Oak is used in winemaking to vary the color, flavor, tannin profile and texture of wine. It can be introduced in the form of a barrel during the fermentation or aging barrel periods, or as free-floating chips or staves added to wine fermented in a vessel like stainless steel. Oak barrels can impart other qualities to wine through evaporation and low level exposure to oxygen.J. Robinson Jancis Robinson's Wine Course Third Edition pg 91-93 Abbeville Press 2003
The chemical properties of oak can have a profound effect on wine. Phenols within the wood interact 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 as well as the aggressive wood flavors.K. MacNeil The Wine Bible pg 41 Workman Publishing 2001 The hydrolyzable tannins present in wood, known as ellagitannins, are derived from lignin structures in the wood. They help protect the wine from oxidation and redox.J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 492 Oxford University Press 2006
Wines can be barrel fermented in oak or placed in oak after fermentation for a period of aging or maturation. Wine matured in oak receives more oak flavors and properties than wine fermented in oak because yeast cells present in fermentation interact with and "latch on" to oak components. When dead yeast cells are removed as lees some oak properties go with them.K. MacNeil The Wine Bible pg 45 Workman Publishing 2001
Characteristics of white wines fermented in oak include a pale color and extra silky texture. White wines fermented in steel and matured in oak will have a darker coloring due to heavy phenolic compounds still present.J. Robinson Jancis Robinson's Wine Course Third Edition pg 93 Abbeville Press 2003 Flavor notes commonly used to describe wines exposed to oak include caramel, cream, smoke, spice and vanilla. Chardonnay is a varietal with very distinct flavor profiles when fermented in oak, which include coconut, cinnamon and cloves notes. The "toastiness" of the barrel can bring out varying degrees of Cafe mocha and toffee notes in red wine.D. Sogg " White Wines, New Barrels: The taste of new oak gains favor worldwide" Wine Spectator July 31, 2001
The length of time a wine spends in the barrel is dependent on the varietal and finished style the winemaker desires. The majority of oak flavoring is imparted in the first few months the wine is in contact with oak, while longer term exposure adds light barrel aeration, which helps precipitate phenolic compounds and quickens the aging process. New World Pinot noir may spend less than a year in oak. Premium Cabernet Sauvignon may spend two years. The very tannic Nebbiolo grape may spend four or more years in oak. High end Rioja producers will sometimes age their wines up to ten years in American oak to get a desired earthy cedar and herbal character.
In France, both the Quercus robur (common oak) and Quercus petraea (white oak) are considered apt for wine making; however, the latter is considered far superior for its finer grain and richer contribution of aromatic components like vanillin and its derivates, methyl-octalactone and tannins, as well as phenols and volatile aldehydes. French oak typically comes from one or more primary forests: Allier, Limousin, Nevers, Tronçais 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.T. Stevenson "The Sotheby's Wine Encyclopedia" pg 33-34 Dorling Kindersley 2005 have had a long history of using oak from the Quercus robur which is known for its tight grain, low aromatics and medium level tannins. Slavonian oak tends to be used in larger barrel sizes (with less surface area relative to volume) with the same barrels reused for many more years before replacement. Prior to the Russian Revolution, Quercus petraea oak from Hungary was the most highly sought after wood for . The trees in the Hungarian Zemplén Mountains grow more slowly and smaller in the volcanic soil, creating fine tight grain which sequentially lends itself to a very delicate extraction.
The hemicellulose in the Hungarian oak breaks down more easily, and conveys an exceptional selection of toasted, vanilla, sugary, woody, spicy and caramel-like flavors – imparting these aromas with less intensity, and more slowly than American or French oak.
Many winemakers favor the softer, smoother, creamier texture that Hungarian oak offers their wines. French winemakers preferred to use Hungarian barrels until the early 20th century, then – because of world wars, supply cut – the French wine industry was forced to find its own source in France, similar to the unique quality, legendary Hungarian Zemplén oak.
However, after the fall of the Iron Curtain, the cooperages from France again became major consumers of the exclusive Quercus petraea/Sessile Hungarian Oak trees originating in the Zemplén Mountain Forest.
The Russian oak from the Adygey region along the Black Sea is being explored by French winemakers as a cheaper alternative to French and Hungarian oak.D. Sogg " French Barrelmaker Turns to Russian Oak" Wine Spectator October 15, 2002 have been experimenting with the use of Canadian oak, which proponents describe as a middle ground between American and French oak even though it is the same species as American oak.K. Ebjich " Canadian Oak Barrels Get the Nod From Winemakers" Wine Spectator November 11, 2003
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 barrels. The trees are typically in the winter months when there is less sap in the trunk.J. Robinson Jancis Robinson's Wine Course Third Edition pg 92 Abbeville Press 2003
Even though American coopers may use a kiln-dry method to season the wood, almost all others will season American oak in exactly the same way as French. Open air seasoning has the advantage of leaching undesirable chemical components and bitter tannins, mellowing the oak in a manner that kiln-dry methods are incapable of replicating. Even though sun, rain, and wind may suffice in most cases to season oak, in drier climates coopers - such as Tonelería Nacional - apply up to 2000 mm (80 in) of water a year to their wood stacks in order to facilitate the seasoning process.
Since French oak must be split, only 20 to 25% of the tree can be utilized; American oak may be serrated, which makes it at least twice as economical. Its more pronounced oxidation and a quicker release of aromas help wines to lose their astringency and harshness more quickly, which makes this the wood of choice for shorter maturations - six to ten months. Because of American oak’s modest tannin contribution, the perfect first fill is a wine with abundant tannins and good texture; it allows the fruit to interact harmoniously with the wood, which contributes a wide array of complex aromas and soft, yet very palatable tannins.
French oak, on the other hand, generates silky and transparent tannins, which transmit a sensation of light sweetness combined with fruity flavors that persist in the mouth. Spices and toasted almond are noteworthy, combined with flavors of ripe red fruit in red wines, and notes of peach, exotic fruits and floral aromas like jasmine and rose in whites, depending on the grape variety employed.
New barrels impart more flavors 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 flavors imparted on the wine. In addition, oxygen transport through the oak and into the wine, which is required for maturation, becomes severely limited after 3–5 years. The cost of barrels varies due to the supply and demand market economy and can change with different features that a cooperage may offer. As of late 2007 the price for a standard American oak barrel was US$600 to 800, French oak US$1200 and up, and Eastern European US$600.World Cooperage Product Information 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.D. Sogg " Oak Flavorings" Wine Spectator Sept.20th, 2002
The staves are then heated, traditionally over an open fire, and, when pliable, are bent into the desired shape of the 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 flavor 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 coloring of some wines. During the process of toasting, the furanic 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 smokey and spicy notes that in some wines are similar to the aromatics of oil of cloves.T. Stevenson "The Sotheby's Wine Encyclopedia" pg 33 Dorling Kindersley 2005
Prior to 2006, the practice of using oak chips was outlawed in the European Union. In 1999, the Bordeaux court of appeals fined four wineries, including third growth Chateau Giscours, over $13,000 USD for the use of oak chips in their wine.J. Mann " Bordeaux Chateaus Fined for Use of Wood Chips" Wine Spectator November 29, 1999