Whisky Magazine Issue 103
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Ian Wisniewski asks how much influence does an oak cask and ‘wood chemistry' have on the character of a malt whisky?
p to 70 per cent of a malt's character develops during aging, and this relies on three aspects of ‘wood chemistry.' Additive maturation sees the spirit gaining flavour and colour from the oak; subtractive maturation means the spirit loosing pungency; while interactive maturation refers to other complex reactions, including oxidation, which creates additional characteristics.
The particular range of flavour compounds the spirit gains from a cask depends on the type of cask used, the usual choice being casks that previously aged Bourbon or sherry.
Bourbon barrels contribute vanilla, honey, various fruit flavours, coconut and spices such as cinnamon, with a light, dry sweetness. Sherry casks add rich sweetness with dried fruit notes including raisins and prunes, fortified wine, spices such as vanilla and ginger, as well as chocolate tones.
These flavour differences are principally due to the species of oak, with Bourbon barrels made from American oak (Quercus Alba) whereas sherry casks are European oak (Quercus Robur).
Another vital factor is the way casks are treated before being used to age Bourbon or sherry. A flame is applied to the interior of Bourbon barrels to briefly ignite the oak, before being extinguished with water. This creates a surface layer of char, around two mms deep, with the heat also toasting an underlying two to three mms layer of oak. A flame is also used to toast (but not ignite) the interior of sherry casks, creating a toasted layer.