Cask wood and its importance in whisky maturation
The cask plays a crucial role in shaping the flavour of a spirit – as a whisky stays in barrel for an average of 10 years its interaction and relationship with the wood is intense. The choice of wood used for the barrel, the age of the barrel, and the previous contents of the barrel all impact the aroma, taste, and colour of the final whisky.
During the first few years of barrel maturation, the new spirit becomes softer and rounder, and its less desirable smells and flavours vanish. The barrel breathes, allowing the harsh volatile alcohols to escape from the cask. These are replaced with air to allow the spirit to oxidise. Oxidation produces many new compounds that lead to a complex aroma spectrum in the spirit.
It is the key factors of wood structure, the distillate, the interaction of the distillate with the wood and the ambient air, that determine the key differences in aroma and taste of finished whiskies.
Why oak casks?
Oak is the wood most commonly used in the maturation of whisky. Law states that Scotch whisky can only be defined as such if the clear ‘new make spirit’ to emerge from distillation is matured for at least 3 years in oak casks (with a maximum volume of 700 litres in a Scottish bonded warehouse).
Oak is flexible and highly durable and an oak cask will last for decades. It contains no resin channels, which makes the wood more porous, enabling the barrel to ‘breathe’. Undesirable flavours are also avoided in wood without resin.
There are over 400 different oak species of which only 12 are favoured in spirit maturation. American white oak grows faster and is much denser than its European counterparts. It has a low tannin content and produces vanilla and coconut aromas making for a sweeter profile. Oaks, mainly from France and Spain, are more porous, allowing for more intense maturation. This wood is darker with a complex spectrum of aromas, including leather and nut flavours. The high tannins in European oak provide a more robust flavour.
Japanese oak, which is much softer, imparts more aromatic wood tones such as sandalwood, cedar and agarwood.
New vs aged oak casks
New wood has a lot more influence on a spirit than wood from a second fill cask where the tannins and other flavour compounds have been depleted.
One of the many differences between Bourbon and Scotch whisky is that American law states that Bourbon must be aged in new casks.
This key difference emerged when the early Kentucky distillers would transport barrels by the Mississippi River. The long journey helped in maturing the whisky but returning the barrel became problematic. The cost of return passage for empty barrels was more than it cost to make new ones, so the tradition started.
Fortunately, Bourbon needs a significantly shorter maturation period, typically between 3 months and three years so the use of new wood can be tolerated. This is advantageous for the Scotch whisky industry as they often use surplus bourbon barrels from the USA. This is important as Scotch whisky must be matured for a minimum of 3 years, and fine whisky typically ten or older and new wood would not be suitable.