Riesling: an essay by Raül Bobet

1. Formation and evolution of terpenic compounds in Riesling

Terpenic compounds (such as linalool, geraniol, or nerol) are one of the main aromatic families present in Riesling, although in more discreet amounts than in varieties such as Muscat or Gewürztraminer. They are formed during grape development, especially between veraison and ripeness, within the glands of the skin.

A large portion of these compounds are not found in free form but are glycosylated: bound to a sugar molecule. This makes them aromatically inactive, yet potentially releasable later on, both through yeast activity and through chemical transformations in the wine.

Under ideal conditions (cool areas, moderate sunlight), Riesling builds a balanced and elegant terpenic profile, which can lead to floral perfumes (orange blossom, linden) and subtle citrus notes.

2. Limited assimilation of these compounds by yeasts

Yeasts do not actively metabolize terpenes, nor do they have an efficient capacity to hydrolyze the glycosylated fraction. In other words, during fermentation, most latent terpenes remain hidden.

Only some yeasts (non-Saccharomyces) or exogenous enzymes can partially release these compounds. This explains why Riesling can express its true aromatic potential after months or years of evolution, when wine chemistry, especially under acidic conditions, begins to act.

3. Acid hydrolysis and its importance in Riesling

Riesling must and wine, thanks to their low pH (typically < 3.2), provide a very suitable medium for a slow acid hydrolysis of terpenic glycosides.

This process breaks the sugar–terpene bond, releasing free monoterpenes, which are far more volatile and aromatically perceptible. The main released compounds are:

Linalool → orange blossom, lavender
Geraniol → rose, geranium
Nerol → sweet floral notes
Alpha-terpineol → pine, citrus”

4. Impact of Botrytis cinerea (noble rot)

The presence of noble rot can profoundly transform Riesling’s aromatic expression through:

a) Enzymatic action by the fungus:

Enzymes such as fungal glycosidases can release terpenic compounds that would otherwise remain hidden.
In addition, it breaks down sugars and other precursors, generating new aromatic compounds.

b) Formation of characteristic compounds:

Phenylacetaldehyde → rose, honey
Furfuryl-derived thiols → saffron, black tea aromas
Lactones and benzaldehyde → almond, dried peach
Sotolon → curry, nuts (especially in aged wines)

When well managed, botrytis can bring extraordinary complexity, especially in naturally sweet wines or late-harvest styles.

5. Great aging potential of Riesling in cold regions and its link to pH

The great age-worthy Rieslings (Germany, Alsace, Austria, Clare Valley…) come from cold regions, where grapes can ripen slowly and retain very high acidity.

This acidity:

Acts as a natural preservative (inhibits oxidation and microorganisms).
Stabilizes aromatic compounds and the wine’s redox balance.
Favors slow acid hydrolysis, releasing aromas over time.
Allows the development of “petrol” notes (such as TDN, derived from carotene), which are typical of Riesling aging.

The lower the pH (2.9–3.1), the longer and safer the aging trajectory can be. At 10, 20, or even 30 years, a great Riesling can offer a unique and vibrant aromatic profile.

Written on 01/02/2026 by Raül Bobet