Chapter 21

The Role of Oak in Winemaking

Benefits of Oak Barrels in Wine Production

The aging of wines in oak barrels is a process used to preserve wine but also to enhance its properties. It is a process of great interest, mainly because of the additional properties it adds to wines and because of its economic implications. Throughout the aging process, a series of reactions take place resulting in changes in chemical composition and organoleptic properties. The most relevant reactions that take place during aging in oak barrels are oxidation, anthocyanin and tannin condensation, the release of aromatic compounds from wood to wine, and clarification.

Oxidation

While oxidation usually involves negative implications, the slight uptake of oxygen during barrel aging (élevage) is generally viewed as desirable, especially for red wines, as a way of improving wine properties (e.g., color, aroma, texture). Wine exposure to oxygen occurs principally as a consequence of periodic cellaring procedures, such as topping and racking.

Anthocyanin and Tannin Condensation

Oxygen promotes the polymerization of anthocyanins with tannins and, therefore, color stability. Young wines contain many short tannins that contribute unpleasantly aggressive, green, stemmy characters. On the palate, these tannins seem coarse, bitter, and astringent. Over time, these short tannins combine with anthocyanins—color pigments from grape skins—to form longer polymer chains in a process called polymerization, which leads to the formation of a anthocyanin-tannin polymeric pigments. As a consequence of these chemical reactions free anthocyanin concentration decreases with a corresponding decrease in astringency and an increase color and stability.

Release of Aromatic Compounds

Another fundamental aspect of aging wines in oak concerns the aromatic compounds that are extracted. When these compounds marry perfectly with a wine’s intrinsic aromas, they make a significant contribution to the richness and complexity of the bouquet, as well as improving the flavor. The subtle flavors imparted to wine as it matures in the barrel include aromas of vanilla, clove, smoke, tea, tobacco, and coconut, therefore increasing the complexity of the wine.

Clarification

Barrel maturation can also help to stabilize the wine, particularly if the wine is aged on lees, since the time required to extract the oak and lees characters also allows the wine time for natural stabilization to occur due to the action of natural colloids and the slow precipitation of potassium bitartrate. Clarity is easier to obtain when wine is aged in the barrel rather than in the vat, due to the smaller volume.

Best Wines to Age in Oak Barrels

Oak aging is often employed in the production of premium red wines such as those with good body, complex flavor, supple texture and long finish. The varieties of red wines that tend to benefit most from use of oak wine barrels include Syrah, Tempranillo, Nebbiolo, Zinfandel, Chianti, Pinotage, Merlot, Cabernet Sauvignon, and Pinot Noir. Many whites undergo maturation in oak barrels too. White wines that can benefit from oak wine barrels include Chardonnay, Sémillon, Sauvignon Blanc, Pinot Blanc, and Pinot Grigio.

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