Terpenes are organic compounds found naturally in many plants, including grapes. These compounds are a major component of the essential oils that give plants their distinct aromas. In grapes, terpenes contribute significantly to their unique fragrance and the aromatic profile of wine. They are present in small concentrations but have a considerable impact on the sensory properties of grapes and wine.
Understanding Terpenes in Grapes
Terpenes are organic compounds derived from five-carbon isoprene units (C5H8), classified by the number of units they contain. In grapes, the most common classifications are monoterpenes, sesquiterpenes, and norisoprenoids. Monoterpenes are composed of two isoprene units (C10H16), while sesquiterpenes consist of three isoprene units (C15). Norisoprenoids are C13 compounds formed from the oxidative cleavage of carotenoids.
These compounds are primarily found in the exocarp, or skin, of the grape berry. Monoterpenes, often alcohols, are stored as glycosides in grape berry cell vacuoles. Sesquiterpenes accumulate as hydrocarbons in the epicuticular wax layer. Terpenes exist in grapes and wines as either free volatile terpenes (FVT), which contribute directly to aroma, or as odorless glycosidically bound potentially volatile terpenes (PVT). PVTs are two to eight times more abundant than FVTs.
The Scent and Flavor Contribution
Grape terpenes are responsible for distinct aromatic characteristics in grapes and wine. These compounds contribute to the varietal character of grapes such as Muscat, Gewürztraminer, and Riesling. Muscat grapes, for instance, are known for high concentrations of monoterpenes, imparting a characteristic floral aroma.
Specific terpenes contribute to particular aromas. Linalool, geraniol, and nerol are prominent monoterpenes found in Muscat grapes and related cultivars, imparting floral, rose, citrus, and spicy notes. Linalool, a common grape terpene, is found at concentrations significantly above its sensory perception threshold in Muscat varieties. Geraniol, nerol, and citronellol also contribute floral, rose-like, and citrusy aromas.
Norisoprenoids also contribute to the aromatic diversity of wine. TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) is a norisoprenoid that imparts a kerosene or petrol aroma, particularly noticeable in aged Riesling wines. Other norisoprenoids, like α-ionone and β-ionone, are derived from the oxidative cleavage of carotenoids and contribute fruity and floral notes.
How Terpenes Develop in the Vineyard
Terpene development in grapes is influenced by several vineyard factors, starting with grape variety. Different grape varieties have a genetic predisposition for varying terpene concentrations and profiles. Aromatic varieties like Muscat and Gewürztraminer naturally accumulate higher levels of these compounds.
Climate plays a role in terpene biosynthesis. Adequate sunlight exposure to the fruiting zone increases monoterpene concentrations in grapes like Riesling. Temperature also influences terpene development; cool climates help maintain the fresh character of terpenic wines, preventing overripe or heavy notes.
Viticultural practices also impact terpene content. Trellis systems, vigor control, shoot trimming, and leaf thinning enhance light penetration to grape clusters, promoting terpene accumulation. Grape ripeness at harvest is also a factor, with terpene content notably increasing as grapes reach around 21 Brix.
Terpenes and Winemaking
Winemaking significantly influences the final concentration and expression of terpenes in wine. Maceration, involving skin contact during grape crushing, is important because terpenes originate in grape skins. Extended maceration, as used for Gewürztraminer, leads to higher terpene concentrations in the resulting wine compared to juice with minimal skin contact.
Fermentation, particularly yeast action, is a step for terpene release. Many terpenes exist in grapes as odorless glycosidically bound precursors. During alcoholic fermentation, certain yeast strains, including Saccharomyces and non-Saccharomyces varieties, possess enzymatic activity that hydrolyzes these sugar-bound molecules, releasing volatile, aromatic terpenes. This enzymatic cleavage transforms the odorless precursors into aromatic compounds.
Aging also plays a role in terpene evolution. For example, compounds like TDN, which contribute a petrol aroma, form during bottle aging from norisoprenoid precursors. Different winemaking techniques can enhance, preserve, or diminish the aromatic potential from grape terpenes, allowing winemakers to shape the wine’s final aromatic profile.