|Malted barley, yeast and water: as individual ingredients they don't exactly set the pulse racing, but put them together, make beer then spirit and mature it in the presence of good quality oak and wondrous things begin to happen.|
Oak is a strong, slow growing tree with wood that is malleable enough to be shaped into casks. The resulting casks remove impurities and undesirable flavours such as sulphur from the spirit. Oak, additionally, is rich in its own flavour compounds which are easily accessible by charring or toasting the wood, giving the spirit a pleasant vanilla, spicy, toasty flavour. Oak casks provide protection but are porous enough to allow oxygen molecules to pass through to perform beneficial oxidation reactions. Finally, compounds within the oak itself react with compounds in the spirit helping to develop it into a full flavoured whisky.
The size of cask determines how quickly these reactions occur, when more whisky is in contact with the wood maturation will be quicker. A list of whisky cask types and their capacities can be found here.
|The effects of oak on whisky spirit are major. Both new make bourbon, called White Dog, and new make malt spirit, are colourless. All of the colour in bourbon comes from the cask, and a high proportion of the colour of Scotch does (although some European distillers add caramel). The fundamental differences between the approaches used in Scotland and the US are the length of maturation and type of cask used. Bourbon must be matured in oak for at least two years and Scottish single malt for a minimum of three years (although in practice most will be matured for much longer). US laws stipulate that a new (virgin oak) barrel must be used for every fresh batch of bourbon.|
Malt whisky tends to be too delicate to be matured in virgin oak because the tannins and spicy flavours in the wood can, quite quickly, overwhelm it, so casks which have been used before are favoured.
The two systems make for a happy partnership. Scotch whisky is normally matured in casks which have previously contained bourbon (or sherry) � and as the bourbon industry is dumping them in their hundreds of thousands, a grateful Scotch whisky industry can buy them for a snip. For this reason, bourbon casks, made with white American oak, are the most common form of maturation vessel for malt whisky. All bourbon casks will be made of American oak but it does not follow that all casks previously used for sherry production will be made from European oak. Sherry styles such as oloroso are matured in American oak, so there are on the whole three main types of casks for whisky making: bourbon cask, European oak sherry, and American oak sherry. The differences between European oak (Quercus robur) and America oak (Quercus Alba) are so pronounced that you can discern them with the naked eye. The tighter grain of the slower-growing European variety imparts its flavour more slowly to the spirit.
Even within Europe there is variance with, say, Swedish and Spanish oak imparting different flavours. American oak tends to have more compounds called lactones which give the coconut flavour often present in bourbon and more vanillins which give a vanilla flavour.
Considerable experimentation has taken place in recent years. It is not unusual to find whiskies that have been matured in part in casks used to produce Madeira, rum, brandy, red or white wine or port. Although the rules forbid the addition of anything to a single-malt whisky, some do take on distinctly ruddy and pink hues from their contact with the spirit- or fortified wine-soaked wood; such casks can have a major impact on flavour. In short, wood is not used to simply store the whisky as it matures of its own accord. Wood gives whisky much of its character.