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The Maturation of Whisky in Casks

Whisky matures over many years in casks of different origin. Which influences are there during maturation? Why do whiskies from an individual distillery sometimes taste so differently?

Numerous experts are looking for the taste substances with physical and chemical methods. Already the smallest amounts of them affect large differences in taste. That’s how sensitive our senses are. The scientists measure the different substances such as ester, tannins, lactones, etc. in ppm and ppb. In full: ppm = parts per million and ppb = parts par billion. That means: 1:1 million to 1:1 billion. Those are inconceivably small quantities. Perhaps a comparison helps using not amounts but time. 1 ppb corresponds to one second in 31.7 years. A nearly inconceivable small portion.

Small changes during the maturation process thus have an enormous influence on the later taste of a whisky. Let’s look at the wooden cask first, the most important component of the maturation process.

The Wood

A casks is a small work of craftsmanship art. Although nowadays more and more machines are used during the cask production, the actual cask production process still remains in human hands. The boards for the staves can’t simply be cut like timber right-angled from one log. You have to pay attention to the direction of the fibers in the wood, so that none of the radial vessels of the wood penetrate the cask wall. In the latter case too much alcohol evaporates or the cask even begins to leak. Only oak wood is suitable for the cask production. Softwood contains e.g. resin, which prevents the respiration of the cask. Other woods deliver unpleasant flavours, which make the whisky unenjoyable. Oakwood from trunks with an age of 70 to 200 years is ideal.

There are two fundamentally different kinds of oak. The American white oak and the European oak. The American white oak grows faster, is more moderate, finer and more reserved in flavour, in contrast the European oak delivers a full, intensive and tannin-rich flavour. While an American oak can already be cut after 70 years, the slowly growing European oak must grow at least a 100 years longer.

Wood contains not only annual rings, but also vessels (xylem vessels), which lead from the core star shaped out to the bark. These vessels transport the water and nutrients in the tree. For the whisky these vessels are unfavorable, since they make the cask stave leaky. The wood must therefore be cut using special samples (star cut, mirror cut or rift cut), so that the annual rings stand vertically. With this kind of cut the yield is much smaller. The cask stave thereby becomes more expensive, than a normal lumber.


Diagramm Lumber Cut vs. Star Cut

Staves are now produced from these boards, which receive trapezoidally sloped wanes according to the roundness of the planned cask. The still fresh staves have to be dried afterwards to under 10% rest moisture. Whether one leaves this to the natural sun warmth or does it by fast modern drying chambers, does not have an influence on the quality of the cask.


Drying Staves in Kentucky

If you would make the casks from this wood now, you would get a closed container. However the whisky would not mature. At this point the wood is maturation-wise seen dead. Life only comes into the wood with the following thermal treatment. This is a combined process. Only by the heat the wood can bend into the cask-typical form. During that 30-minutes heating on 200 degrees Celsius in a large oven, the specialist calls it Toasting, the firm wood structure is broken open and the cask begins to live. After the cask is brought in form, it is burned out on the inside for approximately 3 to 5 minutes and extinguished with water.

Burning of the Casks

If you cut through a thermal treated stave, you recognize beside a several millimeter thick charcoal layer a red ring in the wood. The specialist calls this layer 'Red Layer'. It represents the separating layer between 'activated' and 'naturally-left' wood. Up to this layer the warmth penetrated into the wood and activated it.


 Staves with Red Layer

Charcoal is an extremely good filter and filters sharp components out of the whisky. This is the domain of the American barrel manufacturers and the Bourbon distillers, who even indicate the thickness of the charcoal layer for their orders of barrels (grade 1 to 4). Sherry or Portwine casks are toasted, but only rarely burned out on the inside. This remains a domain of the Americans, who put by far more value in a 'mild and mellow' whiskey.

The Different Casks

The casks however do not only differ by the kind of wood and the thermal treatment. Also the size has a crucial effect on the maturation process. Thus a whisky in a smaller cask matures faster, since there is relatively more surface per liter of whisky. The exchange of materials between wood and whisky is faster in smaller casks.

Most important in the whisky production is the American Standard Barrel (ASB). It contains about 200 litres. ASBs are also 'raw material' for the production of Scottish Hogsheads. These are casks with approximately 250 litres contents. If you divide a ASB into individual staves and use larger rings (hoops) for the assembly, you can make casks with larger diameter using the same staves. From five ASBs about 3 Hogsheads can be made. Even larger casks cannot be made from the staves, since the trapezoid form of the staves, originally meant for 200 litres, prevent a larger diameter. The casks would sooner or later become leaky at the rifts.

 Trapezoid Form American Standard Barrel in Hogshead Bending

European oak is made to casks with 500 to 600 litres in Spain and Portugal. These casks are ideal for maturation of Sherry and Portwine. The Scots calls these casks 'Butt or Sherry Butt' and 'Port Pipes'. Only few Butts are made of American Oak.

Re-use of Casks

The cask costs make up a significant share of the costs for the production of whisky. About 10% of the manufacturing costs are allotted to the cask. The intensified demand for casks in the whisky industry is additionally bringing the cask prices in an upward motion. For Scots it thus makes sense to use casks several times. About three to four times whisky can be matured in casks, then most flavour substances are extracted from the wood.

