The stages of the cask maturation

Maturation in Casks

Whisky is matured for several years in casks of different origins. What influences the maturation process? Why do whiskies from one distillery sometimes taste so differently?

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Numerous experts try to identify the different flavours with physicochemical methods. Even smallest amounts can account for big differences in taste. That's how sensitive our senses are. Researchers measure the various substances like esters, tannins, lactones etc. in ppm (parts per million) and ppb (parts per billion). These small quantities are hard to grasp. Maybe a comparison in terms of time makes it easier: 1 ppb equals 1 second in 31.7 years – a quantity almost inconceivable.

The Lids of casks at the Glenmorangie distillery
Lids from Glenmorangie

Smallest changes in the maturation process also have an enormous influence on the taste of the whisky. Let's first have a look at the wooden cask, the most important component of the maturation process.

Wooden barrels at the Dalwhinnie distillery
A wooden cask at Dalwhinnie

The Wood

A wooden cask is a small masterpiece of craftsmanship. Although more and more machines are used by coopers today, the actual manufacturing is still done by hand. The planks for the staves mustn't simply be 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. Otherwise too much alcohol evaporates or the cask even starts to leak. Only oak wood is suitable for cask production. Softwood contains resin, which prevents the cask from breathing. Other types of wood emit unpleasant flavours that make the whisky awkward or even unenjoyable. 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 in general). American white oak grows faster and has a mellower, finer and more contained aroma, while European oak provides full, intense aromas and more tannins. An American oak can be cut down after 70 years, while the slower-growing European oak must at least grow for 100 additional years.


Wood doesn't only contain annual rings but also vessels that lead from the core to the bark radially. The tree transports water and nutrients through these vessels. For whisky, however, these vessels are inconvenient since they make the cask staves leaky. Therefore the wood must be cut according to special patterns (star cut, mirror cut or rift cut) so the annual rings stand vertically. With this method far less usable wood can be cut from one log, so a cask stave is much more expensive than a normal plank.

Diagram timber cut vs. star cut

These planks are then made into staves with trapezoidal wanes (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. Whether this is left to nature and solar heat or done quickly in modern drying chambers doesn’t affect the quality of the cask.

A pile of staves in Kentucky
Drying staves in Kentucky

If the casks were made from this wood, you would get a tight container, but the whisky couldn't mature. From a maturation standpoint, the wood is still dead. Only the following thermal treatment breathes life into the wood. This is a combined process. Only with heat the wood can be bent into the typical cask shape. During 'toasting', the wood is heated up to 200°C in a big oven for approximately 30 minutes, and the firm wood structure is broken up, cellulose is split into wood sugar and caramelises, and the Lignin is partially converted into Vanillin. The cask begins to live in terms of maturation. After the cask has been bent into shape, the inside of the cask is burned (charred) for 3 to 5 minutes and extinguished with water.

the casks being burned
Charring of the Casks

If you cut through a stave that was treated that way, you can see a red ring in the wood beside a charcoal layer of several millimetres depth, the so-called 'red layer'. It's the separating layer between the 'activated' and the 'natural' wood. Up to this layer the heat has penetrated the wood and has activated it for flavour extraction.

Staves with red layer

Charcoal is an extremely good filter and extracts tangy components from the whisky. This is the domain of the American coopers and bourbon distillers, who classify the depth of the charcoal layer (grade 1 to 4) when ordering casks. Sherry and port casks are also toasted but only seldom charred. This remains reserved for the Americans, who attach much more importance to 'mild and mellow' whiskey. 

Different Casks

However, beside the type of wood and the thermal treatment also the size of the casks influences the maturation process. Whisky matures faster in small casks since the surface area is bigger in relation to the content. The exchange of substances between wood and whisky happens faster.

 

The most important cask type 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 trapezoid 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.

 

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.

