As stated in the Scotch Whisky Regulations 2009, Scotch whisky has to be matured in oak casks for at least 3 years. But most distilleries mature their whisky for much longer to achieve the best flavours possible. Why do distilleries go the extra mile by letting their whisky sit in a wooden cask for years and years to come? Why do they spend so much money and effort to get the best quality casks?
Let’s explore the last part of every whisky production. Find out why maturation is considered one of the most important production steps by distillers and whisky drinkers alike.

A bit of maturation history

Historically speaking the benefits of maturation were actually discovered by accident. Wooden casks have been used to store literally everything. The shape of the cask made its transport a lot more practical. Instead of lifting the heavy goods you could roll them from A to B. Oak proved to be the best wood for casks to store liquid in as it is extremely dense and tight. So when distilleries needed to store whisky, oak casks were the obvious choice.

Up to this point people drank whisky mostly straight from the still. It was quite a rough and harsh drink back then. In the 1800s wine and spirit merchants selling whisky have noticed that whisky stored in wooden casks changed in flavour over time. Their whisky became milder, more pleasant and easier to drink. Occasionally merchants and distilleries started making use of the improvements and stored their whiskies for some time in casks. The most used casks were wine or sherry casks as they were easily available.

But it wasn’t until 1915 that ageing whisky became a law. The man putting it forward was prime minister David Lloyd George whose aim it was to reduce drunkenness amongst people. He introduced a law that whisky had to be matured for at least 2 years before it was legally whisky. The minimum age requirement was increased to 3 years in 1916. Lloyd George thought if he increased the production time it will make people drink less during the draining years of World War I.
Ironically his law on the long term achieved the exact opposite. It laid the foundations for making Scotch whisky more refined in flavour, more palatable and resulting it to gain even more popularity.

Casks outside the Speyside ready for maturationCooperage
Casks outside the Speyside Cooperage

The maturation process

Casks in maturation warehouse at Cardhu
Warehouse at Cardhu

After filling casks with fresh new make, they mature in a dark, cool warehouse. Over time the whisky sinks into the first layers of wood. That enables the liquid to extract flavours and gives it the iconic golden colour. That happens because wood is not airtight and there is a molecular exchange between the whisky, the cask and the air surrounding it. When temperatures rise the pores of the cask open up and the whisky sinks deeper into the wood. This allows more flavour extraction. Lower temperatures tighten the pores and press the whisky back out. The wood basically acts like a sponge, slowly releasing more flavours and more colour over time. As a matter of fact 60-90% of flavours in your whisky come from the cask it is matured in (numbers vary from source to source).
Unfortunately whisky also falls victim to evaporation while it matures. The damp and cool warehouses in Scotland provide a very stable storing climate to limit the affects. But even under those optimal conditions 0.5-2% of alcohol evaporate each year. Distillers call these losses the angels share. Other lighter molecules evaporate as well, leaving less and less whisky in the cask over the years. That’s why older whiskies are so much more expensive. After 30, 40 or even 50 years, there is only a very small amount of whisky left in the casks. The remaining drops are extremely valuable and very dense and rich in flavours.

During maturation the stored casks are monitored regularly by nosing and tasting samples. When the age is right and the flavours match the expectations and standards of the brand, they empty the cask and prepare the whisky for bottling. Before entering the bottles whisky is often diluted with water to the preferred alcoholic strength. Most distilleries bottle at an ABV around 40-46%. If distillers don’t add water to whisky and fill it straight into bottles, it is bottled at cask strength. In that case the ABV can be as high as 60% or more.

Oak varieties

Oak Tree from a distance
Oak Tree seen from underneath

For centuries oak has been the preferred material for casks. It is a very strong and dense wood. It can withstand heavy impacts, is water resistant due to its tight pores and is less prone to rot. All of these attributes make it the perfect container to store whisky in for very long periods of time.

