Exactly what it is which forms the final flavour profile of a wine is complex, multi-faceted and remains in the most part unknown. However, there are a number of varietal wines which have come to be known by a particular dominating aroma or flavour. Petrol in Riesling is one of them. I don’t know why, but I just can’t get enough of it, I’m a petrolhead. But what exactly is it? A fault? A varietal characteristic? Whatever it is, it‘s aroma that divides wine lovers and mystifies the casual wine drinker. In this post I will explore its origins and discuss in more detail viticultural and climatic factors affecting its presence and concentration.
The aroma, in particular its origins, is a polarising topic. Spending time in the Mosel this year (arguably the spiritual home of Riesling) with WSET Diploma students was a unique opportunity to gain insight. A great deal of time was spent talking about Riesling (sounds fun, huh?) the petrol aroma was of course the subject of much of the discussion amongst both producers and trip attendees. It was clear to me that there was a general deficiency, and disagreement, in understanding across the board. So, what do we know?
1,1,6,-trimethyl-1,2-dihydronapthalene, or TDN for short, is the member of the C13-norisoprenoids family known to be responsible for the kerosene/petrol aroma in Riesling. These norisoprenoids (a class of aromatic compounds responsible for many characteristics in wine) originate in large carotenoid compounds found in grapes. However, pre-fermentation these compounds are bound to sugars rendering them aromatically inactive. Post-fermentation, formation of aromatically expressive TDN (free not bound) comes primarily as a result of acid hydrolysis (using wine as an acid medium) of these carotenoids, eventually developing in to aromatic norisoprenoids. There are a range of norisoprenoids, some of which accumulate primarily after fermentation, TDN reaches its highest concentration after extended periods of ageing.
Whilst the petrol aroma is almost exclusively noted as being present in Riesling, TDN can actually be found in a number of non-Riesling wines, particularly white varieties like Chardonnay, there is a variety of reasons why the compound is present but not noted. The sensory threshold (the threshold at which we sense a compound) of TDN is around 2ug/L, in aged Riesling wines it has been known to reach as high as 50ug/L, whilst in Chardonnay (and several other white varieties) it is present at levels just below the sensory threshold. Additionally, the expression of TDN depends upon on the precursor carotenoid, in particular β-carotene and lutein. Unsurprisingly it is known that Riesling not only has a much higher carotenoid content than many other grape varieties but also has a lower ratio of lutein to carotene, it is broadly acknowledged (including by the AWRI) that this contributes to increased expression of TDN in Riesling.
There are climatic and viticultural factors which have been shown to affect the concentration of TDN. Exposure to sun is the most investigated factor known to affect concentration of TDN in a finished wine. Research has found a spike in the levels of one carotenoid in particular (zeaxanthin) mid-season. This spike correlated well with high TDN levels in the finished wine. Scientists were already aware that zeaxanthin forms in direct response to sun exposure; at lower temperatures its formation is strongly suppressed. This increase in concentration relating to sun exposure likely occurs due to the fact that carotenoids assist the grape tissue in protecting itself from ultraviolet light
In a further study researchers measured TDN concentrations in Riesling at increasing levels of sun exposure, they found that anything higher than 20% of full sun exposure on the grape cluster from veraison onwards increased TDN concentration. As climate change knocks ever-louder on the doors of many wine-growing regions the aforementioned knowledge of when in particular TDN precursors increase could prove crucial in shaping the pruning decisions of vintners wanting to control TDN expression. Furthermore, it has been noted that lower yields at harvest result in higher concentration of TDN in a finished wine, whilst no research is available on this topic it would be rational to deduce (and aligned with additional understanding of the impact of lower yield) that a smaller harvest would increase overall concentration, anecdotally it has been noted that the petrol aroma is much more prominent in top wines opposed to high-yield cheaper alternatives.
Studies have also shown TDN concentrations to be related to storage temperature. In one study, wine samples stored at 30°C showed considerably higher concentrations of TDN than in samples stored at 15°C, this increase is likely related to the known effect increased temperature has upon reactions. Lower pH in the wine (more acidic environment) has also been shown to increase TDN concentration, this could be explained by the lower pH providing a more acidic environment, better supporting acid hydrolysis over time. Closure type has also been reported to show connection to TDN concentration, cork and synthetic closures are found to ‘lose’ more TDN than screw closures.
A relationship between water stress and nitrogen deficiency given their relationship to canopy development. Partial drying of the root zone has been found to have an indirect affect on the production of TDN, it reduces canopy size and therefore increases sunlight penetration. Nitrogen deficiency in soil has also been examined as a possible cause for high TDN levels. Some researchers have hypothesised that fertilisation has an effect by encouraging more leaf cover and berry shading thus reducing TDN concentration as a result of reduced sun exposure. Given the research examined thus far it is reasonable to suggest that vintners could employ practises in both the vineyard and the winery which could to some extent seek to manage TDN levels. Both yeast strain and clonal selection have been shown to marginally impact TDN concentration.
In 2011, whilst presenting his new range of Alsace wines, M. Chapoutier declared that the petrol aroma in Riesling was the result of a ‘mistake’ during winemaking, he further claimed that decomposition of the veins within the grape (particularly during crushing) were to blame for the fault. Whilst it is true that the carotenoid precursor related to the formation of TDN is almost entirely located in the skin of a grape it does not seem plausible, given the wide range of variables shown to be related to TDN, that the aroma would be either a fault or that crushing would be entirely responsible. Given the extent of available research and relationships already shown to exist (sun exposure etc.) it seems unlikely that crushing (considering many producers likely choose not to crush at all) would be a primary factor in the formation or concentration of TDN, although it would be interesting to see a study attempted. Despite there being no specific research on the topic of crushing/pressing pressure and concentration of TDN it would be logical to suggest that vinification methods could lead to an altered expression of TDN.
Whilst a great deal of work has already been done exploring the factors affecting concentrations of TDN, further work is required to understand how vintners can effectively manage TDN in both the winery and the vineyard.