Trapezoid shape of American Standard Barrel in hogshead bending

Re-use of Casks

The production costs of casks account for a considerable part of the overall costs of whisky production (about 10% to 20%). Increased demand for casks in the whisky industry has also contributed to the rise of cask prices. That's why it makes sense for the Scots to use casks several times. Casks can be used three to four times, then the wood has lost most of its flavours.


1st fill casks are therefore most interesting. ‘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. This 1st fill extracts the strongest flavours from the wood.


Remember: by toasting, the wood is activated, which is signified by the red layer, which separates the active from the inactive wood. So why not scrape out the inside of the cask and toast it again? After toasting the cask is charred anew, and voilà: You have a new, activated cask.


A bourbon cask, used for 2 to 4 years in the USA, can be used for another 30 years in Scotland. After the remanufacturing process described above, called 'rejuvenation', the cask can again be used for 30 to 40 years. And afterwards it might just be rejuvenated again... How old casks can get that way hasn't been tested yet. Casks have only been rejuvenated for about 10 to 20 years. However, not every part of the cask is rejuvenated. The cask usually receives new ends, since the old ones don't survive the rejuvenation and break apart.


Rejuvenation is the future. With little effort the life of the cask can be prolonged significantly. But saving money is not the only factor. It's mainly the flavour characteristics that make rejuvenated casks so popular in the whisky industry.

Recycling of casks in the cooperage

The place where casks are recycled is called 'cooperage'. The coopers have numerous tasks in connection with the casks. Here, casks are not only refurbished, but also newly manufactured. This begins with a block of oak wood, which is sawn into staves and tied into a cask with metal rings. Of course, a lot of intermediate steps are necessary. A detailed illustrated description of the production of casks can be found here using the example of Brown Forman's barrel factory or the Speyside Cooperage. Scottish distilleries, on the other hand, usually use casks that have already been filled before. This is because the storage of Scotch whisky in Bourbon casks has established itself as a quasi-standard and the use of Sherry casks is still very much in fashion. Coopers prepare these reused casks as described above.

Maturation and Taste

During maturation three fundamentally different effects in the cask play a role. Let's have a look at the following diagram.

You see a decending line red (Immaturity), a rising green dottet line (cask to strong), a rising flat green dotted line( cask to weak), a green line in between (cask maturation), blue line starting high decending slowly (distillery character)
Cask maturation diagram

Subtractive Maturation

New make whisky has a tangy, metallic taste. If you ever tasted a sample from a pot still or a spirit safe you will always remember this rather unpleasant taste.

With increasing maturation time, this unwanted taste diminishes more and more along the red line. After 5 to 8 years it is usually gone. Since we're dealing with a reduction of taste, the special term is 'subtractive maturation'. That's also the reason why there are only few whiskies younger than 8 years.

Additive Maturation

With increasing maturation time the whisky takes up flavours from the cask. Initially it's a general wood flavour but especially vanilla, toffee and oak. The wood passes over its character to the whisky. This additive maturation is displayed by the green lines. An old, not very active cask would influence the whisky along the lower dashed line. A 1st fill European oak cask would be represented by the upper dashed line.

Interactive Maturation

Not only maturation and the cask character affect the taste of whisky. The basic character of a whisky is shaped by the special distillation and the malt that has been used. The upper blue line describes the distillery character, which remains pretty constant over time. (Actually peat smoke is reduced more or less over the years, which leads to a diminished distillery character.)

With increasing age, the cask taste is combined with the distillery character. When distillery character and cask influence join to form a harmonious whole, it is called interactive maturation.

The Cask Determines the Result

The green lines show the various cask influences during interactive maturation. Since the alcohol in the whisky extracts flavours from the cask, we're actually dealing with dissolution processes. First the curves rise steeply and then flatten more or less strongly. Only the middle curve is more or less ideal. Interactive maturation is the ideal combination of cask and distillery character. The upper dashed line rises much too steeply, and the cask begins to dominate. The lower dashed line shows a cask that has already lost most of its flavour substances. Even after many years the distillery character is dominant.

