Chapter 21

The Role of Oak in Winemaking

Compounds Extracted from Wine Oak Barrels

TThe chemical composition of the oak wood is important because of its effect on wine flavor and bouquet. Many of the oak’s constituents are modified by seasoning and toasting the wood during the process of barrel making. The compounds extracted from wood into wine can further be modified during the maturation of a wine. Thus, the flavor of the wine is influenced by many volatile and nonvolatile oak wood extractives and their interaction with wine constituents. The nature and amount of chemicals extracted from oak into wine varies depending on a number of factors, such as the wood moisture level, wood origin, extent of seasoning, toasting level, age of barrels, and duration of exposure. The key compound classes and their precursors in oak are listed in Table 21.1.


The major odorous compound imparted by wood is lignin, and over some months in barrel, it oxidizes to a few aromatic aldehydes. The most important aldehyde is vanillin, which has the most easily recognizable aroma in barrel-aged wines. A significant increase in vanillin level occurs in toasting the barrels.


With judicious barrel aging, vanillin creates aromas and flavors complementary to most red and white grape varietals. It is generally considered to be an important contributor to the character of barrel-aged wines.


Eugenol possess a very similar spicy, clove-like aroma. Present in raw oak, eugenol is reported to increase during open-air wood seasoning. Release into wine is reported to increase with toasting level.

Guaiacol and 4-Methylguaiacol

Guaiacol and 4-methylguaiacol impart smoky, charred and spicy aromas to the wine; guaiacol is smokier whereas 4-methylguaiacol is spicier.


Furfural and 5-methylfurfural are generated by the breakdown of carbohydrates, in particular cellulose and hemicellulose, during the barrel toasting. They possess sweet, butterscotch, light caramel, and faint almond like aromas. They may contribute these characteristics to wines aged in oak and are also markers for the whole family of caramelization compounds. The concentration of these compounds can decrease during heavy toasting levels.


Among the volatile compounds found in oak wood, lactones, especially cis- and trans-lactones, are known for their remarkable sensorial impacts in red wines. They are responsible for a variety of sensory characteristics and are chiefly responsible for the greater intensity of coconutlike aromas found in wines. The cis isomer is a more powerful aromatic than the trans isomer.

Hydrolyzable Tannins

Hydrolyzable tannins are generally extracted from oak barrels used in wine aging and are present in low concentrations in comparison to condensed tannins foundin grapes (Section 1.3). Oak tannins are also referred to as ellagitannins or gallotannins and are potent antioxidants. These tannins undergo polymerization contributing to a softer, rounder, silkier mouthfeel.

Tannin Potential of Oak Staves

The concentration of ellagitannins within an oak barrel will also vary by tannin potential (TP) of the oak stave. Tannin potential refers to the concentration of total ellagitannins within a single untoasted oak stave and is predicted using near-infrared spectroscopy (NIRS) (Nikolantonaki et al., 2019). NIRS measures the micrograms of ellagitannins per gram of dry wood (µg/g) and helps categorize oak staves into low tannin potential (LTP), medium tannin potential (MTP), or high tannin potential (HTP).

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