The Whisky Club's Guide to Casks

The Whisky Club guide to casks

Whisky casks are more than storage – they’re flavour creators. From the type of oak to the previous contents, every detail influences the aroma, colour, and complexity of your dram. In this guide, we’ll explain the science behind maturation, explore the main types of oak used in whisky casks, and break down popular cask styles like Bourbon, Sherry, and Port. Whether you’re a whisky enthusiast or just starting your journey, this is your essential resource for understanding how casks shape whisky.

Casks

Casks play a vital role in shaping the flavour, aroma, and colour of whisky. This guide explores the different types of wood, oak, and casks used in whisky maturation and how each contributes to the final spirit.

Maturation Basics

Wooden casks are needed to make whisky and have a huge effect on the flavour of any final product. During time in an oak cask, we see three types of maturation.

• Additive maturation – this is what the oak gives to the whisky. Namely colour compounds, tannins as well as flavour and aroma compounds.
• Subtractive maturation – this is what we lose from a cask. Namely volatile compounds equating to about 2% of the volume of a cask each year. The alcohol volume will also change depending on the climate. In a humid environment like Scotland, the alcohol drops. In a dry rickhouse in Kentucky, the ABV can increase as more water is lost through osmosis. This is known as the angels’ share.
• Transformative maturation – this is the development of flavour through other chemical reactions over time. Basically flavours will develop and become more complex the longer you leave them.

All of these types of maturation are vital to the way a whisky tastes. Additive injects new flavour; transformative develops these new flavours as well as those already present and subtractive removes unwanted notes, but also concentrates the flavours left behind. 

What are the main types of wood used for making whisky casks?

The main wood type you will come across is oak. Oak is unique because it is watertight and it breathes. So, the casks hold water and allow the whisky to evaporate. The evaporation from a cask is known as the angels' share.

Oak is the only wood the Scottish can use when making whisky.

Ireland and Australia can use any type of wood. Acacia, applewood, chestnut and walnut have all been experimented with in the past. But oak remains the wood choice of the vast majority of distillers.

What are the main types of oak used for making whisky casks?

The main types of Oak for maturing whisky are:

• American oak – Quercus Alba – this is the main type of oak used for making casks. It has less tannin than its European cousins and gives aromas of vanilla, coconut, orchard fruits and lemon citrus.
• European oak – Quercus Robur – contains 5 x more tannin than American oak, and therefore delivers a darker colour and richer flavours including notes of dark chocolate, dried fruit and orange citrus.
• French oak – Quercus Petrea - less tannic than Spanish, more tannic than American. This delivers gingerbread notes as well as orange citrus.
• Japanese oak or Mizunara – Quercus Mongolica – more coconut, sandalwood and aromatic notes. Mizunara casks are difficult to find, difficult to work with and notoriously expensive. But the flavour profile is unique.

What are the main types of casks used for making whisky?

These are the main types of casks you will find on a whisky label:

• Virgin oak - virgin casks have never been used before, so the spirit takes on all of the flavours in the wood. Where the wood was grown and its genetic makeup will have a profound effect on the whisky maturing in the casks. Most American whiskies will be aged in virgin oak casks.
• Bourbon barrels - to call anything a Bourbon, it must be matured in a virgin oak cask. This means there are a lot of these second-hand casks available on the open market. Usually made from American oak, they deliver flavours of vanilla, fresh orchard fruit and lemon citrus to a whisky. Their abundance, affordability and flavour make them great choices for maturing whisky.
• Sherry casks - these are casks that have been seasoned with the Spanish fortified wine Sherry. The wine used for seasoning these casks is usually oxidatively aged, removing aromas and flavour profiles that are less desirable in whisky. They can give citrus, dried fruit and chocolate notes. Apera casks are those seasoned with an Australian version of Sherry and can deliver similar characteristics.
• Wine casks - these are casks that have once contained table wine, usually red. They give the whisky a pink colour, a tannic structure and often nice active oak characteristics. The low ABV of the previous contents do make them vulnerable during transport.
• Port casks - similar to Sherry casks, but seasoned with Port wine from Portugal. The style of Port (tawny, ruby or white) will deliver different characteristics. Expect white chocolate, berry fruits and in the case of Australian Tawny (not technically Port), deep rich plum pudding notes.

