In the USA, white oak, from Ohio, Kentucky, Virginia and Missouri, is the species most commonly used in creating barrels. There are several reasons white oak is preferred over black or red oak, including a tighter grain to minimize evaporation of the product from the oak barrel, the wood’s resistance to shrinking after removal of the wine, which also means the wine is less likely to leak from the newly filled oak barrels, and a higher tannin content that helps to preserve the oak barrels and provides a positive benefit for the wine that is aged inside.
The oak used in French oak barrels is also white oak, but harvested from several forests in France, including Nevers, Troncais, Vosges, Alliers, and Limousin. The oak barrels from each of these forests imparts a slightly different aroma or flavor to the wine. The tightness of the wood grain determines the rate at which the aromas and flavors are extracted and also varies by the location the oak was grown. Typically, winemakers choose a mix of oak barrels from different French forests to take advantage of the characteristic imparted by the different oak barrels using French oak.
However, among winemakers preferring American oak barrels, there is not a notion of differences due to region. Instead, those who use American oak barrels are more concerned with the cooper’s reputation than the forest from which the wood used in the construction of the oak barrels.
In addition to the wood variations found between forest, the character of oak barrels varies due to the trees within the forest due to factors including age and growth conditions. Traditional 60 gallon oak barrels will contain about 31 staves that come from several different trees. The reputation of the cooper is based on his ability to create oak barrels that are uniform from one year to the next. While winemakers sometimes look for a degree of variation from one vintage to the next, knowing that there is a consistency in the new oak barrels they buy from one year to the next is critical.
As wineries continue to seek methods of reducing production costs, the demand for American oak barrels has dramatically increased. More winemakers are substituting American oak barrels that cost 60% less than the French oak barrels that can cost over $500 each. The trend has caused a renewed scrutiny of the actual differences seen in American and French oak barrels.
Although oak barrels from both sources contribute both tannin and aroma, American oak barrels are seen as more aggressive and as providing an immediately apparent aroma and taste. American oak barrels are higher in vanillin and other odorous compounds. French oak barrels are higher in tannins and other flavor components with a less obvious oak smell and flavor. While winemakers once ruled out the American oak barrels for use with white wines and felt it only appropriate for robust wines that would not be overpowered by the taste and aroma of the wood, coopers in California have worked to reduce these characteristics by using the methods traditionally used by French coopers. For example, newly cut stavewood is allowed air dry exposed to the elements for 18 months or more instead of drying the wood in a kiln or oven. With this more natural process the wood is exposed to periodic rain and drying which helps to leach the harsher tannins while retaining more desirable components including vanillin, that is, to a large degree, lost by artificial drying. Charring of the oak barrels can be carried out using a lower heat that allows the flame to penetrate deeper. In the past, American oak barrels were often flash-fired, creating a heavy char used when aging bourbon.