The types and role of whisky casks
Casks are essential for maturing and storing whisky. Explore the various vessels and cask types of the whisky world.
Author: Dave Broom, World-Renowned Whisky Expert & Writer
Estimated reading time: 5 minutes
After making whisky, distillers either store it or sell it. In both cases, they must put the spirit into an oak cask, which can be used for storage or maturation.
There is, however, a difference between storage and maturation. The latter refers to deliberately putting the spirit in the cask to alter its flavour. This became standard practice for whisky in the 19th century.
Scotland’s distillers drew on the experiences of cognac and rum, which had to be transported long distances to the UK. In the process, the spirits acquired colour, mellowed, and developed new flavours. By the mid-18th century, producers were deliberately ageing rum and cognac for quality reasons.
Oak was preferred for casks because it’s watertight and porous. It is also strong but can be coopered easily. The trunk of a 100-year-old tree is sawn or split into staves, which are seasoned in the open air to remove astringent compounds. The staves are then bent into shape by heating and wetting.
As the wood is being toasted, its sugars caramelise. This toasting must happen if you want to get flavour. At this point, the cask ceases to be just a vessel for storage or shipping and becomes an active participant in a whisky’s final complexity.
Signature Flavours and Characteristics
There are different types of oaks, and each gives its own signature character.
- American white oak - The widely used species is Quercus alba, aka American white oak. Most casks used for maturing Scotch are made from this. It is high in vanillin (giving vanilla flavours) and oak lactone (giving coconut flavours), as well as sweet spice and marzipan.
- European oak - The other main oak type is Quercus robur, aka Spanish (or European) oak. It has higher levels of tannin, a richer colour, and aromas of incense, dried fruit and clove. These casks are first seasoned with sherry.
- French oak - The third species is Quercus sessiliflora, aka French oak. It has a vanilla undertone and a more pronounced spiciness. These casks will have held either wine or cognac.
Cask and Barrel Guide
People tend to call wooden casks, barrels; however, a barrel is a specific size and shape of cask. Put simply, a barrel is a cask but not all casks are barrels. These are the most widely used:
- Gordas are ex-sherry and probably made of European oak.
- Butts (500l) are ex-sherry and mostly made from European oak but can be made from American White oak.
- Hogsheads (250l) are nearly always American white oak. They have extra staves and new heads.
- Barrels (180 -200l) are always American white oak; most used to mature bourbon.
Note that the largest cask permitted by law today is 700 litres.
The Marriage of Spirit and Cask
The interaction between the spirit and cask impacts 60% of a whisky’s final flavour. Initially, the cask removes the aggressive elements and sulphur compounds in a new spirit. Some of these evaporate, while others are absorbed into the charcoal layer on the inside of ex-Bourbon casks.
Simultaneously, the cask is breathing. Air pressure pulls the whisky into the wood and out again. With each inhalation, the liquid absorbs flavour and colour. In time, the oak-derived flavours and those of the spirit begin to move together, producing greater complexity.
The intensity of those flavours will depend on two things:
- The length of time the whisky has been in the cask.
- The freshness of the wood.
Bourbon can only be aged in brand-new casks, whereas Scotch whisky takes these once-used casks and reuses them.
Knowledge Leads to Better Flavour
Knowing what flavours are released in a specific type of cask allows distillers to build up a greater range of complexity in the final bottling. A first-fill sherry cask will be different to a first-fill ex-Bourbon, and a refill cask of each type will be different again.
In single malt, the key is to not allow the oak to overwhelm the distillery character. Most Diageo malts use refill casks to accentuate the distillery character. You can pick out the rich fruity signature of sherry casks in brands such as Mortlach, Singleton of Glen Ord 18-year-old, and Johnnie Walker Black Label.
A brand-new cask is filled with flavour. Each time it is filled, there will be less in the wood to share with the maturing spirit. Each fill gives different flavour possibilities, and this is one reason why age is not a determinant of quality.
An age statement simply tells you how long the youngest whisky in a blend has been in a cask. It’s a measurement of time. Maturity is a measurement of character and that is determined by how active that cask has been.
- Oak is the preferred wood for making casks as it’s porous and watertight.
- There are three types of oak wood that are mostly used: American, European, and French.
- The interaction between a cask and the spirit affects 60% of the final flavour.
- Age is not a measurement of quality but signifies the age of the youngest whisky in a blend.
- Bourbon can only be aged in brand-new casks, while Scotch whisky can re-use ex-bourbon casks several times.
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