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Saturday 27 August 2022



What gives whisky its unique taste, colour, mouthfeel and aroma? The barrel — more specifically, the inside of the barrel. You can get all the steps right from the very beginning — the best grain, a pristine water source, a flawless distillation sequence — but it’s the maturation process that gives the spirit its character. And the barrel’s interior plays a big part in that. This article comes up again in a later post.

In Scotland, the spirit must mature in casks for a minimum of three years to be legally called whisky. In this time frame, the spirit is influenced by the cask it’s been matured in, which is why it is important to understand how these casks contribute. Obviously, the longer the time spent in the cask, the more the influence and hopefully, the better the quality and final taste of the whisky. The type of wood used, age, size and the previous liquid in the cask all matter. Different types of wood add different elements to the contents. The innate qualities of the new make are given the required finesse and final colour and taste by the cask.

Given the advancement in technology, it is possible to identify the different flavours using physicochemical methods. Even the smallest of variations can account for big differences in taste. That's how receptive our senses can be. Researchers measure the various substances like esters, tannins, furfural, congeners, lactones, etc., in ppm (parts per million) and are now reaching out to ppb (parts per billion) in a nanometric world.


A wooden cask is a small masterpiece of craftsmanship. Although machines are increasingly used by coopers today, actual manufacturing is still done by hand. The planks for the staves aren't simply cut at right angles from a log, like construction timber. The grain direction of the wood must be taken into account so none of the radial vessels of the wood penetrates the side of the cask. Not only does wood contain annual rings but also vessels that lead from the core to the bark radially. The tree transports water and nutrients through these vessels. However, these vessels are inconvenient for whisky since they would make the staves leaky and allow excessive evaporation. Therefore the wood has to be cut in special patterns (star cut, mirror cut or rift cut) so the annual rings stand vertically. This is why far less usable wood can be cut from one log; a cask stave is thus much more expensive than a normal plank.

Yet another aspect has to be factored in- how the new make reacts with the staves of wood. Alcohol is insidious-it attacks from within, albeit at some micrometres per day. As it reaches into the wood, it meets both intractable and amenable wood. The alcohol slowly absorbs minute quantities of specific substances it can dissolve and integrates them with the new make. The more the reaction, the more the absorption. That said, the longer the stay in the barrel, the greater the evaporation as the Angel's share.


These planks are then made into staves with trapezoidal vanes (according to the roundness of the planned cask). The newly made staves must then be dried until they reach a level of less than 10% residual moisture. This may be left to nature and solar heat or done quickly in modern drying chambers without affecting the quality of the cask.

If the casks were made from this wood, you would get a tight container, but the whisky cannot mature. From a maturation standpoint, the wood is still dead. Specific thermal treatment breathes life into the wood, as will be explained in depth later. The wood can be bent into the typical cask shape only with heat. The wood is heated up to 200°C in a big oven for approximately 30 minutes, and the firm wood structure is broken up. The cask begins to live in terms of maturation. Once the staves have been bent into shape, the cooper completes his job of assembling that cask.


A cut through a treated stave will reveal a red ring in the wood beside a charcoal layer of several millimetres depth, the so-called 'red layer'. This layer separates the 'activated' from the 'natural' wood. It is up to this layer that the heat has penetrated the wood and activated it for flavour extraction.

Only oak wood is suitable for cask production. Soft wood contains resin, which prevents the cask from breathing. Other types of wood have congeners that emit unpleasant flavours, making the whisky unpalatable. Oak wood from trunks with an age of 70 to 200 years is ideal. There are two fundamentally different species of oak: American white oak (Quercus alba) and the various European oak species (Quercus general). American white oak grows faster and has a mellower, softer and sweeter taste with notes of vanilla and caramel. In contrast, European oak is spicier with full, intense aromas and more tannins and has a stronger wood input that is prominent on the nose.

European oak grows in northern Spain and Portugal. French oak is used to age wine and cognac. An American oak tree can be cut down at 70-100 years, while the slower-growing European oak must grow for at least 150-200 years. Five major factors influence the Whisky in the cask: 

  • Type of predecessor liquid
  • Size of the cask
  • Type of wood
  • Level of charring
  • Reuse of the casks (First Fill vs Refill)



Prior Spirit

Alteration of Taste

Colour Alteration 



vanilla, sweetness, caramel, creamy




very fruity, slightly sweet, slightly dry

dark red


Fortified Wine

spiciness, light fruitiness, sweetness, dryness

dark, amber

Port (sweet)

Fortified Wine

sweet, dried fruit, spiciness


Port (semi-dry)

Fortified Wine

lightly sweet, dried fruit, spiciness


Port (dry)

