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The Basics Series: Let’s Learn All About Casks

An essential feature of whisky is that it must be aged in a cask. Regulations in Scotland, England, Ireland and Japan specifically provide that the spirit cannot be labelled as whisky unless it has been aged for a couple of years (usually at least 3 years) in wooden casks.

In fact, casks are responsible for up to 60 percent of the resulting whisky’s flavour. This is due to the complex process of maturation where various chemical reactions occur.


(Image Source: Wikimedia Commons)

 In this post, we shall first discuss what happens during the complex whisky-maturation phase that occurs in casks. We shall then discuss the three main factors that can influence the taste of the whisky during the maturation process: (1) the predecessor liquid- whether bourbon or sherry, (2) the type of wood, whether American or European, and (3) the toasting and charring of the wood.


A deep dive into the whisky-maturation process

So, what exactly happens during the barrel-aging process? Maturation smoothens out the harsher notes in the spirit and creates more complexity in the flavour of the spirit. It is a complex process involving an interaction between the alcohol, aromatic substances in the spirit, the barrel wood, and remnants of the liquid that the barrel used to store (e.g. bourbon, sherry or other wines).

Three main types of reaction occur – (1) additive, (2) subtractive and (3) interactive.

Addictive. The spirit takes up flavours of the cask. Compounds from the oak (wood lignins, tannins and cellulose) impart flavours to the spirit lending notes of vanilla, caramel, toffee and even coconut. The bourbon or sherry residue in the wood also imparts flavours into the spirit, from the bright crème brulee sweetness of bourbon (due to high corn content), to the dark fruits, nuttiness and dryness of sherry.

Subtractive. New make spirit are harsher tasting and sometimes contains a tangy, metallic taste. The oak absorbs undesirable elements of the spirit including the pungent sulfur compounds, such as dimethyl sulfide. This removes the more aggressive new-spirit character and makes the spirit smoother.

Interactive. The compounds also interact with each other. And as oxygen diffuses into the porous barrels, reactions take place between the molecules in the spirit, and between the spirit and the wood. Alcohols and aldehydes are oxidised, and acids react with ethanol to form esters – which are some of the most aromatic of whisky flavour compounds responsible for fruity aromas. The lignin from the wood undergoes “ethanolysis”, where ethanol breaks down lignin into other flavour compounds including aromatic aldehydes. The by-products of earlier reactions also feed off other reactions down the line.


Casks upon casks of whisky at Bunnahabhain Distillery during a chilly January

To add to this complex equation, terroir certainly also affects the flavour of whisky. Apart from the source of the barley, the air and weather conditions in the area where the casks are stored have an impact too. During warm summer months, the barrels warm up and expand, while in the cold winter months the barrels contract, thereby "breathing" the surrounding air through the pores of the oak wood. Two things happen:

  • The air outside the cask is absorbed by the wood and oxidises the liquid, stimulating the creation of aromatic esters.
  • The environment and quality of the air also has an effect. For instance, on Islay with a coastal environment, the salty sea air is absorbed into the casks. This is why many seaside-distilleries (think Talisker) have a hint of maritime profile.



Three things that influence the whisky during the maturation process 

It is clear from the above that the barrel-aging process is extremely complex- It is impossible to fully grasp what happens.

That said, flavours imparted to the whisky during the maturation process are mainly affected by three main factors:

  1. Type of predecessor liquid (eg bourbon or sherry)
  2. Type of wood (eg American, European or Mizunara oak)
  3. Treatment of wood (toasting or charring)


Type of predecessor liquid


Sherry is a fortified wine made from white grapes that are grown near the city of Jerez de la Frontera in Andalusia, Spain


The peculiar thing about whiskies is that they are since time immemorial aged in casks that used to store another kind of alcohol – either Spanish sherry or American bourbon.

  • A brief history of ex-Sherry casks. As the story goes, whisky was once aged in neutral casks that did not store any predecessor liquid. However, Iberian casks of sherry found their way to British shores during the 1700s. Around the same time, industrious Scottish whisky distillers learned that the dark, sweet remnants in the empty sherry casks could do wonders for their whisky. Unlike neutral casks, these empty sherry casks imparting sweet flavours of dark fruits and nuts, giving young whisky a more complex flavour. Since this discovery, used sherry casks were relished and re-used by Scottish distillers.
  • A brief history of ex-Bourbon casks. The American coopering (barrel-making) and lumber industries suffered greatly during the era of Prohibition in the 1920s to 1933, and through the Second World War. At the end of the Second World War, the US Congress passed law requiring all American bourbon to be matured in new virgin American oak casks in order to give the coopering and lumber industries a boost.

    Indirectly, this US bourbon law resulted in a huge excess supply of empty
    bourbon casks had to be discarded by American bourbon makers after only one use. The same industrious Scottish distillers began buying these ex-bourbon casks at very low prices and ex-bourbon casks became the new thing in the whisky world.


Bourbon is a type of American whiskey made primarily from corn and matured in virgin American oak.


Ex-bourbon barrels are today the most common cask used for Scotch whisky. Why? Sherry casks are very expensive (up to 10 times the cost of a bourbon barrel). Although ex-sherry casks are very popular, many distilleries could not afford to use them on a regular basis. 

The choice of using ex-bourbon or ex-sherry casks greatly affects the taste of the whisky:

  • Using ex-bourbon casks tend to cause the spirit to take on a golden colour. It also imparts a light sweetness with notes of honey, vanilla, caramel and cream.
  • Using ex-sherry casks tends to cause the spirit to take on an amber red colour. It imparts more intense sweetness, spiciness and notes of dark fruits (figs, plums, raisins, prunes and black cherries).
    To go one step deeper – there are 3 main types of sherry - Pedro Ximenez (PX) sherry, oloroso sherry and fino sherry.
    • Pedro Ximenez sherry casks impart the sweetest flavours with dark fruits, raisins and syrup
    • Oloroso sherry casks impart a moderate sweetness with some dark fruits and earthy and nutty notes
    • Fino sherry casks are the least sweet, impart a leathery dry flavour notes of orchard fruits and light oak



Type of wood (eg American, European or Mizunara oak) 



Another factor affecting the final taste of the spirit is the species of the oak used.