Mainly two types of oak are used for maturation:

American White Oak (Quercus alba)

Casks made from American oak do not interact as much with the spirit stored in them because of their extremely tight pores. Wine and Sherry manufactures prefer that since they want to store their products with less wood influence.
If more interaction is preferred, for example by Bourbon distillers, they have to char the inside of the cask to break open the first layers and allow liquids to sink in. But more about that later.
Generally American oak is said to give vanilla, caramel and coconut flavours to whiskies.

European Oak (Quercus robur)

There are different types of European oak that are used for whisky maturation.

  • European Oak (Quercus robur) – Also referred to as French oak and the most common of the European varieties
  • Sissile Oak (Quercus petraea)
  • Spanish Oak (Quercus pyrenaica)

European oak is less dense than its American relative. Therefore casks made from this wood can absorb liquids easier and interact more with them. Due to the easier interaction more flavour compounds can be extracted without charring the surface. This results in more spice, pepper notes and dry tannins in your whisky.

Charred and toasted casks


Because oak is such an incredibly dense wood, liquid normally wouldn’t penetrate it very much or not at all. For possible interactions between whisky and oak, the first layers inside the oak casks need to break up.
To achieve that, the inside of casks are fired up for a short amount of time. The tightly woven structures of the oak break apart and the surface cracks open. The longer the exposure to fire and the higher the temperature the more the oak gets burned which often results in a heavily burned and charred layer. That’s why this process is called charring.

Those charred cracks allow the whisky to sink very deep into the wood and it creates more surface for the whisky to interact with. But this is only one of many reason why distillers want to char their casks.

charred wood

The important compounds in the wood for flavours are hemicellulose, lignin, tannins, and oak lactones. The long sugar chains of hemicellulose caramelise when exposed to temperatures of 140°C or higher. This makes the sugars soluble and can be absorbed by the liquid filled into the cask.
Lignin contains vanillin (vanilla and some spices). Oak lactones are also called whisky lactones because they are very apparent in whisky. They show themselves through coconut and cream flavours.
Tannins are the dry, bitter notes we know mostly from wines and the most harsh ones are filtered out by leaving the wood dry outside for a long time, exposed to the weather. High alcohol spirits like Bourbon also strip out tannins from wood. Sherry on the other hand leaves more tannins inside the cask due to its lower ABV. This results often in drier notes in Sherry cask matured whiskies.
Last but not least the charcoal particles filter out unpleasant compounds. That keeps the spirit inside the cask “clean” and also affects the colour, providing a much darker amber and brown tone.


On the downside, heavy char breaks apart a lot of lignin, containing the desirable vanilla flavours. It also breaks down the oak lactones with sweeter wood notes and coconut flavours, which distillers want to extract as well. To achieve that a more gentle heating process has to be applied: toasting.
When toasting a cask, the temperature applied is lower than for charring but the exposure time is increased a lot. It allows the heat to crawl deeper into the cask without burning the surface. It’s like when you are cooking a steak at home. With higher, intense heat you get a charred crust and the inside can still be raw. Or you can slow cook the meat first to get an even cooked steak to ensure it’s fully cooked but not overdone.
So in a way distillers slow roast the cask to get a deeper heat penetration without destroying the outer layer of the wood. The gentle heating breaks free the wanted vanilla flavours and spices from the lignins. The sugars of the hemicellulose on the other hand do not caramelise at the applied temperatures yet. If distillers want more vanilla instead of sweet toffee and caramel notes, toasting is the way to go.

When to apply which method?

American oak casks are heavily charred when used for Bourbon maturation. This way the spirit can interact with the wood, which is exactly what whisky distillers want. Additionally the US Federal Regulations for “Labeling and Advertising of Distilled Spirits” states that casks must be charred before use.
European oak casks on the other hand are just toasted, usually for wine maturation. European oak does not need as much opening to let liquids in and only the vanilla and wood notes are wanted from the wood.
Sherry casks of both oak varieties, American and European oak usually get a light or a heavy toast, since Sherry is not supposed to interact too heavily with the cask.