The upper curve is typical for fresh European oak casks. The cask becomes dominant very quickly, and the whisky is beat by the wood. The lower curve stands for depleted casks that were used too often. While depleted cask can be reactivated through rejuvenation, there's no ‘cure’ for European casks that are too strong. The cask starts to dominate much too fast, while the immaturity might not have been reduced yet.

But there's a trick, namely ‘finishing’. First the whisky is matured in normal American casks until its immaturity has reduced and it has already taken on a basic aroma. 8 to 10 years are enough for that. Then the whisky is filled into European oak casks that held various wines before. You need 1st fill casks for finishing. After 1 to 2 years the whisky is ready. The distillery character and the casks have joined and formed a harmonious whole.

A convenient by-product of this process are European oak casks that have lost their extreme character and are now suitable for longer maturation. However, these refill casks must also not be used too long so they don't get depleted.

Fresh European oak is 'almost unbearable'. Only for Glenlivet French Oak new and unused French Limousin oak casks are used for finishing. Pure European oak maturation is very rare in whisky production.

Even the winemakers have come to appreciate the extreme taste of this wood. More and more barrique casks are used to mature Southern European red wines for a short period of time – something the Scotch whisky industry can profit from, too.

Small Batch and Single Cask

You have probably come across the terms Single Cask or Small Batch on Whisky.com at one time or another. Bottlings that carry these terms on their label are something special. Because for a Single Cask or Single Barrel bottling, only one single cask is filled. Depending on the size of the cask, this results in only about 100 to 300 individually numbered bottles. To get hold of such a bottle is of course something special. There are very few on the market compared to Whiskies from the standard range, which are always available. A 'Small Batch' bottling also involves a limited number of released bottles. Only a relatively small number of selected casks are married and bottled together. This occurs primarily in the USA.

Types of Warehouses: Dunnage Warehouse or Racked Warehouse

A dunnage warehouse is an old, traditional warehouse without windows, usually one, sometimes two floors high. On these floors, a maximum of three casks can be stored on top of each other. In the dunnage warehouses, air circulation is said to be particularly beneficial for a good Whisky maturation. The construction of most of the dunnage warehouses is very simple and traditional: clay floors, thick stone walls and a slate roof are enough to give Whisky casks a place to stay. Logically, this type of warehouse is less efficient for distilleries, since it can accommodate only a few casks. Dunnage warehouses are therefore less common in distilleries than so-called 'racked warehouses'. The name says it all: In these larger, modern warehouses, there is room for huge racks on which 15 to 20 casks can be stacked on top of each other. Accordingly, racked warehouses are built higher, with thinner walls and tin roofs. Often the casks are stored there upright and not horizontally, so the warehouse workers can move them around more easily. This is done on a large scale with forklift trucks. In the so-called 'palletised warehouse', the name also says it all: pallets loaded with casks are simply stacked on top of each other.

The main difference between the two types of warehouses is the regulation of temperature. The racked and palletised warehouses with their thinner walls provide a more varied climate that changes with the seasons. The dunnage warehouses with their thick walls and roofs allow for less outside influence. Therefore, it tends to be wetter there than in racked warehouses. These factors influence the maturation process: the higher the temperature, the more the Whisky expands inside the cask. Thus the Whisky has more contact with the cask wall, from which it extracts many aromas. A Whisky generally matures more slowly in an environment with higher humidity. However, these are only tendencies. In fact, many people believe that the climate in both types of warehouses is very similar and therefore the warehouse in which a Whisky matures is irrelevant. So, there is much that speaks for both types of warehouses. There is no 'right' or 'wrong' - as is so often the case in the artisan trade. What matters in the end is the finished Whisky. And in this respect we trust in the work of the Master Distillers and Master Blenders, who are committed to bringing the best possible product to the market.