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Tuesday 28 May 2024



By now you know how the type of grain, distillation and ageing process affect the flavour of your favorite tipple. The science behind whisky is a crazy and convoluted process that can make all the difference in your drinking adventure. The lowly barrel your whisky is aged in plays a massive role in shaping its taste. By understanding the science behind whisky ageing, you can develop a deeper appreciation for this fabulous spirit.

Types of Whisky Barrels

There are so many options when it comes to ageing whisky, it's enough to make your head spin! We're talking about different types of barrels, each with their unique personalities that can impact the final flavour and aroma of your beloved whisky.

The realm of whisky ageing is a perplexing and bursting one, with a plethora of options to choose from. The most prevalent vessels used for this purpose are oak barrels, which provide a porous surface for oxygen to interact with the whisky, producing unique flavours and aromas. American oak is the most commonly utilised variety, while Scotch and Irish whiskies are known to be aged in American and European oak barrels.

But why limit oneself to the ordinary? Bourbon barrels, for instance, are oak barrels that have already been employed for ageing bourbon. The law mandates that bourbon must be aged in new charred oak barrels, and hence these barrels are used just once for bourbon production before being sold to whisky distilleries. This results in an unpredictable and exciting ageing process for the whiskey.

The most common casks are made of oak, specifically American oak, Quercus Alba, also known as white oak. Europe also has many varieties of oak, also referred to as Quercus robur, Quercus petraea, Quercus sessiliflora, Quercus pedunculata, aka European oak, English oak, French oak, Slovanian oak, Polish oak, common oak or pedunculate oak. In Japan, distilleries also use Japanese oak, known as Mizunara oak (Quercus mongolica).

Put differently, there are approximately 600 species of oak tree present across the world, including hybridised versions. North America houses the greatest concentration of oak tree species, with 90 different varieties growing in the United States and 160 growing in Mexico (though 109 of these are endemic). The second largest centre of diversity in oak species is China, which has 100 species. An oak tree that produces acorns on a stalk (peduncle) is pedunculate; a stalkless species native to the UK is named the Sessile oak.

                                AMERICAN OAK                                                                     EUROPEAN OAK

Both European and American oak are cultural icons, adorning coats of arms, emblems, flags and pub names throughout Europe and America. Both are hardwoods sharing the same scientific genus, Quercus. And, like all oaks, both species reproduce with acorns, each carried in a shallow cup. But, to the more discerning eye, there are a number of key differences between these two types of oak.

White American oak differs from European oak in its vanillin content and wide grain. A wide-grained oak imparts more flavour into the spirit, and its high vanillin content lends vanilla, coconut, and spicy notes typical of oak cask whisky.

                              EUROPEAN OAK                                                                     AMERICAN OAK

Boasting warm, honeyed golden browns, European oak generally has a darker, richer complexion than American oak, which tends to display a lighter tan colour with some pinkish and yellow hues.

European oak also has a more distinctive, wavy grain pattern with more prominent knots and swirls, providing character and rustic charm. You may see some instances of burr, a unique and eye-catching type of figuring. On the other hand, American oak’s less pronounced, straighter grain pattern is more uniform.

With a natural product like timber, some variation in tone is to be expected. In this regard, European oak is typically more consistent, with tight grain — American white oak can display a greater variation from board to board. The visual appeal of American oak is down to this colour variation rather than its grain.

Most new barrels produced are used to finish and age bourbon, as new charred white oak barrels are a designated requirement. After a single use, the barrels are resold, often used to mature other spirits like whisky, tequila, rum, sherry, etc.

As a spirit spends time in a barrel, it undergoes incredible changes in flavour, texture, and bouquet. Depending on the age of the barrel, the whisky will take on varying degrees of “extract” from the wood. The younger the barrel, the more extract it will produce. Thus, the whisky (or other spirit) will be more intensely flavoured.

Indrink: Then there is, of course, the Devil’s Cut, or indrink. This is the amount of spirit a barrel soaks up depending upon its porosity and period of holding liquid. A bourbon or sherry butt may hold a fair amount of bourbon or sherry in its inner recesses, but these are met up with in time by the spirit new in that butt. In the former case, most distillers try and extract the bourbon to the limit possible. Most distillers retain the sherry soaked in the barrel as it adds its distinctive colour and taste to the maturing spirit.