Sometimes both methods are applied to a cask, having vanilla and spices freely available all the way to the centre of the wooden staves. Then the outer layer are heavily charred, destroying some of the vanillins in the outer layers but in exchange caramelising the sugars for the sweet caramel and toffee notes. And it leaves the vanilla notes deeper inside the wood fully intact. Then the whisky can reach both layers and extract all the wanted flavour compounds equally.

Bourbon casks

The most commonly used casks are made from American White Oak and are used for Bourbon whiskey maturation before being shipped to Scotland. With the end of Prohibition a new law was enforced in 1935 by which American distillers have to mature their Bourbon in new, heavily charred oak casks. This way the unions ensured the American barrel makers (coopers) to always have enough demand to keep their jobs. But instead of throwing the casks away after they have been used, Bourbon distilleries sell their used, their ex-Bourbon casks, to Scotch whisky distilleries who are not subject to the same restrictions.

Benefits of Bourbon cask maturation

In fact the Bourbon takes out most of the harsher flavours of the new oak (also called Virgin Oak) that would otherwise overpower the flavours of Scotch whisky. After the extraction of these rougher flavour compounds, the cask interacts more gently with the spirit. After each use casks interact less and less with the spirit, providing less flavour and colour than before. It’s like using a tea bag 2 or 3 times.
After the ex-Bourbon cask arrived at a Scotch whisky distillery, they are filled with fresh new make spirit. Now the whisky matures in a 1st fill ex-Bourbon cask. If distillers use the same cask again for another maturation cycle, it is labelled as Refill or 2nd fill ex-Bourbon cask. This system applies to all casks in whisky maturation. Distilleries use the casks 2-4 times, as long as the wood yields flavours of interest for them and as long as the wood holds up. In between the periods of maturation they get checked for damage, such as cracks or leaks. In that case coopers have to repair casks by substituting broken staves with intact ones. Those staves are made from the same oak and has been used the same amount times.

Inside Speyside Cooperage
Inside the Speyside Cooperage: This is where whisky casks get repaired and prepared for the next maturation cycle.

The flavours distillers expect to get out of ex-Bourbon casks are mostly vanilla, caramel, cinnamon and coconut. Vanilla and caramel develop inside the wood during the charring or toasting process through caramelisation and get absorbed when the spirit soaks into the little cracks of the wood. Bourbon already absorbs many of these very sweet compounds, and it can take intense flavours easier without getting overpowered by them. Scotch whisky is more delicate and would be way too sweet to actually taste the spirits “house” character.
When these flavours mix with the sweet, fruity character of a Scotch distillery new make it can form flavours like honey, apple, pear, banana sometimes citrus notes from acids developping during fermentation and then combining with fruity esters and caramelised sugars inside the cask. Also cereal flavours of different sorts can come through from the malt itself.

Sherry casks

Casks in Glenfarclas warehouse for maturation
Big sherry butts with a capacity of 500 litres, now filled with whisky in a Glenfarclas warehouse

The use of Sherry casks for maturation before goes back to the mid 1800s, when it was more popular than today. Sherry traveled in wooden casks from Spain to the UK to be then  bottled by local companies. Distillers use the emptied Sherry casks afterwards to store their whisky. Eventually merchants and distillers noticed that the flavour of their whisky greatly improved when stored in Sherry casks. And when in the 1860s the Phylloxera vastarix pest destroyed vineyards all over Europe, disrupting the wine and brandy industry, the normally used wine casks became scarce. That’s when Sherry casks gained another boost in popularity. Sherry manufacturers made good financial use of the high demand for their casks, even though the popularity of Sherry itself kept dropping from the early 1900s onwards.
But with the end of Prohibition in the US and the new law regarding their cask usage in 1935 most Scotch whisky distillers turned towards the much cheaper and suddenly much wider available Bourbon barrels, leaving the Sherry industry in a big crisis. Eventually in 1981 a law was introduced to stop bulk shipments of Sherry to the UK and from 1986 Sherry had to be bottled in Spain to be legally called Sherry. This could have become the final nail to the coffin of Sherry production, but luckily it was not!

