Scotch Whisky - The Importance of the Cask

I’m often asked questions about all aspects of whisky, no area more so than the cask itself and the effect which it has on a maturing a Single Malt and the finished product.

“A cask is a cask, is it not?”

“Does the cask really matter?”

“Other than size, does the cask type have significance?”

“What type of cask should I look for?”

The simple answer is that the cask is one of the most important factors and affects the end product immensely. It is certainly the most important aspect of the maturation process.

As governed by Scotch Whisky Regulations, to legally be called Scotch, the spirit must mature in an oak cask for a minimum period of 3 years. In truth, the maturation period is much longer for the vast majority, especially with Single Malt. The simple fact is that you can’t have a 12 year old Single Malt without that spirit sitting in an oak cask for 12 years (not yet at least, though I’m sure there’s a mad scientist or nutty professor out there somewhere trying to find a way)!

All casks are made from oak, two main variants being American Oak and the European Oak species (there are variations of European oak). Whilst Japanese oak is another variant, it is rarely used in Scotch. Ideally the wood used for casks has been grown for anywhere from 70 to 200 years, with American Oak trees being cut down from age 70, European Oak from 100 years. Huge significance is put on the quality of wood before it is used to create a cask, even being cut in special patterns specifically for the cask building process, to ensure that the cask can breathe and won’t leak. The wood is then heated and shaped to create a cask.

Casks are charred or toasted by the cooperage to release the various sugars within the wood, known as tannins and lignins (there are others), in turn giving flavours of vanilla and caramel, bitterness and sweetness in varying degrees.

When newly distilled spirit is first filled into the cask, it is clear and tastes very little of the finished product. That’s where the cask comes in. The cask itself is said to contribute as much as 70-80% of the character and taste of a Single Malt, coming directly from the wood used. As the spirit interacts with the sugars extracted from the wood and as the cask breathes, it essentially shapes the flavour profiles of the spirit, hence, two casks containing the same spirit from the same distillery can taste different.

Common Cask Types - Virgin Oak, Bourbon, Sherry, Wine, Rum

Whilst it’s true to say that casks can become tired, a good quality cask can be used for upwards of 30 years without issue, so it is very common to re-use casks. As such, not all Scotch casks are first filled in virgin oak, in fact quite the opposite is true.

Casks are expensive to produce, the Scotch Whisky market requires a lot of them. On the contrary, Bourbon regulations in the USA state that Bourbon must be matured for at least 2 years in new oak casks which have been freshly charred. Once the bourbon has been emptied from the cask between 2 and 4 years after filling, that cask has become redundant for the bourbon market. Hence, selling these casks to the Scotch distillers allows some revenue generation for the Bourbon market and a low cost cask for the Scotch market. The abundance of supply and lower cost means ex-bourbon casks are evidently very popular to fill with Scotch, and the Scotch benefits from the residual liquid left behind by the bourbon within the wood.

Casks which have previously held other alcohols are also commonplace and in a lot of instances, preferred by the end consumer. Generally these casks provide additional value, rarity, desirability and ultimately command a higher end price to the consumer. Some of the more popular alternative casks are Sherry, Port, Wine, Madeira, Rum and even Beer including IPA. Each of these options offers a welcome alternative to the end user and flavours the Scotch with certain characteristics, flavour profiles. These have all been adopted by the top distilleries in Scotland with certain bottles within these brands generating a real cult following.

Cask Sizes

Casks vary massively in size from the smallest, a blood tub, holding between 30 and 50 litres, through to Puncheons which can hold anywhere from 500-700 litres of fluid. Having said that, the most common cask sizes that you'll see in the market are 200 litre Barrels (also known as American Standard Barrels or Bourbon Barrels), 250 litre Hogshead Casks which are generally constructed by the cooperage from these Barrels. By increasing the size of the ring/hoop they can insert additional staves which allows for more capacity - around 4 or 5 Barrels can produce 3 new Hogshead casks. Finally the last cask size which is more commonplace is a 500 litre Butt cask, generally having previously housed Sherry and known commonly as Sherry Butts.

Coopers are masters of their trade and through their expert craftsmanship not only do they create the larger diameter Hogsheads from the smaller barrel casks, they can expertly review, treat and fix tired casks to ensure the longest life possible and the best results for the maturing whisky. One very common method of rejuvenating a cask prior to it being refilled is a de-char/re-char process where the inside of the cask is shaved slightly to release the new layer of tannins and lignins within the wood, then re-charred or baked before filling.

Finishing – Re-Racking

Whiskies are regularly changed from one cask to another during their maturation period. The process of changing the cask is known as re-racking, with the new cask used to “finish” the whisky. This could be moving a whisky from a Bourbon cask to a Sherry cask for the last couple of years of maturation, or alternatively moving a whisky from for example a Pedro Ximénez Sherry cask to an Oloroso Sherry cask to add further dimension and enhance the spirit and the taste of the Sherry. This is commonplace and is seen in a large portion of the bottle market, whether as a double cask, triple cask or a cask which states finished in… This again has risen a lot in popularity over the years and these bottles are seen on a shelf in every supermarket. 

The Angel’s Share

Maturing whisky within oak, a porous wood, and allowing the surroundings and the cask itself to “breathe”, ultimately means that you’re allowing the casks environment to play a part in the profiles. This is notably seen in certain bottles where there is a real taste of sea-salt from certain distilleries where casks have been matured by the sea. 

Similarly, this leads to an element of evaporation from within the cask as it interacts with its environment and the air, known as the angel’s share. The name comes from the fact that the angels want their quota from every cask of maturing whisky. This isn’t a bad thing, it is simply a slight reduction in bulk volume as the whisky within the cask gets better. The loss of spirit per annum is between 1-2% in bulk volume and approximately 0.5% per annum in alcohol percentage. With standard casks being filled at between 200-500L of liquid at an alcohol percentage of 63.5% to 69%, there’s enough to go around and it’s a small sacrifice to get the angel’s blessing.

Conclusion

Casks are extremely complex and a topic of breadth beyond the scope of this article though are an extremely important part of the maturation journey. Each step of every casks journey from tree to sitting in a Scottish warehouse, involves masters of their trade and is truly remarkable. This craftsmanship is hugely important in the make-up of each bottle of Scotch sitting on a shelf either in a home or supermarket.