Technical Stuff Whisky

Types of Casks and How They Influence Whisky

There are many factors that contribute to the final flavour of whisky. In Scotland the spirit must mature in casks for a minimum of three years before it is legally allowed to be called whisky. In this time the spirit will be heavily influenced by the cask it’s been in, therefore it is important to understand how these casks contribute. The type of wood used, age, size and the previous liquid in the cask all matter. It is said that 60–80% of the taste comes from the cask.

Below you will find further information on just how each cask can influence whisky. Anne-Sophie aka The Whisky Lady also kindly offered some recommendations on whiskies influenced by a range of cask types.

Type of wood

Most whisky casks are made from either American white oak or European oak. American oak gives a softer, sweeter taste with notes of vanilla and caramel, while European oak is spicier and has a stronger wood input. European oak grows in northern Spain and Portugal. French oak is used to age wine and cognac. It will bring notes of vanilla, pepper and subtle spiciness.

Cask influence

Cask size

The bigger the cask, the longer it takes the liquid inside to mature.

Barrel, 180–200 litres, also known as an American Standard Barrel (ASB), is commonly used for bourbon. These are made from American oak.

Hogshead, 225–250 litres, is made by taking part ex-bourbon barrels to create one bigger cask. The Scotch whisky industry often prefers larger casks for ageing as this allows more whisky to be aged in the warehouse.

Madeira Drum, 600–650 litres, is made from French oak and used for Madeira wine.

Port Pipe, 550–650 litres, is made from European oak, and as the name suggests, is used to age port wine.

Sherry Butt, 475–600 litres, is made from American oak, although some are made from European oak. Butt is the most common size used for ageing sherry.

Barrique, 250–300 litres, is widely used in the maturation of wine and cognac. Barrique is mainly made from French oak.

Toasting and Charring

All casks are toasted, but not necessarily charred. A new oak cask goes through a heat treatment, toasting, before any liquid is put inside. Toasting will caramelise the wood sugars. This process brings those lovely vanilla and caramel notes from American oak and releases more tannins and spices from European oak. These flavours will eventually get mixed into the liquid.

The next step is charring of the casks. The charcoal inside the cask will help to mellow the sharp flavours from the distillation. There are different levels of toast and charring, and each distillery has its own preferences. Bourbon barrels are often charred for around 40 seconds, but in some cases up to three minutes. The higher the char level, the more it allows the spirit to get into the pores of the oak.

The charcoal will eventually stop working and the cask will need to be treated again. As casks are very expensive, rejuvenating them allows further usage of the wood. Obviously, these casks will have a different impact on the flavour of the next batch, and the colour can also vary. It is said that a good oak cask can last up to 100 years, so you really get a few fills with that!

the macallan toasting
Photo credit: The Macallan

Number of fills

Bourbon must be aged in new oak, so when whisky is aged in a 1st fill ex-bourbon barrel, this means it is the first time that malt whisky has gone into that barrel after bourbon. In this case, the whisky will be heavily influenced by the wood, and therefore the spirit is only left in the barrel for a short period to avoid heavy wood influence. This can be around five years.

In Scotland, the same cask can be used for years and years, although it is uncommon for the cask to be refilled more than three times. So obviously, after the 1st fill you will have a 2nd fill, which averages around eight to twelve years, and, confusingly, the third and final fill is known as the refill. After the 3rd fill, the inside of the cask can be shaved to show new wood before re-toasting and charring.

Previous liquid

Bourbon

Bourbon barrels are made from American white oak. They go through both toasting and charring before the bourbon is added. Also, the ABV level of the bourbon will impact the flavours of the barrel – the higher the ABV, the bigger the impact on the wood. Basically, the higher alcohol level will strip the barrel of the benefits of toasting and charring, leaving less flavour for the next spirit.

After Scotch whisky has spent some time in an ex-bourbon cask, the Master Blender will mix together whisky from various barrels. These can be from the 1st, 2nd or the 3rd fill. Whisky aged in ex-bourbon barrels has a golden colour.

Tasting notes: coconut, vanilla, honey, caramel, oak, leather, nutmeg, almond, butterscotch

As most whiskies are aged in ex-bourbon, recommending is a bit tricky as the options are endless. However, if you need some ideas check these – Kavalan Solist ex-bourbon cask, Scapa Skiren, Bruichladdic Classic Laddie, The SMWS Peat Faerie.

whisky casks

Sherry

The sherry industry avoids using new oak, and most of the casks used in Solera are at least ten years old. These casks have been seasoned with wine before being used for sherry. Different varieties of sherry get their unique flavour through a range of ageing methods. For example, oloroso has contact with air, which contributes to the fruity and nutty flavours, while fino is protected from the air by a layer of yeast, also known as flor, and this keeps the sherry light and crisp. Pedro Ximenez (PX) gets its sweetness from the raisins; the grapes are left to dry in the sun before being fermented.

