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Trying to understand Riesling. Part II

by Tom Stevenson, 09/08

In part II of his investigation into the 'petrol' aromas in Riesling, Tom Stevenson looks at the role of bottle development, and how the petrol character of Riesling is regarded in different styles of Riesling and different cultures.

Riesling's bottle-aromas

In addition to the intensifying and complexing of terpenoid aromas, bottle-ageing induces Riesling's famous petrol (kerosene, gasoline or paraffin) aroma. Anyone who has ever siphoned-off petrol will know that its head-splitting vapour has nothing in common with the classic petrolly aroma of a mature Riesling. It does not literally smell or taste of petrol. Yet for those who know and enjoy the zesty-honeyed richness of a great Riesling, petrolly is one of the most precisely defined, exquisitely evocative words in the wine tasting vocabulary. Some critics might come up with a veritable fruit-salad of descriptors, only some of which may be recognised by equally experienced tasters, but there is not one iota of doubt in the mind of anyone who has ever experienced the classic petrolly aroma of a mature Riesling that it is an instantly recognisable term, and one that is used without pretension. But what causes it, and is it possible to have too much of this good thing?

Riesling's so-called petrol aroma has been identified as trimethyldihydronaphthalene or, to be precise, 1,1,6-trimethyl-1,2-dihydronaphthalene (Simpson, 1978). TDN is a C13 norisoprenoid, which some class as a terpene, but others do not. It is a moot point; a semantic argument upon which chemists continue to differ. TDN is rarely found in grapes or, indeed, young wine, although its precursors are, and they are primarily carotenoids (may chemists state definitely beta-carotene, others probably alpha-carotene, while some think quite possibly both). Beta and alpha carotenes (and lutein a by-product of alpha carotene metabolism) are all antioxidants. These precursors exist in all grape varieties, thus TDN can be found in almost every wine, although the ratio of lutein to carotene is usually low, thus the TDN potential for most wines will be well under the perception threshold level of 20ppb. However, the ratio of beta carotene to lutein is higher in Riesling than for any other grape variety. If ,as most scientists believe, beta carotene is the primary precursor for TDN, this may explain why, after lengthy bottle-maturation, the wines from this variety can accumulate as much as 200ppb TDN, ten-times the perception level. This is why mature Riesling is famed for its so-called petrolly aroma.

TDN can also be produced by the hydrolysis of two megastigma-3,6,9-triols linked to a sugar molecule (Strauss et al, 1986) and it has been theorised that the hydrolysis of a sugar molecule called 2,6,10,10-tetramethyl-1-oxaspiro[4,5]dec-6-ene-2,8-diol can create TDN (Winterhalter, 1991; Silva Ferreira & Guedes de Pinho, 2004)
  

Too much of a Good Thing

The first time I judged in Australia, I could not understand why the petrol aroma was considered to be such a fault, yet it was expected that any Riesling exhibiting this characteristic should be marked down. However, I eventually realised that the vast bulk of Rieslings going through the Australian competition system are relatively young wines, and it is the fast accumulation of high concentrations of TDN that the Aussies truly frown upon. And I know what they mean. At a Negociants USA tasting at Purple1 held on the Sunday before, but in conjunction with, the 2008 "Riesling Rendezvous" in Seattle, I tasted some splendid Rieslings from Pewsey Vale. Potentially the greatest of those on show was The Contours 2003, but it had more petrol aromas than an oil refinery.

There are some wines I will not put in my mouth because of the way they smell, but I like TDN, albeit in much smaller, longer evolving doses, so I did not even hesitate, and was handsomely repaid by an absolutely sensational wine on the palate.


   Talk about racy, The Contours 2003 has such wonderfully ripe acidity that the intensity of terpene-laden fruit raced across the palate faster than Usain Bolt. When I told winemaker Louisa Rose it was a pity about the excessive TDN she looked at me as I was being unreasonable. After all, Pewsey Vale's flagship Riesling is kept for five years before being released and is well-known for its petrol aromas. However, from my perspective, this Riesling is grown in Australia, where TDN is typically so precocious that judges are expected to mark down wines showing too much petrol too quickly. Louise might feel that five years is not a short time, but for so much TDN to have built up that the bottle developed terpene aromas, which are so evident on the palate, do not even get a look in on the nose, five years is indeed a short time. And if someone is making a Riesling designed to be released as a museum vintage, should he/she not feel compelled to minimise the volume of TDN, and maximise its complexity and finesse?

The conditions that promote the highest TDN levels are well documented, and include low yields, warm weather (particularly high temperatures during the growth period), increased fruit-exposure, water stress, and high acids in either fruit or the wines. Some Riesling clones are more susceptible to TDN than others (Sponholz and Hühn, 1997) and, more recently, we have discovered that low nitrogen levels in the soil also increases TDN levels (Linsenmeier and Löhnertz, 2007). Some of these factors are linked, and involve practices and conditions that encourage the production of high quality Riesling in other respects, but this does not make it impossible to introduce a relatively low TDN regime in the vineyard. In the winery acidification increases TDN levels, but it also encourages the conversion of bound terpenes to free. TDN increases with ageing on yeast lees (which is why it is found in Champagnes) and warm storage conditions increase TDN levels.

It is easy to imagine how, from all factors involved, TDN levels will be naturally higher in Australia than northern Europe, or in Eden Valley than Tasmania, or indeed the steepest vineyards of the Mosel than the Rheingau. And why a drought year like 2003 promoted the most precocious of petrolly aromas even in the Rheingau or Alsace.

Interestingly, according to tests conducted by AWRI, natural cork closures absorb as much as 50% of the TDN found in any wine, while Altec and "One + One" technical corks can remove as much as 70%, and the most absorptive synthetic closures no less than 98%! Screwcaps actually preserve TDN, but I'm a screwcap fan, and TDN is not the only thing that corks absorb. Cork also absorbs numerous other flavour volatiles. So much so that the industry employs a "flavour scalp factor" as a measure of cork's flavour volatile absorption potential, and this scale is measured by highly accurate SPME-GC-MS analysis. It is very hard to understand why anyone has taken the trouble to put lovely flavours into a wine would want to seal it with a molecular suction pump called cork. Combine this with cork's oxygen ingress issue (the oxygen ingress of natural cork can vary 1,200-fold), whereby all wines sealed with the most pristine, untainted, corks are guaranteed to mature in bottle at entirely different, completely unpredictable rates, and it is surprising that the cork-closure industry survives in the 21st century. So I certainly do not advocate that Australian Riesling producers should reverse the trend of the last 5-8 years, and return to cork, but individual producers can easily factor in the screwcap's TDN retention capacity when devising their own low TDN regime, which they must do if they are to avoid a petrol crisis.
  

1 The epitome of Seattle's café culture, Purple is fabulous set up on the corner of 4th and University, with an impressive central tower of wines around which a staircase spirals. There are 70 wines by the glass, and great food - even the roast turkey and bacon rolls we snacked on during the tasting were more'ishly delicious. See their web site

go to part I