The complete science of barrel-aging whisky is something we have only recently come to understand. In the 1970s we began to uncover the full details of what happens when the Glenfiddich distillery men lay the base spirit to rest in oak casks and store it in one of our 46 warehouses. Our Malt Master, Brian Kinsman, told me that every new cask of spirit has the same opportunity of becoming a 12-year-old Scotch as it does of becoming one of the extremely rare casks that make up our incredible 50 Year Old.
While we cannot predict what will happen in each individual cask, we do know that approximately 70% of the flavor characteristics of Glenfiddich come from the wood and the time spent maturing in it. In Scotland, we predominately use two types of wood to mature whisky–American white oak (Quercus alba) and European oak (Quercus robur). American wood is the most popular, accounting for approximately 90% of casks used today, but it is important to remember that each different wood gives its own unique characteristics to the whisky. For example, Q. alba produces vanilla flavors (fruity, sweet) while Q. robur yields a rich, tannic kick. At the Glenfiddich distillery we take the base spirit, which comes off the still at 65.6% ABV, and add it into the casks. The wood casks perform three important functions to the aging of the whisky: ‘subtractive,’ ‘additive,’ and ‘interactive.’ The relationship between the liquid and the wood is one of the many intriguing aspects of whisky. After all, that mysterious and storied past is part of the allure of Scotch whisky!
by Mitch Bechard, Glenfiddich Ambassador
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“Every new cask of spirit has the same opportunity of becoming a 12-year-old Scotch as it does of becoming one of the extremely rare casks that make up our incredible 50 Year Old.”
Brian Kinsman
Glenfiddich Malt Master
THREE MODES OF CASK AGING
SUBTRACTIVE:
Wood casks remove impurities within the whisky. The inside of American oak barrels is usually charred. Charcoal, with its billions of tiny pores, is a great purifier. It has the ability to remove unwanted components such as sulfur-containing compounds. On the other hand, European wood is only lightly toasted and therefore does not have the same pronounced subtractive qualities.
ADDITIVE:
Wood gives flavor and character to the whisky. Oak wood contains hemicellulose, lignin and tannins. These elements cultivate chemical reactions with the base spirit, adding aromatic molecules to produce flavors such as coconut, vanilla and fruit. Wood also varies the color of the whisky. We find that Scotch whisky matured in European wood will have a darker, more reddish hue to it than that which has spent most of its life in American wood.
INTERACTIVE:
Wood is semi-porous, meaning that the cask is always breathing and interacting with its warehouse environment. Around 2% of the volume of each cask—water and ethanol—evaporates from the cask each year. We call this “the angels’ share.” As we age the whisky, the alcohol content of the liquid drops. We must be mindful of this, as spirit below 40% ABV can no longer legally be called Scotch whisky.