American white oak (scientifically known as Quercus alba) genetically has much high levels of oak lactones. These oak lactones tend to impart flavours of vanilla, caramel, light spice and coconut. American oak also contains more of a substance called monogalloyl glucose which enhances the sweet vanilla bourbon taste.

European oak (scientifically known as Quercus robur) on the other hand provides a more intense earthy and oaky profile. It also contains some Gallic acid that imparts slight chocolatey bitterness.

Finally, there is the famous and extremely expensive Japanese Mizunara Oak (Quercus crispula) that provides a very distinctive and unique mixture of sandal wood, coconut and oriental spice flavours.



Treatment of wood (toasting or charring)


Every picture of the barrel-charring process looks like a heavy-metal concert (Image Source: Bulk Wine & Spirits)


The third important factor is how the insides of the barrel is heat-treated.

Why is heat necessary? Burning the barrel’s interior primes the oak and makes it easier for the spirit to interact with natural wood compounds. Heat causes structural elements within the oak- hemicellulose (a type of complex sugar occupying the spaces between plant cell walls) and lignin (a material that forms cell walls), to break down into simple sugars. These sugars are subsequently caramelised by the heat (via the Maillard reaction), resulting in a toasted caramel note. Heat also releases a variety of potent flavour compounds, including vanillin, oak lactones and eugenol which give flavours of vanilla, coconut-like notes and spicy cloves respectively.

Finally, heat also causes tannins within the oak to become less astringent, and creates a burnt layer of wood that absorbs sulphur and other pungent notes in the spirit.

Barrels are heat-treated in two ways. 

They can be intensely charred. This is typically the case for bourbon casks – which are charred over a flame for about 1 minute. The charred wood acts as a filter, changing or eliminating various unpleasant flavour compounds in the spirit, such as sulphur. Charring also tends to impart darker colours and sweeter flavours because the intense heat caramelizes more wood sugars and releases more vanillin, lactones and tannins, which are then imparted into the spirit.

How long the barrel is charred determines the char depth. The amount of char also determines the intensity of the flavours imparted. A barrel’s char level can range from No. 1 (medium char) to a greater char depth of No. 4, also known as “alligator char” due to its resemblance to the skin of an (duh) alligator. 

They could be gently toasted. This is typically the case for sherry casks – which are gently burnt over a flame so that the heat can penetrate deeply into the wood. Toasted barrels have a mellower flavour, imparting a spiced vanilla flavour.  

Without the charred layer to remove pungent components in the spirit, younger spirits tend to have a lighter colour and a sharper taste. 

Would a heavier heat treatment be better? Not necessarily, although the flavours imparted are much more intense, like equivalent of a dark roast coffee. However, over-charring can mean less complexity in the resulting spirit either because the wood flavours become overwhelming, or because the flavoursome compounds have degraded.



Shifting cask preferences 

Traditionally, whisky afficionados have associated the best whiskies with ex-sherry European oak maturation. Sherry casks add more flavour than bourbon, with their sweet, intense earthy flavours that provide fine foundation for an oily full-bodied Scotch. Sherry casks are also 10 times more expensive and therefore seen to be of more value.

Does this preference have more to do with historical weight than objective superiority of the Sherry cask over bourbon cask? We would think so. Preferences are somewhat subjective and constantly shifting as new drinkers enter the market.


(Image Source: Gear Patrol)


For instance, when Suntory experimented with the Japanese domestic market, it found that the Japanese preferred much lighter tasting malts that are delicate with gently fragrant and floral notes. Culturally they fitted in better with Japanese life. The typical Japanese serves whisky with meals or as refreshment even during hotter seasons.

Industry experts have also observed that they could appeal to the Asian palate with lighter and mellower styles. An expert notes that Some of the older vintages, say Yamazaki 25, are very rich. But overall, they’re looked at as friendly to the palate, easier to drink, a whisky you can drink with food.


(Image Source: Dewar’s)


Dewar’s is an example. As it seeks to appeal to the Asian palate, it releases its double-aged 15 Year Old blend that is sweet yet creamy flavour with hints of toffee, honey and fruit, with

a slight peat to it, but not too much that it overwhelms. It has explicitly stated that this profile is intended to cater to the Asian markets.


(Image Source: Chivas)


The Chivas Mizunara Cask Edition is yet another clear example. It comes with very light notes of pears and apples on the nose. On the palate, bright fruity and sweet notes akin to fresh orange zest. Notably, it is lighter-bodied compared to the other Chivas bottles that are catered to western audiences. 

Recently Irish whiskies have also seen a revival amongst millennials and young female pop stars owing to their light-bodied and bright flavour profiles.


(Image Source: Liquid Irish)


The popular shift towards lighter-bodied and smoother malts also means that the bourbon cask is set to become much more popular (if not already) than the expensive traditional sherry cask. Going beyond bourbon and sherry, the Irish whiskey industry has an even bigger appetite for experimentation with other cask styles and different wood. Tullamore D.E.W. has found significant success with its Caribbean-Rum- and Cider-Cask-finished blended whiskeys. Midleton Distillery’s Method & Madness range experiments with French chestnut wood (which is non-oak), and ex-Cognac barrels.

What are the cask styles of your favourite whiskies? Do read their labels and check them out!