A Sherry revival

With the new rise of Scotch whisky in the 1980s the demand for Sherry casks grew again. Some distilleries started to finish their whiskies in different casks while other distilleries, like Glenfarclas and Glendronach for example, preferred to fully mature their spirit in Sherry casks providing a different style from the Bourbon cask matured whiskies. In fact the demand grew so much over the past decades that some producers make very simple Sherry just to season their casks with it and then sell them for £1000 or more to Scottish distillers. The cheap Sherry itself sometimes even is poured away as it brings no profit at this low quality level.

Sherry casks can be made from both American oak and European oak. Sherry manufactures want less interaction with the cask and prefer American oak due to its tighter pores. That being said European oak casks were the preferred choice for shipping the big bulks of Sherry to the UK up until 1986. And because they absorb more liquid and as a result give more flavour during whisky maturation, Sherry casks made from European oak are whisky distillers preferred choice. So Sherry manufacturers use European oak more often for shorter Sherry “seasoning” to meet specific flavour demands by whisky distillers. A beneficial agreement for both Sherry and Scotch whisky producers.

Flavours from Sherry cask maturation

Since different Sherry provides different flavours, the flavours that eventually will end up in your whisky depend a lot on the type of Sherry that has been stored in the cask before.

Oloroso: Probably the most popular of the available Sherry casks. It provides rich, nutty notes like almonds or walnut, spicy flavours along with intense red and dark fruit notes.

Pedro Ximenez: A very, very sweet Sherry. Casks that held this style of Sherry before, also referred to as “PX cask”, deliver much sweeter notes than Oloroso casks do. Whisky matured in PX casks usually contains flavours such as dark toffee, fig, raisin and molasses.

While these flavours are generally associated with those types of Sherry, all the factors mentioned on this page have their own impact on the flavour of your final whisky. If you char a casks more or use a different oak type can make as big of a difference as the former content of the cask.

Other casks

Aside from Bourbon and Sherry casks distilleries use a wide variety of different casks to compliment their whisky, to experiment and to offer new flavours and styles to whisky drinkers. Some of the more popular ones are listed below with their characteristic flavours whisky can get from them:

  • Port casks (fortified wine from Portugal): Berry flavours like strawberry, raspberry and dry spices like cloves and nutmeg.
  • White wine casks (mostly French Sauternes): Peach and apricot, pineapple and other tropical fruits and citrus.
  • Red wine casks: Red fruits like cherry, grapes and raspberry. Often goes along with dry spicy, peppery notes, too.

But also Champagne, Brandy, rum and even beer and cider casks are used from time to time, constantly giving people something new to try and more to explore.

Perfect visualisations of the effects of different casks on whisky over 30 years (top left to bottom right). Pictures taken at the Glengoyne distillery.

Some words about colour

After we have been exploring all the effects the cask has on the flavours of whisky it should also be noted that 100% of the colour is coming from the cask. The longer it sits in the cask, the darker the colour gets. Bourbon casks impart a golden to amber colour while Sherry casks give whisky a darker, more red tone. So the colour of the whisky can give you some information about how long it has been maturing for and what type of cask it was matured in.
While this is a good rule of thumb, some distilleries add caramel colouring (E150a) to their whisky to make it appear darker. There is no need to worry though as it has no impact on the flavour. E150a is basically burned carbohydrates which turn into caramel. It is burned to such a dark colour that it doesn’t have the attributes of sweet caramel anymore and if anything it would just taste bitter if tasted purely. A few drops can darken a liquid a lot without any impact on the flavours.
The reason some companies do that has to do with the assumptions of some cultures and their markets, that darker whisky equals older and therefore equals better whisky. Especially in the US and Japan people respond better to darker whiskies. Another benefit is consistency. It makes sure that every bottle of the same whisky in each shelf around the world will have the exact same colour.
Some whisky enthusiasts are not a big fan of the use of colouring. They think that the taste is all that should matter and there are a some heated debates about that.