TDN, the Petroleum Note Explained

The aroma of petroleum in wine is one of the most curious and debated nuances in the world of enology, particularly in the case of Riesling. This aroma, which some describe as kerosene, is linked to a chemical compound known as TDN, whose formation depends on various factors in both the vineyard and the wine's aging process. With climate change and evolving viticultural practices, this compound is becoming increasingly relevant.

TDN, short for 1,1,6-Trimethyl-1,2-dihydronaphthalene, has a relatively low sensory detection threshold, between two and four micrograms per liter, according to Dr. Hans Schulz, president of Geisenheim University in the Rheingau region. However, at higher levels—above 50 micrograms—it can become so dominant that it is unpleasant for some consumers. Research at Cornell University has found that young Rieslings typically contain between 2.6 and 10.2 micrograms of TDN per liter, while other grape varieties generally present levels below 2.1 micrograms. As Riesling ages, its TDN concentration tends to increase, sometimes exceeding 50 micrograms after 15 years of storage.

The precursors to TDN originate from carotenoids, compounds present in many plants that assist in light absorption for photosynthesis and protect chlorophyll from excess solar radiation. In Riesling, carotenoids degrade during grape ripening, forming non-volatile precursors called C13-norisoprenoids, which eventually transform into TDN. According to Dr. Josh Hixson of the Australian Wine Research Institute, this chemical process is lengthy and influenced by factors such as wine acidity, which accelerates TDN formation.

Managing TDN levels begins in the vineyard. Greater sun exposure increases carotenoid production, which explains why young Australian Rieslings often exhibit higher TDN levels than their German counterparts. In regions like the Rheingau, traditional defoliation occurs about 30 days after flowering to reduce disease risks, but this timing is linked to higher TDN formation. Recent studies suggest that early defoliation, immediately after flowering, could limit TDN formation, likely because grapes better adapt to sunlight exposure at this stage.

Other factors, including water stress and nitrogen deficiency in the soil, also contribute to TDN precursor development by reducing foliage growth and allowing more sunlight penetration. However, once the grapes reach the winery, options for controlling TDN are limited. While some yeasts can transform precursors into other compounds, the effect is minimal. Additionally, TDN continues to develop in the bottle, and storage conditions, particularly temperature, significantly influence its concentration.

The type of bottle closure also plays an important role in TDN expression. According to Hixson, cork-sealed wines tend to absorb TDN into the bottle's airspace, reducing its perception. In contrast, screw caps, commonly used for Australian Rieslings, do not absorb TDN, allowing it to remain in the wine. A study revealed that a 15-year-old Riesling under screw cap contained over 200 micrograms of TDN, compared to 50 micrograms in a cork-sealed bottle of the same wine and age.

Consumer perception of the petroleum aroma varies widely. While some consider it a flaw, particularly in young wines, others appreciate it in aged Rieslings, where it adds to their aromatic complexity. According to Hixson, TDN represents a transition in Riesling's character, shifting from the fruity and floral aromas driven by monoterpenes in youth to compounds like TDN that dominate with age. For many, this evolution is one of the unique aspects that makes Riesling such a fascinating grape.