The use of oak in wine plays a significant role in winemaking and can have a profound effect on the resulting wine, affecting the colour, flavour, tannin profile and texture of the wine. Oak can come into contact with wine in the form of a barrel during the fermentation or ageing periods.
It can be introduced to the wine in the form of free-floating oak chips or as wood staves (or sticks) added to wine in a fermentation vessel like stainless steel, though these techniques are frowned upon by many. Oak introduced in the form of a wine barrel can impart other qualities to the wine through the process of evaporation and low level exposure to oxygen.
Sources of Oak
The species of oak typically used for American oak production is the Quercus alba
which is a white oak species that is characterized by its relatively fast growth, wider grains and lower wood tannins. It is found in most of the Eastern United States as well as Missouri, Minnesota and Wisconsin where many wine barrels are from.
In Oregon the Quercus garryana
white oak has started to gain usage due to its closer similarities to European oak.
In France, the main winemaking oak species is the Quercus petraea
which is known for tighter grain, high tannins and lower aromatics than its American oak counterpart. French oak typically comes from one or more primary forests: Allier, Limousin, Nevers, Troncais and Vosges. The wood from each of these forests has slightly different characteristics.
Many winemakers utilize barrels made from different cooperages, regions and degrees of toasting in blending their wines to enhance the complexity of the resulting wine.
Oak trees are typically between 80-120 years old prior to harvesting with the ideal conditions being a cool climate in a dense forest region that gives the trees opportunity to mature slowly and develop a tighter grain. Typically one tree can provide enough wood for two 59 gallon barrels. The trees are typically harvested in the winter months when there is less sap in the trunk.
Distinctions between French and American Oak
American oak tends to be more intensely flavoured than French oak with more sweet and vanilla overtones due to the American oak having two to four times as many lactones. Winemakers that prefer American oak typically use them for bold, powerful reds or warm climate Chardonnays.
Besides being derived from different species, a major difference between American and French comes from the preparation of the oak. The tighter grain and less watertight nature of French oak encourages coopers to split the wood along the grain rather than saw. French oak is then traditionally aged or "seasoned" for at least two years whereas American coopers will often use a kiln-dry method to season the wood.
Long periods of outdoor season have a mellowing effect on the oak that kiln-dry methods have difficulties replicating. The sawing, rather than splitting, of American oak also enhances the differences between the two styles due to the rupture of the xylem cells in the wood which releases many of the vanillin aromatics and lactones responsible for characteristics like the coconut notes.
Wine barrels have long been used as containers in which wine is aged. Aging in oak typically imparts desirable vanilla, butter and spice flavours to wine. The size of the barrel plays a large role in determining the effects of oak on the wine by dictating the ratio of surface area to volume of wine with smaller containers having a larger impact.
The most common barrels are the Bordeaux barriques style which holds 59 gallons (225 litres) followed by the Burgundy style barrel which hold 60 gallons (228 litres). Some New World wine makers use the large hogshead 79 gallon (300 litres) size.
New barrels impart more flavours than do previously used barrels. Over time many of the oak properties get "leached" out of the barrel with layers of natural deposits left from the wine building up on the wood to where after 3 to 5 vintages there may be little or no oak flavours imparted on the wine.
The cost of barrels varies due to the supply and demand market economy and can change with different features that a cooperage may offer. Due to the expense of barrels, several techniques have been devised in an attempt to save money. One is to shave the inside of used barrels and insert new thin inner staves that have been toasted.
Barrels are constructed in cooperages. The traditional method of European coopers have been to hand split the oak into staves (or strips) along the grain. After the oak is split it is allowed to "season" or dry outdoors while exposed to the elements. This process can take anywhere from 10 to 36 months during which time the harshest tannins from the wood are leached out. These tannins are visible as dark gray and black residue left on the ground once the staves are removed.
The longer the wood is allowed to season the softer the potential wine stored in the barrels may be but this can add substantially to the cost of the barrel. In some American cooperage the wood is dried in a kiln instead of outdoor seasoning. While this method is much faster, it doesn't soften the tannins quite as much as outdoor seasoning.
The staves are then heated, traditionally over an open fire, and when pliable are bent into the shape of the desired barrel and held together with iron rings. Instead of fire, a cooper may use steam to heat up the staves but this tends to impart less "toastiness" and complexity to the resulting wine.
Following the traditional, hand worked style a cooper is typically able to construct one barrel in a day's time. Winemakers can order barrels with the wood on the inside of the barrel having been lightly charred or “toasted” with fire, medium toasted, or heavily toasted. Typically the "lighter" the toasting the more oak flavour and tannins that are imparted. Heavy toast or "charred" which is typical treatment of barrels in Burgundy wine have an added dimension from the char that medium or light toasted barrels do not impart.
Heavy toasting dramatically reduces the coconut note lactones, even in American oak, but create a high carbon content that may reduce the colouring of some wines. During the process of toasting, the furanic aldehydes in the wood reach a higher level of concentration. This produces the "roasted" aroma in the wine. The toasting also enhances the presences of vanillin and the phenol eugenol which creates smoky and spicy notes that in some wines are similar to the aromatics of oil of cloves.