Fortified Wine

dry, dried fruit, spiciness




deep, dark, nutty, dark ripe fruits

red, amber

Pedro Ximenez


very sweet, dark fruits, raisins, syrup




light fruits, sweetness, dryness, light wood




salty, dryness, sea flavours, fresh, some fruit




sweetness, nutty, dry, fresh, acid


Palo Cortado


rich, sweet, dry, sweet spices, fruits




sweetness, zest, acidity, light fruits

bright to amber

Bordeaux (red)


strong red fruits, grapes (wine), berries




light fresh fruits (citrus, mango), very sweet,

bright to amber

Ruby Port

Fortified Wine

very fruity, dark fruits, berries




fruits, tannins (bitter), dry fruits, heavy aromas




lean, crisp, acidic, tropical fruits




floral , sweet, citrus, peach



Fortified Wine

very sweet, dark fruits, raisins, syrup


Rum (white)


sweet, molasses, vanilla, tropical fruit, almond


Rum (dark)


sweet, syrup, dark fruits, oak, caramel, vanilla




tannins (bitter), dry, raisins, ripe fruits



Fortified Wine

sweet, complex, spices

dark red

Virgin Oak


vanilla, cloves, caramel

dark brown


The sizes of casks are awkward to define because there is no ISO standard regulating the volume of a standard cask. In the interim, a cask type taken as the baseline is the American Standard Barrel (ASB), which holds approximately 200 litres. ASBs are also the 'raw material' for the production of Scottish hogsheads, which hold approximately 250 litres. If you disassemble an ASB and use slightly bigger rings (hoops) for reassembly, you can produce casks with a bigger diameter from the same staves. Out of four to five ASBs, you can make three hogsheads. Since the trapezoidal shape of these staves is designed to contain 200 litres, you can’t make casks with even bigger diameters from them. The casks would eventually become leaky at the rifts.

There is another problem with the volume of the casks as the cask sizes are also a unit of measurement. Take the Butt for example. The normal butts come in sizes of 500 litres (132 US gallons). But there is also a measurement unit called a butt, which is 1/2 a tun and is 122 US gallons (477 litres).

Besides the type of wood and the thermal treatment, viz., the size of the cask influences the maturation process. Whisky matures faster in small casks since more of the content is in contact with the wood, as compared to large barrels. The exchange of substance between wood and whisky takes place faster.

In Spain and Portugal, European oak is made into casks holding 500 to 600 litres, which are ideal for the maturation of Sherry and Port. The Scots call these casks 'butts' or 'Sherry butts' and 'Port pipes'. Today more and more butts are made from American oak for financial reasons.

The following table shows the actual sizes the most barrels come in, not the sizes of the measurement units.



US Gallon

Imperial Gallon

American Standard Barrel /Bourbon Barrel








Quarter Cask




Standard Hogshead








Madeira Drum




Port Pipe (tall)




Sherry Hogshead




Cognac type




Bordeaux type




Barrique cask








Cask Sizes Displayed at Edradour Distillery

Casks are also grouped by volume. The Scotch Whisky Association has ruled that no wooden cask in excess of 700 litres may be used. The breakdown is listed below:

Size of Casks

Type of Casks


> 400 Litres (>132 US gallons)

Butt, Port Pipe, Puncheon, Madeira Drum


200 - 400 Litres (53 - 106 US. Gallons)

ASB, Bourbon barrel, any Hogshead, Barrique cask, Cognac cask, Bordeaux cask


<200 Liters (53 US. Gallons)

Quarter cask, Bloodtub


Type of Wood

Effect on Taste

American white oak (Quercus alba)

mellow, soft, vanilla, caramel

European oak (Quercus robur and petraea)

spicy, bitter, strong on the wood

Mizunara Oak (Quercus crispula)

sandalwood, coconut, oriental spices

Maple (Acer)

sweet, maple syrup

There are major differences between the two primary types of oak. American white oak grows in the east of the United States of America and a few parts of Canada. The tree grows rather fast for an oak tree and is therefore a bit less expensive than the European counterpart. Its wood is very dense (770 kg/m³) and contains a lot of monogalloyl glucose. This is later evinced as the typical Bourbon vanilla taste.

European oak grows all over the European continent far into Russia and Turkey. It grows slower than its American counterpart and is less dense (720kg/m³). It contains Gallic acid which is considered a pseudo-tannin. This acid in combination with water gives the whisky a slightly bitter note. The European oak has also a lot of other components that also add to the spiciness of the whisky.

