“The role of oak in wine”
The oak cask evokes wine production as immediately as the keg does for the hops lover, even though it is not unanimously employed in wine making. During the cask aging process, some varieties of red, such as pinot noir, and whites, like chardonnay, will be stored in an oak barrel. The intention is to alter the overall structure of the wine, including subtleties in palate and color.
Predating modern tanks constructed of materials ranging from metal, cement, and plastic, wood barrels made by coopers were the norm. Oak (genus Quercus) ranked high in favor for several reasons: it is strong, fairly easy to work, and almost fully watertight. Along the way, it became known for its positive ability to facilitate chemical processes occurring within the wine, improving taste and hue.
Vintners over centuries experimented with a variety of oaks and concluded that amongst a diversity of species three are most beneficial in their use as wine cask. In Europe, the French oaks of preference are Quercus sessiliflora and Quercus robur, nearly identical white oak species. The American white oak Quercus alba remains the predominant choice for casks among a number of other North American oaks. Distinctions based on the region of origin, age and the parts of the tree used are all taken into consideration when the wine maker chooses a barrel for a specific wine.
Cooperage construction ranks in equal importance as to how the young wine will mature. Coopers build barrels according to the type of wood, then season and toast it to change the chemical activation the oak will have on the grape. Coopers use the hard and durable center of the tree, the ‘heartwood,’ to make the staves that bound together will form the cask. The seasoning process of allowing the wood to dry, can take 2-3 years and alters the way the wood produces certain acids and other compounds. Toasting to bend the staves into the correct shape occurs just prior to or during assembly; this process of firing over wood for different lengths of time and varied heat intensity also influences overall wine body.
Effects on wine
Two specific processes occur during the resting period in which the wine remains casked. The first is exposure to certain flavor compounds that already exist in the variety of oak used and to those properties that have been enhanced through toasting the staves. Some of these are: lactones, imparting oak and coconut aromas; vanillin, which contributes to aroma; and ellagatannins, or tannins absorbed by the wine from the wood, modifying structure and boosting color.
The second is that during ageing the oak barrel allows a very slight and controlled exposure to oxygen. Micro-oxygenation, as this transfer is called, alters its essence by “adding very low levels of oxygen to a developing wine over an extended period,” (Goode, 97) thus building optimal wine structure, reducing herbal or ‘green’ characteristics, and overall helping color and flavor. Colors intensify during the reaction between tannins – bitter tasting compounds in the grape itself and introduced in part by the oak – and anthocyanins, which impart color to the wine. In addition, some tannins will be “softened by polymerization,” (Goode, 104) or the process by which small molecules bind together to form larger ones; these tannins will dissipate, resulting in reduced astringency. To learn more, consult “The Science of Wine” by Jamie Goode, which explores extensively the effects of oak barrels on wines.
Thanks to the oak barrel, wine – and many other spirits as well – matures as delicious and as subtly flavorful as the craft of the vintner can create it to be. From the barrel to the bottle, corked by the bark of the oak Quercus suber, the oak tree plays a vital and delightful role in glasses poured and toasted with around the world.