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.
Several factors affect the impression of a Riesling wine’s sweetness on the palate. As residual sugar increases, the impression of sweetness increases. The sweetness from the sugar interacts with:
The acidity (malic and tartaric acids) and the level of dissolved carbon dioxide, which offers a sense of freshness
The phenolic content, which offers astringency and body
The ethanol content, which brings a feeling of weight, warmth and dryness on the finish
The pH, which accentuates the expression of the acids and reduces the sensation of sweetness as it goes down
For example, a Riesling with 1.5% (grams/liter) residual sugar can appear either dry or sweet. If the wine is 12.5% alcohol with a pH level below 3.0 and an acid level above 9 g/L, the wine will appear to be quite dry. If the wine is 10% alcohol with a pH level of 3.3 and an acid level around 6 g/L, the wine will seem relatively sweet.
The most expensive and complex wines made from Riesling are high-sugar dessert wines. To craft these wines, the first technique is to concentrate the grapes through the evaporation of water caused by the fungus Botrytis cinerea or “noble rot.” The second technique is to concentrate the grapes allowing them to freeze, as in the case of ice wine (in German, Eiswein). These concentrated wines have more sugar (in extreme cases, several hundred grams per liter), more acid (to give balance to all the sugar), more flavors and more complexity.
The resulting wines are among the most age-worthy of all wines. The beneficial use of “noble rot” was discovered in the late 18th century at Schloss Johannisberg in the Benedictine Abbey of Fulda. At the abbey, they once started picking rather later than had been their custom and the grapes had begun to rot. It turned out that the wine made from these grapes was of exceptional quality and greatly exceeded expectations. The first Eiswein is rumored to have been made in 1794 in Franken.
Riesling is a very hardy grape variety that thrives in cool climates and relatively poor soils. The grape requires a long growing season to ripen properly. In warmer climates, Riesling can lose its acid backbone, which is a main characteristic that helps make the wine bright, complex and balanced. Riesling wines generally do not respond well to newish, small oak barrels, preferring to be vinified and aged in stainless steel or larger neutral wood containers.
Riesling is a very aromatic and expressive grape variety, offering impressions of fresh flowers like honeysuckle and jasmine. Riesling’s aromas and flavors are often compared to orchard fruits like apple, peach, apricot (the apricot aroma is often a characteristic of the wonderful “noble rot”) and even the tropical-scented lychee or guava. Riesling wines are often described as tasting clean, racy and bright. Aged Riesling can take on more complex aromas which, in high quality wines, can include an aromatic expression of petrol, diesel or linalool.
Riesling’s compact bunches and small berries make it prone to rot (some of which is beneficial and necessary to make certain styles of wine). Noble rot (Botrytis cinerea) dehydrates ripe grapes, concentrating them to a point where they are capable of producing very memorable, rich, long-lived dessert wines.
What is this minerality thing? Most rocks when ground to smithereens do not have much taste or flavor. Nevertheless, few would dispute that Rieslings grown from differing soil profiles often exhibit extremely different organoleptic characteristics. Because of the lack of interference of other elements – malolactic fermentation, oak, elevated alcohol levels, etc. – one can very easily learn to distinguish the flavor of Riesling grown on schist as compared to slate or limestone. Because Rieslings are grown in such minimal soil on the steep slopes of the greatest vineyards in Germany and Austria, they must derive their moisture from exploring the fissures of the fractured subsoil, mining for water. Vines grown under these extreme conditions, as well as older vineyards (with the deepest roots) will tend to exhibit the highest degree of minerality. It is also believed by some that the putative elevated mineral content of Riesling is implicated in the wine’s prodigious longevity.