Mizunara oak is very common in the forests of northeast Asia, where it is used primarily for high-quality furniture. Due to its high density and thin fibres, the staves must be cut along the fibres, which never grow perfectly straight. The Oak also lacks waterproofing oil enzymes, so much more whisky evaporates during ageing than when stored in American or European oak. Despite these characteristics, which make it much more difficult to use for whisky barrels, Mizunara oak gained popularity for whisky ageing in the early 20th century, because of the special aromas it gives off to whisky, reminiscent of sandalwood, coconut and oriental spices, for example.

Maple grows in many parts of the world, including Eurasia, North Africa and Central & North America. With its relatively low density of 653 kg/m3, it is used primarily in the production of Tennessee Whiskey and is not used elsewhere.


The barrel itself must be prepared beforehand. This is done by burning the insides. While this process may relatively straightforward, there are distinct two types of burning procedures that whisky makers use when crafting their barrels: toasting, and charring.

Heat causes hemicellulose within the wood to break down into natural sugars, resulting in toasty caramel notes, colour and aromas, while oak lactones add woody, coconut-like notes.

When exposed to heat, tannins become less astringent while oak lignin (a fibre within the wood) breaks down into flavour molecules like eugenol (spiced/cloves) and, more famously, vanillin (vanilla).

Charred Barrel

When comparing the two methods, the fundamental variable is the degree to which the wood is burned. Charred barrels, for example, are heavily burned and resemble the remains of a campfire after it has been extinguished. The interior of charred barrels is black and has much more ash residue, resulting in a much darker colour for the whisky. As far as flavour goes, charred wood imparts sweeter flavours like caramel and honey. The reason for this is that the wood sugars are caramelised when heavily burned, and thus they leach into the whisky.

Moreover, the carbon in the ash acts as a filter for the harsher elements of the liquor. Ageing whiskey (like bourbon) in charred barrels results in a smoother, mellower flavour.

At times, a barrel is given a number reflecting the degree it has been charred to on a scale of 1-4, i.e., a new American white oak barrel with a #4 char. This “barrel char level,” as it is called, basically means the higher the number, the deeper the burning char into the wood. There’s a lot of science around this; in essence, the darker the char, the more different the flavour profile and the more colourful the whisky will likely be.

Toasted Barrel

Unlike their charred brethren, toasted barrels are heated much more gently, resulting in a dark brown toast rather than a blackened char. Toasted barrels add a bit more vanilla flavour to the liquor, as well as spicy accents. Because the wood hasn’t been heavily burned, the sugars haven’t had time to caramelise, making the whisky a bit sharper on the tongue. Toasted barrels also do not impart much colour to the spirit, resulting in a lighter shade.

Toasting is a slower process. It involves gently heating the barrel’s interior over an open flame so that the heat can penetrate deep into the wood, which mellows and takes on a dark brown colour.


Oak casks are very durable and can contain Whisky for many decades. However, over the years the aroma that the Whisky can absorb from the wood decreases. The cask is leached out. Whisky that is stored in new casks, therefore, absorbs the most aromas.

In the first (1st) fill casks, the term ‘1st fill’ doesn't mean the original filling of the cask with Bourbon, Sherry or Port, but the first Scotch Malt Whisky that's filled into a cask. No work is done on that cask either. This way, the 1st fill extracts the strongest flavours from the wood.

Bourbon is mandatorily matured in fresh casks and is aromatic enough to be bottled after only two years, leaving a still well-endowed cask. Scotch Whisky is matured in used casks, which have already been used, e.g., Bourbon or Sherry. The cask has already released a large part of the aromas. Therefore the Scottish Malts only become really good after longer maturation. However, this also gives the Whisky more time to reduce the alcoholic spiciness. Nowadays, this historically developed system is an essential distinguishing feature between the Whisky types.

Casks are an expensive commodity. Therefore it is common in the whisky industry to mature whisky in casks more than once. A cask still contains a lot of aromas after ten years of Malt Whisky maturation and is therefore reused for the next Malt. In the industry, they are called 'refill casks'. Refill casks are reused for up to about 30 years.

It is natural that the influence of the cask on the taste weakens progressively. It is no surprise if a whisky from a refill cask has absorbed very little flavour - and also colour. On the labels of some bottlers, you sometimes find indications like 'Refill' or 'First Fill', which reflects the degree of use of the casks. This gives you as a customer an indication of the approximate intensity of the cask aroma. However, it is also common to add colour to whisky with tasteless caramel. In this case, the influence of the cask is hidden.

It is more and more common to refurbish casks, as this is cheaper than buying new ones. This brings out the oak aroma again. Sherry, Port or Wine aromas cannot be reproduced in this manner. These 'rejuvenated' casks make whisky very spicy within a short time. They are most often used to produce NAS whiskies or single malts for blending. 

Data, videos and images courtesy