The extract weakens and passes on more subtle flavours and less colour as the years pass. That said, even an older barrel has a powerful effect on the whisky maturation process. Once a barrel has been used four or more times after reconditioning in a cooperage, it is considered “neutral” in terms of extract, but even a neutral barrel will add to the texture and mouthfeel of the whisky. These old barrels are generally used for grain whiskies.

Because of the evolutionary nature of whisky barrels and since they are hand-crafted and individually charred, no two are alike. This is why master distillers choose to blend whiskies from several barrels. Whereas one barrel might be newer and have more aggressive flavours and char intensity, whiskies from older barrels will provide balance and unique layers of flavour that would not be possible from a single barrel.

The craft of blending whisky is based in tradition and a source of pride for every distiller. Each batch can be as singular, unique, or consistent as they want, based on their meticulously managed barrel programme and masterful blending skills.

Moving on to other unconventional options, sherry casks are barrels that have already been used for ageing sherry. These casks, usually fashioned from European oak, are capable of imparting a range of flavours, from fruity to nutty, to the whisky. Port casks are yet another type, being barrels that have already been used for ageing port wine. Typically crafted from European oak, they can give sweet and fruity flavours to the whisky, providing a truly distinctive experience.

In addition to these, wine barrels offer a multitude of opportunities to experiment with. Whisky can be aged in barrels that have previously held a variety of wines, from red wine to white wine to Champagne. The type of wine used for ageing plays a significant role in influencing the flavour and aroma of the whisky, resulting in a truly remarkable and unparalleled experience. Barrels that have matured other spirits are also used, like Cognac, Armagnac, Tequila and Rum, among others. (See chart.)

So there is a plethora of options to choose from, each with their own distinctive twist on the ageing process. Distilleries love to mix and match different types of barrels to create the perfect blend of flavours and aromas, so don't be afraid to buy a whisky matured in exotic casks. Glenmorangie and Macallan experiment widely.

The Ageing Process

When whisky is first distilled, it's just a plain, colourless liquid with no flavour or aroma, typically called ‘New Make’. Contrary to common belief, all New Make is collected in huge metallic containers and hauled off to either the chill filtration or maturation plant. New Make is then poured into barrels, up to a pre-determined level and closed.

This is where the ageing process comes in to save the day! It's the barrel that gives your whisky its mesmerizing colour and unique flavour profile. The New Make is placed in barrels, and the wood interacts with the liquid, creating a range of chemical reactions. The primary factors that influence the ageing process include the type of wood, the previous contents of the barrel, the level of char, and the length of ageing. Here are the steps in the ageing process of whisky barrels:

  • Extraction: The liquid in the barrel extracts flavours and aromas from the wood.
  • Oxidation: Oxygen in the air reacts with the whisky, leading to the creation of new flavours and aromas.
  • Evaporation: A small amount of whisky evaporates over time, leading to a more concentrated flavour.
  • Filtration: The whisky is filtered through the wood, removing impurities and smoothing out the flavour.
  • Maturation: Over time, the flavour of the whisky becomes more complex and nuanced.

The duration of ageing is crucial! Most whiskies age for at least three years, but some are lucky enough to be aged for decades. Can you imagine the flavours and aromas that come with that? The longer the whisky ages, the more perplexing and bursting with flavours it becomes, but up to a point. The whisky must be extracted while it is still drinkable. Non-stop ageing leads to very low ABVs and the possibility of the wood jarring the overall taste.

The whisky scene is a globally open playing field for distilleries to get creative with their craft. Regulations state that oak must be used for ageing whisky, but distilleries have the freedom to experiment with a variety of different oaks to create unique flavours and aromas in their whisky. And with the minimum three-year ageing rule and cask size limit of 700 litres, distilleries have the freedom to craft their spirits with creativity and innovation, resulting in an explosion of imaginative and diverse whiskies. The result is a whisky scene that's bursting with personality and creativity, where each distillery has its own distinct character and flavour profile.

The Effects of Different Seasons on Ageing

Different seasons affect the ageing process of the whisky. First up, the sizzling summer! As the heat rises, so does the whisky in the barrel, leading to an intense dance between the liquid and the wood. The result? A whisky bursting with more flavour and aroma than ever before! But the heat can also cause more evaporation, leaving behind a higher concentration of alcohol and flavours.

In the the cooler autumn/fall season, the whisky just chills out in the barrel, quite literally! The cooling temperature causes the liquid to contract, which means less interaction with the wood. As a result, the flavours and aromas are less concentrated. But this just means you get to enjoy a more laid-back and mellow whisky.