What size of casks can we use?

All barrels are casks, but not all casks are barrels. The size of a cask is important because the smaller the cask, the more of the whisky is in contact with the wood. This means the additive and subtractive maturation effects can be accelerated in a smaller cask. Transformative maturation effects remain largely unchanged, so the myth of accelerating ageing remains just that, a myth.

Here are the most popular cask sizes used for maturing whisky:

• Octave - 60L
• Quarter cask - 125L
• American standard barrel - 200L
• Hogshead - 250L
• Butt - 500L
• Puncheon - 500L
• Pipe - 500L

Here at The Whisky Club we give our Members as much information about the cask types and sizes we use.

Join The Whisky Club free today for exclusive whiskies you won't find at your local liquor store.

Cask FAQs

Why are casks important in whiskymaking?

Casks are essential because they shape the flavour, aroma, and colour of whisky through additive, subtractive, and transformative maturation processes.

What is additive, subtractive, and transformative maturation?

• Additive: Aromas, flavours and colours added from the wood (e.g. vanilla, spice, tannins).
• Subtractive: Volatile compounds, sulphur compounds and volume are lost over time (the “angels' share”). The remaining aromas and flavours are concentrated.
• Transformative: Flavours evolve and become more complex through chemical reactions during ageing.

Why is oak the most common wood used for whisky casks?

Oak is watertight yet breathable, making it ideal for maturation. It also imparts desirable flavours and is durable enough to withstand long-term ageing.

What’s the difference between American, European, and Japanese oak?

• American oak (Quercus Alba): Vanilla, coconut, orchard fruits.
• European oak (Quercus Robur): Richer, darker flavours like dried fruit and chocolate.
• Japanese oak or Mizunara (Quercus Mongolica): Aromatic, sandalwood, coconut, and incense-like notes.

What is a virgin oak cask?

A new oak or virgin oak cask has never been used before. It imparts strong wood flavours and is commonly used in American whiskey production, but more Scotch distilleries are experimenting with its use these days. These non-seasoned casks impart intense aromas of vanilla, coconut and spice into a whisky. The type of oak will impact the types of flavours delivered.

What are ex-Bourbon barrels and why are they popular?

These are second-hand American oak barrels previously used to age Bourbon. They’re affordable, widely available, and impart vanilla, citrus, and orchard fruit notes. An American standard barrel holds 200L or 53 gallons.

What flavours do Sherry casks add to whisky?

Sherry casks add notes of dried fruit, citrus, chocolate, and spice. They’re often used in Scotch whisky for richness and depth. The type of Sherry used to season the cask may have an effect on the final flavour, particularly sweet sherries like Pedro Ximénez and Moscatel. 

Are wine and Port casks used in whiskymaking?

Yes. Wine casks (especially red) add tannins and fruitiness, while Port casks contribute berry, white chocolate, and dessert-like notes depending on the style of Port used.

Does the Whisky Club feature whisky from different cask types ?

Absolutely. The Whisky Club procures whisky from lots of different cask types, many of them created exclusively for our Members. Here is a flight of 5 different cask types created exclusively for The Club:

• Jack Daniel’s Single Cask - a Tennessee whiskey aged in a single virgin oak cask
• The Glenlivet 17 Year Old First Fill American Oak Barrel - a single cask release from The Glenlivet matured for 17 years in a Bourbon barrel
• Westward Lamington Porter Cask - an American single malt matured in Lamington Porter beer casks
• Kilchoman Madeira Cask - an Islay single malt matured in Madeira wine casks
• The Glendronach 2013 Vintage Oloroso Cask - a Glendronach matured exclusively in Oloroso Sherry casks

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