There are two main types of oak used for the maturation of Bruichladdich: Quercus patrea or Quercus robar (European oak) and Quercus alba (American oak).
Wine casks are in general, though not exclusively, made from European oak; Bourbon casks are exclusively made from American oak.
The pores in Quercus alba are more densely knit, ensuring that the casks allow less spirit to evaporate, “The Angel’s Share”, than their European counterparts.
These casks are sawn from oak grown in the Ozark Mountains, Arkansas. It is dried in large kilns, machine assembled, and heavily charred on the inside for the Bourbon industry. Bourbon casks are smaller (200 litres) than wine barriques (250 litres).
Bourbon laws dictate that the casks cannot be reused, and must be discarded once the contents are bottled, usually after only four or five years. The redundant casks are shipped to Scotland for whisky maturation.
The tannins, lignins and vanillins present in virgin oak can dominate a spirit in long term whisky maturation. Consequently, at Bruichladdich we use virgin oak casks with strategic care.
Second hand casks are therefore preferred and, bearing in mind the number used, for economic reasons too.
The flavours imparted by American oak tend to be in the vanilla spectrum, Crème Brûlée. The colour tends to be pale gold for first (whisky) fill casks. Pale yellow for second fill.
French Oak imparts a much more complex range of flavours in the ‘buttered toast’ specrum, as well as a wider variety of oak styles.
90% of whisky maturing in Scotland today is in American oak. Sherry casks, though made in Spain, are also primarily made of American oak - not Spanish oak as often claimed, which is usually unsuitable for this purpose.
Confusingly, an increasing percentage of wine casks made in France are also coopered from American oak.
The influence of the oak on the maturing spirit is reduced with each filling of the cask, as the wood flavours are leached out. More than two fillings and the wood effect is neutral – the whisky continues to evolve, but the effect of the wood’s tannins are minimal.
Bruichladdich buys from Buffalo Trace, discards 30% of bottled casks annually, and 70% of the casks used are first fill American oak Bourbon casks, 30% French oak. Around 10% of these are virgin oak.