In winter, the barrel cools even more, causing the liquid to contract even further. This limits the interaction with the wood, resulting in fewer flavours and aromas being extracted. However, the slower ageing process leads to a smoother and more mellow whisky.

Finally, we have the vibrant and lively spring season! As the temperature rises, the whisky expands, resulting in more interaction with the wood and the creation of new flavours and aromas. Talk about a party in a barrel!

The fluctuations in temperature and humidity throughout the seasons can also impact the wood in the barrel, causing it to expand and contract. This makes the whisky move in and out of the wood, further impacting the flavour and aroma profile.

In the end, each bottle of aged whisky is unique and distinct, thanks to the magical touch of the different seasons.

The Impact of Temperature and Humidity

Temperature and humidity significantly impact the ageing process and can have a considerable influence on the final flavour and aroma of the whisky. The combination of different temperatures, humidity levels, and seasonal variations can create a unique flavour and aroma profile for each bottle of aged whisky making the ageing process both complex and multifaceted.

  • Temperature: One of the critical factors that affect the ageing of whisky. Higher temperatures can cause the liquid to expand, leading to more interaction with the wood and the creation of more intense flavours and aromas. Conversely, lower temperatures can slow down the ageing process, resulting in a smoother and more mellow whisky.
  • Humidity: It can also have a significant impact on the ageing process of whisky. High humidity can cause the wood in the barrel to expand and contract, creating more opportunities for interaction with the liquid. This can lead to the creation of new flavours and aromas in the whisky. On the other hand, low humidity can cause the liquid to evaporate more quickly, leading to a more concentrated and intense flavour profile.
  • Seasonal variations: The changes in temperature and humidity due to season can also impact the ageing of whisky. As the temperature and humidity fluctuate throughout the year, the liquid in the barrel can expand and contract, creating new opportunities for interaction with the wood. This can result in a more complex and nuanced flavour profile for the whisky.

The Role of the Distillery

The entire process of making whisky, from selecting the grains and water to distilling and ageing the spirit is handled by the distillery. It must carefully manage the entire production process to create a high-quality whisky with a unique flavour and aroma profile. Distillery roles include:

  • Selection of grains: This includes selecting barley, corn, rye, or wheat. The grains can impact the flavour and aroma of the final product.
  • Mashing and fermenting: The mashing of grains and adding yeast on it creates a mash that can be fermented. The fermentation process converts the sugars in the mash into alcohol.
  • Distillation: The distilling of fermented liquid removes impurities and concentrates the alcohol. The type of still used in the distillation process can impact the flavour and aroma of the final product.
  • Ageing: Selecting and preparing the barrels that will be used for ageing the whisky. Monitoring the temperature and humidity of the ageing room ensures the proper ageing of the whisky.
  • Blending: The blending of different whisky barrels create a consistent flavour and aroma profile for the final product.
  • Bottling: The whisky is bottled and labelled with the necessary information, such as the age, type, and alcohol content.

The Significance of Cask Size

The size of the cask used for ageing whisky can have a significant impact on the flavour and aroma of the final product. Distilleries must carefully select the size. Here are some of the ways that the cask size can influence the ageing of whisky:

  • Surface area: The larger the cask, the smaller the surface area in contact with the liquid. This can slow down the ageing process and result in a smoother and more mellow whisky. Conversely, smaller casks have a larger surface area in contact with the liquid, leading to a faster ageing process and more intense flavours and aromas.
  • Oxygenation: Smaller casks have a higher ratio of liquid to air, resulting in more oxygenation and the creation of more intense flavours and aromas.
  • Wood interaction: Smaller casks have a more significant impact on the liquid, resulting in more intense flavours and aromas.
  • Temperature: Smaller casks can heat up or cool down more quickly than larger casks, impacting the ageing process.

The Barrel Ageing Timeline

The ageing timeline of whisky can vary depending on several factors, such as the type of whisky, the type of cask used for ageing, and the environmental conditions in the ageing room. The general timeline for the whisky ageing process goes like this:

  • Ageing for a minimum of three years: By law, most whiskies must be aged for a minimum of three years, two in Australia. During this time, the whisky takes on the flavour and aroma of the cask, with the wood interacting with the liquid to create new flavours and aromas.
  • Maturation: As the whisky continues to age, the flavour and aroma profile become more complex and nuanced. The length of maturation can range from three to over 30 years, depending on the desired flavour and aroma profile.
  • Peak flavour: At a certain point, the whisky reaches its peak flavour and aroma profile. This is when the distillery decides that the whisky is ready to be bottled and sold.
  • Over-ageing: After the whisky has reached its peak flavour and aroma profile, it can continue to age in the cask. However, over-ageing can lead to a loss of flavour and aroma, making the whisky less desirable.

