Terpenes: The Building Blocks of Grape Aroma
Terpenes are a vast class of organic compounds found in many plants, acting as the primary components of their essential oils. For grapes, these volatile molecules are the source of many floral, fruity, and spicy notes. While over 4,000 terpenes exist in nature, around 75 have been identified in grapes, with different families of terpenes contributing distinct characteristics.
The presence and concentration of these compounds are influenced by multiple factors, including the specific grape cultivar, climate, soil conditions, and ripeness at harvest. Winemakers can further influence terpene expression during the fermentation process and aging.
The Diverse Family of Grape Terpenoids
Within the broader family of terpenes, several key subgroups play a significant role in defining grape and wine flavor. The most commonly discussed are the monoterpenes, which are particularly abundant in aromatic varieties. However, other isoprenoids, such as sesquiterpenes and norisoprenoids, also contribute crucial aromatic qualities.
- Monoterpenes: These are responsible for many of the strong, floral, and citrus notes. Key examples include linalool (rose, floral), geraniol (rose), and nerol (sweet-floral). In varieties like Muscat and Gewürztraminer, these compounds can be present in concentrations well above the sensory threshold, making their aromatic profile unmistakable.
- Sesquiterpenes: These compounds contribute spicy and earthy notes. A notable example is rotundone, which imparts a distinctive black-pepper aroma to Syrah and Grüner Veltliner.
- Norisoprenoids: While not technically terpenes, these compounds are derived from carotenoid precursors and are highly influential in wine aroma. A prime example is TDN (1,1,6-trimethyl-1,2-dihydronaphthalene), which is responsible for the 'petrol' or kerosene aroma in aged Rieslings.
The Journey from Grape to Glass
Terpenes exist in two primary forms within the grape: free and bound. Volatile, free terpenes are immediately perceptible and contribute to the fresh fruit aroma. In contrast, most terpenes are initially bound to sugar molecules as non-aromatic glycosides, concentrated predominantly in the grape skins.
During the winemaking process, especially fermentation and aging, these bound terpenes are liberated through enzymatic activity and acid hydrolysis. This transformation releases their full aromatic potential, which can be further influenced by factors like skin contact time, yeast selection, and maturation environment. For instance, increased skin contact, as seen with Gewürztraminer, can extract more monoterpenes and amplify floral intensity.
Comparison Table: Terpenic vs. Neutral Grape Varieties
| Feature | Terpenic Grape Varieties (e.g., Muscat, Gewürztraminer) | Neutral Grape Varieties (e.g., Chardonnay, Pinot Noir) |
|---|---|---|
| Free Terpene Concentration | High, often above the sensory threshold. | Low, often below the sensory threshold. |
| Dominant Aroma Profile | Characterized by intense, distinct floral and citrus notes. | Subtler aromas, influenced by fermentation and aging. |
| Main Terpenes | Linalool, Geraniol, Nerol, among others. | Rotundone (in some red varieties) and various norisoprenoids. |
| Winemaking Focus | Often emphasizes methods to preserve volatile aromas. | Often focuses on techniques that build complexity, such as oak aging. |
| Aroma Evolution | Strong varietal aroma is often present even before fermentation. | Develops complexity primarily during fermentation and maturation. |
Terpenes in Red vs. White Grapes
While highly aromatic white varieties like Muscat and Gewürztraminer are most famously associated with high terpene levels, red grapes also contain these compounds. The key difference lies in the specific terpenes and their concentration. For example, the sesquiterpene rotundone is found in the skins of red grapes like Syrah, imparting a peppery note. The winemaking process also plays a role; red wines fermented with the skins on will extract more skin-derived terpenes.
Conclusion
Do grapes have terpenes? Unquestionably. The intricate world of grape aromas is deeply rooted in these organic compounds, which contribute everything from heady floral notes to subtle spicy undertones. While some varieties possess famously high concentrations of terpenes, every grape contains them, and they work in concert with other chemical compounds to shape the unique sensory profile of both the fresh fruit and its resulting wine. This fundamental component of grape biology highlights how a single compound can have a dramatic influence on a food's character, and explains why different wines can offer such a vast and fascinating spectrum of bouquets.
For a deeper dive into the science of wine flavor, including the role of other aroma compounds, consult industry-specific resources such as SevenFifty Daily on the science of isoprenoids.
