Notes on Wood, Barrels and Whisky
Wooden barrels or casks have been used since before Roman-times for both the storage and shipping of goods.
Coopering, the art of barrel making, is an ancient skill that today has been made more efficient and less injury prone for the cooper, by combining both hand and machine tools in the process.
Oak is traditionally the preferred wood used in wet coopering, making the barrel adequate for storing liquids, as it's cellular structure makes it particularly watertight when it is cut correctly. There are primarily two types of oak that are used in the making of barrels, American White Oak (Quercus alba) and European Red Oak (Quercus robur). American white oak grows faster and is considered to add soft, mellow, vanilla flavours to the spirit during the maturation process versus a spicy, bitter flavour, due to the higher tannin contents found in European oak.
The Scottish whisky industry depends on repurposing and reusing staves from either the bourbon industry in the United States or the sherry and wine industries in Europe. In general new casks are produced with oak boards that have been kiln or air dried (i.e. seasoned) to reduce the moisture content and tannins in the wood. These boards are then cut into staves with trapezoidal wanes. In order to bend the wood into its barrel shape without cracking, the wood is steamed. Once shaped into a barrel, the inside is thermally treated in order to ‘breathe life’ into it. Toasting at high temperatures (approximately 200 degrees Celsius for 30 minutes) results in the breakdown of the wooden cell structure releasing sugars and vanillin already present in the wood. Lastly, the inside of the barrel is charred for 3 to 5 minutes and then extinguished with water. This combined thermal treatment (toasting and charring) allows for the extraction of flavour from the wood. Charring creates a layer of charcoal that filters out unwanted flavours in the maturation process. Whisky matures faster in smaller casks because the exchange between the wood and the whisky is accelerated due to the larger surface area relative to the liquid.
Coopers in Scotland mainly reuse staves from the bourbon industry, generally American Standard Barrel (ASB), which hold approximately 200 liters. ASB staves are also reassembled and refitted with bigger hoops to produce the 'Scottish Hogshead' which holds approximately 250 liters. A bourbon cask can only be used once in the Bourbon industry (approximately 2-4 years) and yet in Scotland it can be re-used and the wood re-activated or rejuvenated 3-4 times through re-charring leading to a further use of an additional 30 years.
In the beginning years of maturation, the freshly distilled alcohol (new spirit) loses some of its undesirable flavours. This is referred to as subtractive maturation. Over time this transitions into additive maturation, where the alcohol begins to take on the flavours offered or activated by the thermal treatment of the wood. Vanilla, toffee, oak, are some of the flavours that can be found in whisky that has matured for 8 years or more in a wooden cask that has been repurposed from the bourbon industry. More recently, some distilleries have begun to ‘finish’ their whiskies by pouring whisky that has matured in a bourbon cask for a number of years to a sherry or wine cask for an additional 1 to 2 years. Finishing whisky in this manner adds other flavour and colour characteristics to the maturation process of whisky offering complexity and variety to the final product.
This research trip was made possible with funding from a Mary Elvira Stevens Traveling Fellowship administered by my alma mater, Wellesley College. Importantly, I must thank Richard Chalmers and Nial Mackinlay for facilitating my access to the various sites I visited: Speyside Cooperage (Craigellaiche), Speyside Cooperage (Alloa), Balblair Distillery (Edderton) and Old Pulteney Distillery (Wick). I want to also thank the following people for their generosity, time, and expertise in showing me around: Gill Reid, John MacDonald, Russell Angus, Kathy Csorogi, and Steven Langlands.
Here are some links to see and learn more about these places: