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Oak Barrels – A Pallet of Wine Flavors
In the form of barrel staves, oak has been associated with wine for at least two millennia. The first report of barrels used to store wine is recorded by Pliny the Elder (23–79 A.D.), while Hadrian’s Column provides the first visual representation of barrels (113 A.D.). Use of the term “barrel” in translations from older sources (e.g., Herodotus, 485–425 B.C.) is dubious, at least as applied to watertight cooperage of up to 500 L, composed of wooden slats, possessing curved sides, and held together by hoops. The curved sides enhance both barrel strength (the double arch) and greatly facilitate their movement (minimal surface contact when rolled).
Although several types of wood have been used in cooperage construction, white oak is preferred. In North America, Quercus alba is the principal species used, whereas in Europe, Q. sessilis and Q. robur are the main species. Although oak possesses very large, water conducting vessels in the living wood, these become densely plugged with tyloses as the living wood differentiates into nonliving heartwood – the portion used for barrel construction. This differentiation makes the heartwood highly impermeable to liquids and gases. This property is further enhanced by positioning the stave’s widest (radial plane) outward, around the circumference of the barrel. So, aligned, the large, elliptical collections of ray wood act an additional barrier to gas and liquid diffusion across the stave. Other desirable features of oak include its straight grain (facilitating stave splitting), the comparative rarity of knots or other features that would interfere with the wood’s impermeability, and the wood’s mild fragrance.
The lignins and cellulose of heartwood donate the required structural strength, whereas the accumulation of phenolics (notably hydrolyzable tannins) provides resistance to fungal decay. By serendipity, not only have consumers become habituated to oak’s aromatic constituents but often appreciate their presence. Although these compounds can mask the subtle fragrance of many white wines, they enhance the flavor complexity of most red wines. For these, barrels may be used not only for the wine’s maturation (period between fermentation and bottling), but also as a fermentation vessel. The level of oak character extracted is usually regulated by the duration of contact, or by selecting cooperage of an appropriate capacity (influencing the oak/wine surface area/volume ratio).
Barrel construction requires exposing the staves to moist heat. This softens the wood sufficiently for the staves to be safely bent into their curved shape. For the past century, it has become common to give the inner surfaces additional heating, the duration of which generates various degrees of a toasted character. This induces hydrolytic and pyrolytic changes to the oak’s aromatic character. First modified are the oak lactones, resulting in a progressive reduction in the wood’s coconut-like aspect. Medium toasting generates more of a roasted and vanilla character (derived from lignins). Further heating produces more of a caramel-like flavor (generated from hemicelluloses). If heating continues, the progressive accumulation of pyrolytic breakdown products generates a noticeable smoky, spicy character. Prolonged toasting coats the barrel with a thin layer of charcoal. Although this may be desired for maturing Bourbon, such barrels are not used for maturing wine.
The level of toasting given barrels depends on the specifications of the purchasing winery. The same also applies to the oak species used, and their specific geographical origin. The type of seasoning given staves prior to use also affects the oak’s aromatic characteristics. Additional variations in the oak flavor can be generated by the proportion of new versus used barrels, the number of repeat uses, and whether they have been retoasted, after shaving off a few mm from the barrel’s inner surface. Differences donated by individual barrels tend to be eliminated during blending matured wine before bottling.
Originally barrels were primarily used for storage and wine transport, but are now primarily used as a source of desired flavors (similar to most fortified spirits). Because barrels constitute a considerable winery expense, many producers have sought more economical sources of oak flavor. Occasionally, barrels are used only in locations where visitors can see what they expect to see. At one end of the spectrum is the addition of an alcoholic oak extract. Other alternatives include the addition of oak sawdust (in permeable bags), oak shavings, oak chips, or thin oak slats hung in, or line the insides of, stainless steel cooperage. These alternatives can be produced from oak who’s origin and toasting mimic those of barrels. What may be lacking is the slight and/or periodic exposure to oxygen that favors color stability in red wines. Technologic solutions to this issue can involve plastic cooperage (with known rates of gas permeability), or by sparging the wine with air or oxygen at a desired (minimal) rate.
With all these options at their beck and call, oak provides an additional source subtle flavors to complement those provided by the grapes, fermentation, maturation and aging; and thereby differentiate one producer’s wine from the competition, satisfy their artistic bent, and appeal to the preferences of consumers.
About the Author
Ronald S. Jackson received his bachelor’s and master’s degrees from Queen’s University and the doctorate from the University of Toronto. His time in Vineland, Ontario, and subsequently at Cornell University, redirected his interest in plant disease toward viticulture and enology. As part of his regular teaching duties at Brandon University, he developed the first wine technology course in Canada. For many years, Dr. Jackson was a technical advisor to the Manitoba Liquor Control Commission, developed sensory tests to assess the tasting skills of members of its Sensory Panel, and was a member of its External Tasting Panel.
Dr. Jackson has left his position as a professor and the chair of the Botany Department at Brandon University to concentrate on writing. He is allied with the Cool Climate Oenology and Viticulture Institute, Brock University.
He is also the author of Elsevier books Wine Science: Principles and Applications, Fourth Edition, which won the prestigious OIV Award in 2015, and the forthcoming Wine Tasting: A Professional Handbook, Third Edition.
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Additionally, please click one the links below to listen to Dr. Jackson’s lectures and learn more.
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