The use of oak 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. The use of oak barrels can impart other qualities to wine through the processes of evaporation and low level exposure to oxygen.
The use of oak has been prevalent in winemaking for at least two millennia, first coming into widespread use during the Roman Empire. In time, winemakers discovered that beyond just storage convenience that wine kept in oak barrels took on properties that improved the wine by making it softer and in some cases better-tasting.
The chemical properties of oak itself can have a profound effect on the wine. Phenols within the wood interact with the wine to produce vanilla type flavours 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 flavours.
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. It’s more pronounced oxidation and a quicker release of aromas help wines to loose their astringency and harshness faster; 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 flavours that persist in the mouth. Spices and toasted almond are noteworthy, combined with flavours 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.
Wine barrels, especially those made of oak, 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 hold 220 L followed by the Burgundy style barrel which hold 230 L.
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. In South Africa barrels can cost about R10 000 each. 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, but create a high carbon content that may reduce the coloring of some wines. The toasting also enhances smokey and spicy notes that in some wines are similar to the aromatics of oil of cloves.
Although oak barrels have long been used by winemakers, many wineries now use oak wood chips for aging wine more quickly and also adding desired woody aromas along with butter and vanilla flavours. Oak chips can be added during fermentation or during aging. In the latter case, they are generally placed into fabric sacks and placed into the aging wine. The diversity of chips available gives winemakers numerous options. Oak chips have the benefit of imparting intense oak flavoring in a matter of weeks while traditional oak barrels would need a year or more to convey similar intensity. Critics claim that the oak flavouring from chips tend to be one-dimensional and skewed towards the vanilla extract with the wines still lacking some of the physical benefits that barrel oak imparts.
The use of oak powder is also less common than chips, although they are a very practical alternative if oak character is to be introduced during fermentation. Oak planks or staves are sometimes used, either during fermentation or aging. Wines made from these barrel alternatives typically do not age as well as wines that are matured in barrels.
Throughout history other wood types, including chestnut, pine, redwood, and acacia, have been used in crafting winemaking vessels, particularly large fermentation vats. However none of these wood types possess the compatibility with wine that oak has demonstrated in combining its water tight, yet slightly porous, storage capabilities with the unique flavour and texture characteristic that it can impart to the wine that it is in contact with.
Chestnut is very high in tannins and is too porous as a storage barrel and must be coated with paraffin to prevent excessive wine loss through evaporation. Redwood is too rigid to bend into the smaller barrel shapes and imparts an unpleasant flavour. Acacia imparts a yellow tint to the wine. Other hardwoods like apple and cherry wood have an off putting smell.
