Researchers at The Australian Wine Research Institute (1) have investigated the factors influencing the level of TDN in Riesling wines. TDN is the abbreviation we ordinary petrol lovers can learn as the pronunciation of this aroma’s chemical name is a tongue-twisting exercise: 1,1,6-trimethyl-1,2-dihydronaphhalene. Some love it, others think TDN is a fault.
Many factors interact to give Riesling wine higher levels of TDN. We will get there in a moment. But first some chemistry. Hang on now – this is exciting!
It is an interesting fact that the level of free TDN in grapes and must is almost non-existing. Our lovely aroma is instead created later in the process. The basis for TDN is what is called carotenoids. Carotenoids? What? Well, there are in fact many kinds of carotenoids. You have probably heard of beta-carotene. The compound usually just called carotene, which gives carrots their beautiful orange colour. At least a dozen different carotenoids have been found in grapes, including the ordinary beta-carotene. Most of it is found in the grape skins, about three times as much as in the pulp.
The carotenoids start to build up in the grapes right after fruitset. But when the grapes begin to change colour, at verasion, then the happy days of growth are over and shift into ruthless breakdown. In that process, small molecules are created. These C13-norisoprenoids also hold the compounds that generate TDN.
The C13-norisoprenoids are deeply in love. The object of their worship is sugar molecules and they cling hard to them. “Flavour reservoirs” are created this way. They will slowly release TDN during storage and thus also our lovely petrol aroma.
Higher level of carotenoids in the grape will thus give more petrol aromas. An interesting question is then if the level is possible to influence. Yes, according to the researchers, it is possible. The more direct sun and heat the grapes get, the more carotenoids. If vine leaves shade the clusters, then growth is hampered. The grower can consequently affect the TDN-level by canopy management. Besides the selection of pruning and trimming approach will the amount of fertilisers play a part, as more of it encourages foliage growth and thus more shade to the grapes. Correspondingly can drought and water stress reduce the foliage and as a result give more sun exposure.
Once the wine is in the bottle, the TDN-level grow through the “flavour reservoirs”. Also in this stage are several factors found to influence the development.
Apart from increased levels of TDN just by age, the storage temperature will contribute. Wine bottles cellared at 30° C have a considerably higher level of TDN than those stored at 15° C.
The higher level of acidity, the faster the petrol character develops. That is due to the breakdown of other aroma compounds in the acidity rich wine. Those compounds (monoterpenes and esters) give the wine its citrus, fruit and floral flavours and usually mask a part of the TDN.
Even the bottle closure affects the TDN-level. Studies have shown that natural cork and synthetic closures during a period of two years absorbed more than the half of the TDN. Screw caps on the other hand did not affect the TDN-level at all.
We petrol lovers should thus look for Riesling wines that
- Were made of grapes exposed to heat and direct sunlight. Consequently, warm and sunny vintages are good choices.
- From producers not considering petrol to be a defect and because of that take additional steps to shade their grapes.
- Have a high level of acidity, which most Riesling wines have.
- Have screw cap.
- Can we additionally suspect a warm freight and that the bottles have been stored on warm shop shelves, the chances increase. And why not use that otherwise so inappropriate wine storage place in the hot kitchen to serve as an extra TDN-activator?
We tasted a Tim Adams Riesling 2011, from Clare Valley in South Australia. Clare Valley is, just as neighbouring Eden Valley, well known for its Riesling wines, often showing a great petrol flavour. Even if 2011 was a colder year, with relatively few sunshine hours, we were satisfied with our choice. We found an incipient fine petrol character in terms of bicycle tubes. Dry, with a nice high acidity. Fruity with citrus, pears and barely ripe melon. Good minerality. Lovely long aftertaste with more inner tubes and citrus. Really good!
(1) Black, C. et al. (2012), "Aged Riesling and the development of TDN", Wine & Viticulture Journal, September/October 2012.