Selbach-Oster Wehlener Sonnenuhr Spatlese 2009; the wine that turned me into a petrol junkie. Is it a fault? A varietal characteristic? Whatever it may be it is an aroma that divides wine lovers, mystifies the casual wine drinker and raises many an eyebrow when actively pursued by oenophiles. It is, of course, the aroma of petrol in Riesling.
I have had experience with very few aromas relating to wine which have proven to be as polarising as Petrol in Riesling. This polarisation was made crystal clear at a recent tasting that I attended. At this tasting the petrol debate raged fiercely on, without resolution or agreement. After the dust had settled, the silence was broken with the sound of a question: ‘So what exactly is that petrol aroma?’
When I first encountered it, I found myself asking this very same question, my inquisitive nature led me down a Riesling-fuelled rabbit hole. In this post I would like to share the findings of my research with you in as approachable a format as possible (I tried, honest)
1,1,6,-trimethyl-1,2-dihydronapthalene, or TDN for short, is the member of the C13-norisoprenoids family 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-winemaking process these compounds are bound to sugars rendering them aromatically inactive. Post-winemaking process, formation of aromatically expressive TDN (free not bound) comes primarily as a result of acid hydrolysis (using wine as an acid medium) of terpenes, specifically of the carotenoid precursors.
Interestingly the compound TDN is actually found in many wines, particularly white varieties like Chardonnay. So why you may ask is it only Riesling that seems to either express, or be associated with, this petrol aroma.
As noted above, the expression of TDN depends primarily on the precursor carotenoid, rather unsurprisingly Riesling not only has a much higher carotenoid content than many other grape varieties but also has a lower ratio of lutein to carotene compared to other varieties which would appear to be related to an increased expression of TDN. 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 showing that this expression is far more than just association and has firm rooting in science.
Before discussing the viticulture and climatic factors which may affect the concentration, and noticeability, of TDN it is also worth noting that the inherently acidic nature of the Riesling grape may also go someway toward assisting acid hydrolysis over time, this would seem to explain why expression of TDN tends to be more pronounced in aged Riesling. Additionally the Riesling grape is found to have a relatively low number of overall free terpenes which could mean that TDN is less masked by other terpenes and therefore more prominent.
There are indeed climatic and viticultural factors which appear to affect the concentration of TDN. It seems to be the case that exposure to sun is the key factor relating to concentration of TDN, this relationship is thought not to be caused particularly by an overall increase in the precursor carotenoid but instead an increase in relative carotenoid (zeaxanthin) In one 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. Studies have also shown that the relationship between TDN and sun exposure depends on the point at which the grapes receive sun exposure, therefore strategic shading and pruning may enable winemakers to reduce TDN concentrations in finished wines.
It is also now generally accepted that TDN concentrations are related to storage temperature. In one study, wine samples stored at 30°C showed considerably higher increases in TDN than was found in those stored at 15°C. It is probably worth nothing here that closure type is also reported to be closely linked to TDN concentration. Cork and synthetic closures are found to ‘lose’ more TDN than screw closures.
Finally, given that it was noted that shading and sun exposure were related to TDN concentration, it seems rational to note 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.
In 2011, whilst presenting his new range of Alsace wines, M. Chapoutier (legendary Rhone winemaker) 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 concentration of free-TDN is almost entirely located in the skin of a grape it does not seem plausible to me that the aroma is either a fault or that crushing would be entirely responsible for the presence of TDN. It seems unlikely that given the premium nature, vast experience, geographical location and broad range of winemakers working with Riesling that the majority of these producers would all be subjecting grapes to similar crushing pressure thus all falling victim to this ‘mistake’
Despite there being no research on the topic of crushing it does seem logical to suggest that vinification methods could lead to an altered expression of TDN. However I do believe these vinification methods are more likely to simply increase or decrease expression opposed to eradicate it altogether which is what you may expect if this were indeed caused by a mistake in winery practise.
The jury is certainly still out on exactly what affects the expression of TDN in Riesling but personally I think it is incredibly exciting that wine science is expanding in both quantity and quality so much to allow oenophiles who may be that way included to more comprehensively understand various elements of wine.
So are you a petrol junkie or a kerosene combatant? I would love to hear your views on the topic.