The ageing timeline can vary depending on the type of whisky and the type of cask used for ageing. For example, Scotch whisky is often aged for longer periods than American whiskey. Additionally, the environmental conditions in the ageing room can impact the ageing timeline. Overall, the ageing timeline of whisky is a complex and dynamic process that involves many factors, resulting in a unique and complex flavour and aroma profile for each whisky.


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. Most of the flavours sit in this layer, the section right underneath the char. Liquid stored in deeper-charred barrels takes longer to interact with this layer, meaning the flavours are extracted from the barrel far more slowly. This facilitates a smoother character over long ageing periods. Whisky stored in lighter-charred barrels interacts with the red layer much faster, leading to a quicker rate of flavour extraction. This helps pack the whisky with sought-after notes over a much shorter ageing period.

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 FLUID


Prior Spirit

Alteration of Taste

Colour Change 



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

                                                   SIZE OF THE CASKS

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



Maturation in casks to provide taste, colour and provenance is one of the main reasons whisky is so widely popular across the globe. Interactions between the organic compounds in newly distilled spirit and the chemical compounds in wood produce a wide and complex range of flavours, and aging it in different types of wood can create incredible variations in taste. This is a crucial fact to pay regard to, since the raw make that enters the Spirits Safe has an obnoxious taste which has to be cautiously and continually nurtured, first to an acceptable and then to enticing taste. Evidently, whisky gets much of its flavour from the wood it’s aged in. If many different types are used, what do they add? The key points about whisky maturation are:

  • The number of times the cask has been used–the more times, the less flavour will be imparted.
  • The size of the cask–the greater the surface-area-to-volume ratio, the more flavour the whisky will get.
  • The time spent in the cask–more time means more flavour extraction.
  • The intensity of the spirit–a lighter style will get more influence from the wood than a heavier one.


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.



The ageing process can be broken down as follows:

  • The temperature in the warehouse fluctuates day-by-day and during the changing seasons
  • The resulting pressure change forces the whisky into the pores of the wood by way of the charred interior surface
  • The whisky reacts with air/water and undergoes a chemical reaction that breaks down some of the organic compounds in the wood
  • Some of the ethanol and water evaporates as part of the reaction and exposure to air causing the “Angel’s Share” to leave the barrel
  • The wood constituents and whisky diffuse back into the barrel as the warehouse environment fluctuates
  • Convection currents mix the contents of the barrel and the whisky turns darker brown as the cycle repeats


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.

Some distilleries will even put a cask back into service for a fourth filling, depending on the spirit and intended purpose.  5th and 6th fill casks definitely exist, but it’s considered uncool to talk about them. Most of them land up in the three-year grain whisky maturation process.

With the massive boom in the whisky industry and demand for casks at an all-time high – plus the cost of casks increasing accordingly – tired casks that might once have been discarded are now being rejuvenated to get a second lease on life. The internal surface of the cask is scraped back to fresh wood, then re-charred to re-instate the all-important charcoal filter. “De-char, re-char” is the common phrase and process in Scotland, and many casks will also receive a level of toasting prior to re-charring, in order to better break down the wood, release the vanillins, and soften the tannins. An example of Loch Lomond techniques regarding 'Charring' will follow. Casks or whisky releases noted as STR undergo this process, with STR standing for Shaved, Toasted, and Re-charred.

Long-term maturation in 2nd fill casks allows the magical interactive process to really play out without the additive process becoming over-bearing or upsetting the balance. Single malts matured from 15 to 25 years in 2nd fill casks produce some of the finest results.

Glenfarclas prefers 2nd fill casks, and believes that the best Glenfarclas whiskies come from refill casks.  Glenfarclas, of course, is famed for maturing its whisky in ex-sherry casks, and the distillery is careful to ensure that 1st fill casks aren’t left for too long.  The danger is that the sherry-influence would over-power and dominate the spirit.  Most releases in the Glenfarclas portfolio (e.g. the 10yo, 12yo, 15yo, 17yo, etc, etc) are thus a careful vatting of casks that combine 1st fills and re-fills; American oak and European oak.  Macallan goes down a similar route with its Double Cask range, further introducing the influence of ex-bourbon casks in its Triple Cask Matured range


Maturation of whisky in oak is a three-pronged action that requires three different processes to occur. These three processes may be summarised as being additive, subtractive, and interactive. They are worth elaborating on:

Additive: The cask will add flavour and character to the spirit.  Wood sugars, oaky notes, tannins, the influence of the previous filling (e.g. sherry), and colour will all be added to the spirit.

