cork absorbs TDN into the bottle's headspace and closure material, reducing its concentration and perception in the wine
• Storage temperature accelerates TDN release from precursors; cooler cellar conditions slow TDN development
• Vineyard management is the most effective tool for TDN control, as options at the winery and in bottle are limited

TDN expression varies markedly across the world's Riesling regions, primarily as a function of sunlight and climate. Australian Rieslings, particularly from Clare Valley and Eden Valley in South Australia, historically show higher TDN concentrations due to intense sunlight, longer sun hours, and the widespread adoption of screw cap closures. Highest TDN concentrations globally have been found in Australian Rieslings, where petrol character is considered a scoring positive by many wine judges and consumers. In Europe, the picture is more nuanced: German Riesling producers, particularly volume-oriented ones, have increasingly sought to minimize TDN in response to domestic consumer preferences for fresh, fruity styles, while export and premium aged German and Alsatian Rieslings continue to be appreciated for their petrol complexity. Rising global temperatures mean that formerly cool-climate European Riesling producers face growing pressure to manage TDN through viticultural practices such as canopy management and irrigation.

• Highest TDN levels found in Australian Rieslings (Clare Valley, Eden Valley), driven by intense sunlight and screw cap closures
• German Wine Institute has omitted 'petrol' from its German-language Wine Aroma Wheel, reflecting domestic preference for less TDN in younger wines
• Alsatian and premium aged German Rieslings continue to be valued internationally for petrol complexity developing over 10-plus years
• Climate change is increasing TDN risk in European Riesling regions, prompting research into canopy and irrigation strategies to limit carotenoid formation