Oak is a hard, supple, and watertight wood used in container manufacture, which has a natural affinity with wine, imparting qualities and flavours that today's consumers appreciate.
There are hundreds of species of oak, all of which can be broadly separated into two categories, red and white. The red oaks are porous and so are not used for cooperage.
Three sorts of white oak used in wine:

• Quercus alba, the American white oak: low phenols, but high aromatics, particularly methyloctalactones (coconut)
• Quercus sessiliflora, the sessile oak: a tighter grain and less extractable tannins, but high aromatic potential (e.g. lactones, volatile phenols (eugenol; cloves), phenol aldehydes (vanillin; oaky and vanilla odours)).
• Quercus robur, the pedunculate oak: low odoriferous compounds but high extractable polyphenol content

Due to the differing growth curves of the trees and the different barrel making methods, French oak (for a 225L barrique) is ca. £300 per barrel, £250 for American oak. Hence a wine aged in 100% new oak will cost £1 per bottle just for the oak ageing!
Barrel quality is not just a question of oak species but also very dependent on the barrel-making process, specifically:

• Sawing versus splitting the logs
• Seasoning, how long staves are stored prior to use
• Drying: air versus kiln
• Assembling
• Shaping and toasting
• Finishing

Barrel maturation an increasingly common practice for superior-quality still wines of all colours and styles. The advantages of barrel maturation are that:

• It encourages clarification and stabilisation of the wine
• It helps to deepen and stabilise the colour, to soften the tannins, and to increase the complexity of the flavour compounds.
• It imparts some wood flavour directly into the wine
• It encourages the maturation of the wine due to its slow oxygenation. This uptake of oxygen tends to reduce the fresh, grapey primary aromas and causes small tannin molecules to agglomerate, which changes colour towards gold in whites and softens astringency in both reds and whites. In red wines oxygen aids in the stabilisation of colour.

The flavour effects are often ‘replicated’ with oak in ‘non-barrel’ form, but are usually harsher and less balanced than in true barrel ageing, mainly due to the absence of controlled slow ageing and oxidation that the barrel promotes. Red wines are normally put into barrel immediately after malolactic fermentation and left to mature for up to two years. Racking every three or four months helps clarification, softens the wood flavour, and inevitably involves some oxygenation. Oxygenation is positively encouraged during the first six months by leaving the barrels with the bung up, with regular topping-up. Thereafter they are rotated so that oxygenation is reduced.
Low-tannin wines, such as those from Pinot Noir, are often encouraged to complete their malo-lactic fermentation in the barrel in order to increase the level of integration of oak and wine flavours.
All over the world, many top quality white wines are subjected to barrel fermentation prior to barrel maturation, as this tends to result in much better integration of wood and wine than putting white wine into barrel only after fermentation.
Barrel maintenance:

• Preparation for use: check for leaks and ensure that the barrel offers the right flavours to the wine.
• Storage conditions: If the cellar is too cold, the wine will not develop. If it is too warm, off-flavours and harmful bacteria may develop, and the wine may age too rapidly. If the cellars are too dry, too much wine can evaporate and the barrels themselves can dry out. The ideal temperature is usually around 10 to 18 ° C, with a humidity over 75 per cent. Below 75 per cent, water evaporates, but above that figure alcohol evaporates.
• Storage of empty barrels. Traditionally empty barrels were rinsed, dried, treated with sulphur dioxide, and then bunged up. Empty barrels must be stored under the same conditions of low temperature and high humidity as full barrels.