The petrol note is considered to be caused by the compound 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN), which during the aging process is created from carotenoid precursors (terpenes) by acid hydrolysis. The initial concentration of precursors in the wine determines the wine’s potential to develop TDN and petrol notes over time. From what is known of the production of carotenoids in grapes, factors that are likely to increase the TDN potential are:
- Ripe grapes (accentuated by low yields and/or late harvest)
- High light exposure
- Water stress, which is most likely in regions that don’t practice irrigation. This primarily occurs in dry vineyard sites during warm and low-rain years
- Warm soils (gravel, etc.)
These factors are usually also considered to contribute to high-quality Riesling wines. The petrol note is, in fact, more likely to develop in top Riesling wines than in simpler wines made from high-yielding vineyards. Riesling grown in warmer climates, such as Alsace, will tend to exhibit the petrol character earlier in their post-bottling development. A discreet amount of petrol aromatics is a great enhancer, too much can be a bit of a turn-off.