It is not uncommon for a whisky distillery to buy European oak (mainly Spanish), have the casks coopered in Jerez and filled with any sherry until they have enough sherry influence to be used in whisky making.

There may be up to ten litres of sherry left in the cask. It simply soaks into the wood, which will have a huge impact on the flavour and colour of the whisky. The inside of the cask is usually kept slightly wet while it is being transferred. Other contributing factors are the number of fills and the type of oak used. The more fills, the less of those dried fruit and Christmas spice flavours you’ll get.

The whisky is often ‘finished’ in a sherry butt to allow just enough influence without making the sherry flavour overpowering.

Tasting notes: fig, dates, raisins, nutty, cherry, cinnamon, clove, marzipan, ginger, fruit cake

The Whisky Lady recommends: GlenDronach 12, Aberlour A’Bunadh for a cask strength version, Kilchoman Sanaig (peated)

sherry butts

Port

There are several types of port available and each has its unique style. Tawny port is golden in colour and is mainly aged in older wooden casks to allow gentle oxidation. While tawny port can be aged in oak for up to 40 years, vintage port only spends two to three years in the cask. Ruby port is mainly matured in steel tanks to avoid oxidation, therefore ex-ruby port wooden casks aren’t widely available. And when ruby port is aged in wood, the maturation time is kept short. Port can be aged in large port pipes or smaller port barriques.

Port cask ageing will add smoothness and fruity notes to the whisky. Flavours will vary depending on the style of port previously aged in the cask. Port pipes are mainly used to ‘finish’ whisky.

Tasting notes: plum, blackberry, sultanas, redcurrant, dried apple, oak, cranberry, dark chocolate

The Whisky Lady recommends: Glenmorangie Quinta Ruban, Talisker Port Ruighe (p

Cognac

Cognac casks are made from French oak from the forests of Limousin or Troncais. These casks will bring richness and subtle fruitiness to the whisky. Cognac casks tends to be very old as cognac can be aged up to 50 years.

Tasting notes: sultanas, liquorice, oak, rich fruits, nutty, caramel

The Whisky Lady recommends: Brenne French Single Malt, Teeling Revival V

Rum

Rum cask ageing has been fairly popular in recent years. No wonder, as these casks add yummy, sweet tropical notes into whisky and tend to be easily available. Rum casks are made from American oak. There are a wide range of rum styles, therefore each will have a unique influence on the spirit. Rum casks are used to ‘finish’ whisky before bottling, to allow just enough of those fruity and baking spice aromas. Rum-cask-finished whiskies also make a great addition to cocktail making.

Tasting notes: vanilla, tropical fruit, apricot, caramel, cinnamon, ripe banana, pepper, molasses

The Whisky Lady recommends: Balvenie 14 Caribbean Cask, Ardbeg Drum (peated)

Wine

Wine casks are usually made from French oak. Both red and white wine casks can be used to age whisky, although this is still not that common, and these bottlings tend to be limited editions. The flavour range varies based on the grape variety; some red wines are spicier and full-bodied while others are lighter or fruitier. White wine can bring out a buttery texture and grassy flavours.

Tasting notes: red berries, cherry, plum, bread, caramel, green apple, pear, buttery

The Whisky Lady recommends: White wine – Arran Sauternes Cask, Glenmorangie Nectar D’Or & Red wine – Port Charlotte MRC:01 (peated), Red Spot

WD storage

Storing the cask

During maturation, the flavours of the spirit blend with natural compounds in the wood cask to give the whisky its own specific flavour and aroma. If the warehouse is next to the sea, on an island or in the middle of the Highlands, then the air quality, temperature and humidity will be different and will influence the end product.

The barrel location inside the warehouse will also have an impact depending on the light and temperature it is exposed to. A higher temperature will increase the angel’s share and speed up the maturation time. When the temperature is high the oak’s pores will open up, allowing the liquid to interact more with the wood. During each year of maturation, the angel’s share is about 2%; this means some of it is lost through natural evaporation.

Bunnahanbain barrels

Overall

By playing around with different cask types and sizes, the Master Blender can create layers of flavours. They can use a range of the same casks, for example, different fills of ex-bourbon, or mix and match a variety of cask types from bourbon to rum. When it comes to the age of whisky, longer maturation doesn’t necessarily mean better quality; there are simply too many factors that contribute. Trying whiskies that are bottled at cask strength, without chill filtration or other interference, will allow you to enjoy whisky in a raw, untouched way and to get a better understanding of the cask’s influence.

Which cask types do you prefer? Please leave your bottle recommendations below.

*Some of the links used are affiliate links. If you buy through the links, I may receive a commission for the sale. This has no effect on the price for you.

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