Subtractive: The cask will subtract certain volatiles and compounds from the spirit, making it more mellow, softer, and approachable.  Casks are typically charred before being employed in the drinks industry; the charred wood acts as a natural filter that removes undesirable compounds such as sulphur.  (As an aside, this is why the distilleries that produce a more sulphury new-make often benefit from longer years in the wood).  Other volatiles in the spirit, some of them undesirable, will be lost through evaporation.  The “Angels’ Share” is the lighthearted term given to evaporation, but it’s a serious matter for whisky producers in warmer climate countries:  Filling a 200 litre cask with your precious spirit and only having 100 litres of it left in the cask after five years is a challenging issue.  

Interactive: This is where the magic happens. Certain chemical and molecular reactions occur between the wood, the spirit, the residuals left by previous fillings, and – most importantly – oxygen, to create new flavours/aromas. The casks breathe; the spirit oxidises; and the oak and spirit transform one another. This interactive process can be where the different oak species produce different results, as they vary in the nature of their chemical and biological make-up: The differing characteristics of the wood’s density, permeability, hemi-cellulose, lignins, tannins, oils, lipids, and so on, will all impact the spirit differently. Despite the modern Scotch whisky industry being more than 250 years old, commentators and industry professionals believe that this third interactive process is only just now starting to be understood from a scientific perspective.

Time is therefore of the essence, and due time is needed for all three of these processes to play out.  Three years is the minimum period of maturation stipulated by law in Scotland, which was set as a minimum benchmark for quality….although the climate and production regimens in Scotland generally dictate that most whisky will take 8-15 years to peak.  Climate and environmental conditions (namely heat and humidity) obviously have a large influence on maturation – particularly evaporation – and so different countries factor this into their production methodologies and timescales.  There’s a truism that whisky matures faster in hotter countries, but it’s important to understand the difference between true, three-pronged maturation and simply base evaporation and taking on colour.

Evaporation over time is a complex problem for distillers, and it differs the world over, subject to the local climate – particularly the local humidity.  The conditions in Scotland lead to the alcohol evaporating faster than the water within the spirit, meaning that the alcohol content of the whisky decreases with time.  In contrast to this, the conditions in Kentucky, USA, lead to the water evaporating faster than the alcohol, meaning that the alcohol content of the spirit actually increases over time! 

One of the challenges for the so-called new world whisky producing countries is to find the balance or sweet spot with their maturation programs whilst juggling the many forces, demands, and financial factors at play for start-up operations.  For new distilleries wanting to bring aged whisky to market sooner and bring in much-needed cash flow, there is temptation and a growing tendency to adopt small-cask maturation in active casks to supposedly fast-track the process.  However, as many distilleries in Australia have found to their detriment, aging spirit in 20L or 50L casks for just over two years in ex-wine casks might produce a dark whisky that initially seems ready to some drinkers, but the reality is that only two of the three maturation processes discussed above have partially occurred:  The additive and interactive processes have occurred to some extent, but there simply hasn’t been sufficient time for the subtractive process to fully play out.  The result is often hot and dry whisky that is over-oaked yet under-matured.    

The situation is exacerbated at distilleries who insist on using ex-wine casks that have only been toasted, rather than heavily charred, meaning that their casks are less capable of removing (subtracting!) the volatiles and other aggressive compounds that maturation seeks to address.   (Hence leading to whisky that is described as “hot”, regardless of the actual ABV percentage.)

Fans of peated whiskies should also be aware that peatiness decreases over time.  The phenol levels in the spirit reduce with years in the wood (there’s that subtractive process again!) and so those wanting to experience the real “smoke bombs” should be seeking younger whiskies and not necessarily older releases.

Only oak may be used for the storage of Scotch whisky. There are two main types of oak used for barrel making and another two used for variety, i.e., less often.


The Cooperage And Loch Lomond’s Unique Barrel Maintenance

Loch Lomond’s distillery owns its own cooperage, ensuring that barrel repairs and charring are carried out to an exacting standard so every cask is perfect for maturing the Loch Lomond spirit.

