What the heck is a "Oak's Purpose?"

Sherry barrel showing the aging process                 


Oak drying before being made into barrels

In the United States, white oak, grown in Virginia, Missouri, Kentucky, Oregon, and Ohio, is the species used for barrels. It is preferred over red and black oak due to its tighter grain which minimizes evaporation of wine from the barrel and its resistance to shrinkage after wine is removed, an important feature in preventing wine from leaking out of a newly-filled barrel, and its high tannin content.

French white oak is harvested from several different forests in France. The Forests are Limousin, Alliers, Vosges, Troncais and Nevers, each producing oak that imparts slightly different nuances of flavor to the wine. The tightness of the grain, which determines the rate of extraction of these flavors, also varies among forests. Winemakers typically use a blend of barrels from different forests to take advantage of the unique characteristics of each.

The porous nature of an oak barrel allows some levels of evaporation and oxygenation to occur. In a year, the typical 59-gallon barrel can lose anywhere from 5½ to 6½ gallons of wine through the course of evaporation. This allows the wine to concentrate its flavor and aroma compounds. Small amounts of oxygen are allowed to pass through the barrel and act as a softening agent upon the tannins of the wine. Phenols within the wood interact with the wine to produce vanilla type flavors and can give the impression of tea notes or sweetness. The degree of "toast" on the barrel can also impart different properties affecting the tannin levels of the wine as well as the aggressive wood flavors.