Coopering of casks is an age-old skill, the nuances of which have seen little change over the centuries. Loch Lomond is one of only four distilleries in Scotland to have their own on-site cooperage. Their team of seasoned professionals and apprentices carefully manage the quality of all of the casks to ensure that the whisky is maintained to the highest possible standards.

An expert team examines every new cask that arrives on-site and carry out repairs and maintenance on oak casks from all over the world.  They even rejuvenate casks using a De-char / Re-char (DCRC) process. Each year, this team maintains and repairs over 30,000 casks, which is absolutely vital in creating quality and consistency in whisky maturation.

The De-char / Re-char Process (DCRC)

Many distilleries will have a cooper or two on site to repair the odd cask, however when Loch Lomond says ‘Full service’, they mean that the experienced team of 8 fully qualified coopers and three apprentices will check every new cask that comes on site, repair any casks from Bourbon to Sherry Butt AND rejuvenate casks using their DCRC process.  

This whole process gives exact control over another element in the production of our whisky – which creates consistency, quality and allows Loch Lomond to really showcase their signature whisky style of fruit, honey sweetness and soft smoke.

Most distilleries will tell you that they will use a cask 3 times. But such are the capabilities within Loch Lomond’s cooperage and their confidence in the experience of their coopers that at Loch Lomond they can use a cask up to a maximum of 9 times! Three times for malt, the casks will then come into the cooperage to be checked, repaired if required, DCRC and then used three times for grain. Back into the cooperage to be checked, repaired if required, DCRC and then a further three times for grain. Consider that if the spirit is only in the cask for 6 years each time then they are using that cask for over 50 years.

Coopering of casks is a skill that hasn’t changed much in centuries and this is probably best reflected in the tools the team uses at their stations. Coopering is a very physical job and the few modernisations/mechanisations seen help to remove some of the most manual elements – helping to prolong the career of a cooper but also protecting their H&S.

What is De-Charring?

Loch Lomond Distillery was the first Scottish distillery to have this set up and the first to install this type of de-char machine.  All casks that they receive will have a level of char; most commonly, ex-bourbon casks come with a heavy char. This machine uses a Stainless-Steel wire brush that rotates and is moved up and down the cask as the machine rotates it (the cask). to the goal is to remove the char from the cask whilst taking the minimum amount of wood possible. This leaves a clean, consistent surface ready for a fresh char. The char is vacuumed out during the process but a little is always left at the bottom of the cask.

What is Re-charring?

The re-char machine uses a flame gun to char the inside of the cask. Many factors impact how long this takes and the skill of the Cooper is to use their experience and control the level of char that each cask gets depending on what is required by the distilling team. They char a cask for anything from 2-3 minutes for a medium char up to 5 or 6 minutes for a heavy, alligator char.

During the process, the nature of the flame changes:

Beginning will often be an orange flame with a blue/green tinge. This is burning of any alcohol in the cask

Charring – The team knows the cask itself is charring when the flame is a full bright orange flame and they can hear the crackles and pops. This means the cask itself is now alight. For medium char, the flames are extinguished using a spray of water but for heavy char we will turn off the flame and let the cask burn itself out.

Each cask that is re-charred has a chalk number on both the top and the cask ensuring that when the two are reunited that the correct top is married back to its cask as each top is a slightly different size and fit. They often re-char dozens of casks per day and this is the simplest way to keep them together. The casks are then taken next door to be finally tightened and pressure tested.

De- char / Re-char Vs Shaved Toasted Re-charred Casks

What are the key differences?

Loch Lomond does not ‘shave’ casks – the de-char machine uses steel brushes, removing the minimum amount of wood required, ensuring the integrity of the cask!

Shaved Toasted Re-charred casks have historically been ex-wine; ALL Loch Lomond re-chars are ex-bourbon.

Re-char uses higher temperatures to caramelise the wood sugars faster than the gentler toasting approach.

How The Cooperage Supports Loch Lomond’s Unique Whisky Character

Having one’s own Cooperage has two main benefits. First and foremost – it's all about quality. Having a team of dedicated coopers managing casks means that the Master Blender can ensure more consistent maturation year on year to maintain the signature style of Loch Lomond. Secondly, it helps in innovation and flavour creation. Steam & Fire for example uses heavily charred casks to impart additional sweetness into the whisky – something they do using the skill and craft of the cooperage to elevate the signature character of Loch Lomond Whisky.