Particularly interesting is thus the first filling of the cask (1st Fill). However not the original filling of the casks with Bourbon, Sherry or Portwine is meant, but the first whisky which is filled into a used cask. This 1st Fill extracts the strongest flavours from the wood.

Let us remember: By the heating, the toasting, the wood is activated. Characteristically for it is the Red Layer, which separates the active from the inactive wood. Who prevents us from scraping out the cask on the inside several millimeters and to toast it again? After the toasting the barrel is freshly burned out and voila, we get a new, active cask.

A Bourbon barrel used for two to four years in the USA, can do further services in Scotland for 30 more years. After the above described remanufacturing, the specialist speaks of the Rejuvenation, the cask can be used for 30 to 40 years again. And then the cask is revived again. How old a cask may become is not yet tried out. Just 10 to 20 additional years have rejuvenated casks already been used. But not everything at the cask is reworked. Usually the cask receives new ends, since the old ones do not survive the remanufacturing and break apart.

The future belongs to the retreaded casks. With only a little effort the life of the casks can be extended significantly. Not only the saved money counts. Above all it is the taste characteristics, which make remanufactured casks so popular in the whisky industry.

Maturation and Taste

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

Diagram of Cask Maturation

The taste is put on the vertical axis. On the horizontal axis we find the associated time.

1. Subtractive Maturation

A not matured fresh whisky has a sharp, metallic taste. Everyone who could once taste a fresh sample at a pot still or a spirit safe can always remember this unpleasant taste.

With increasing maturation time in the cask this unwanted taste diminishes more and more along the red line in the diagram. After 5 to 8 years it usually has disappeared. Since it concerns the reduction of a taste, the specialist speaks of a subtractive maturation. This is also the reason why there is nearly no premium whisky with an age below 8 years on the market.

2. Additive Maturation

With increasing maturation time the whisky takes up flavours from the cask. First it is the taste of general wood and in special vanilla and oak. The wood passes its character on to the whisky. These additive maturation is given by the green lines in the diagram. An old and only little active cask will affect the whisky along the lower, dashed line, a 1st Fill Cask from European oak will follow the upper dashed green line.

3. Interactive Maturation

But not only maturation and the cask character determine the taste of the whisky. Above all, special distillation and the used malt make up the fundamental character of a Malt Whisky. The upper, horizontal, blue line describes the distillery character that remains about constant over the time. Actually the peaty taste reduces a little over the years, what leads to a slight reduction of the distillery character.

With increasing age the cask taste combines with the distillery character. The expert speaks about an interactive maturation when they combine to a harmonious ensable.

The cask determines the result

The green lines show the different cask influences during the interactive maturation. Since the alcohol in the whiskey extracts taste materials out of the cask walls, we actually speak of dissolving processes. First the curves increase strongly, but then they start to flatten, as you can see at the upper dashed green line. Only the middle curve is more or less ideal. The interactive maturation connects cask character and distillery character ideally. The upper dashed curve increases too quickly and the cask character is too dominant. The lower dashed line shows a cask that already lost the larger part of its aroma. Even after many years, the distillery character dominates.

The upper curve is typical for fresh casks made from European oak. Very quickly the cask character dominates and the whiskey will be 'killed' by the wood. The lower curve stands for to frequently used and already sucked out casks. While you can activate these casks with an retreatment process, they have no chance compared to the too active European casks. The European casks dominate too quickly and the un-matured taste is perhaps not yet away.

But a trick resolves all. The cue is called Finishing. First the whiskey is matured in normal American casks until it has lost its un-matured taste and already received a basic aroma. 10 years is sufficient for that. Then the whisky is filled into European oak casks, that contained the different types of wines. Important is the 1st Fill. After 1 to 2 years the whisky is ready. Distillery character and the casks combine in an interactive maturation to a harmonious ensamble.

A comfortable byproduct of this maturation are European oak casks, that lost their extreme character and are now suitable for a long-time maturation. These Refill casks however may not be used too long so that they don’t get sucked out.


The most important influences during the cask maturation are listed according to their influence on the taste:

  1. Distillery Character
  2. Wood Activity
  3. Maturation Time
  4. Environment

As a conclusion, you find a list of typical representatives of the different maturations.

1. Pure American Oak (1st Fill)

Glenmorangie 15 Years

2. Pure European Oak (1st Fill)

Macallan 12 Years

3. Finishes

Glenmorangie Port Wood Finish

Balvenie Double Wood

The strong and intense taste of the European oak can be best experienced in the following two bottles: Auchentoshan Three Wood and Macallan 12y. The intensive taste of the oak in the Macallan is covered by the sweetness of the Oloroso-Sherrys and combines with the strong character of the distillery very well. In the Auchentoshan, the European oak ist very stong above the taste of the Ports and Sherries. The distillery character is because of the triple distillation, so fine and delicate, that the European oak hits straight through.

If you want to experience the fine taste of an American oak, you should try the Glenmorangie 15 years and the Glenlivet American Oak Finish. While in the Glenmorangie only 1st fill American barrels are used, only the finishing of the Glenlivet takes place in entirely fresh casks of American white oak. Although only used in the Finish, the American oak can be detected clearly.

Fresh European oak is, please excuse the expression, 'almost not endureable'. Only in the Glenlivet French Oak Finish, entirely fresh French Limousine oak casks are used. There is no entirely pure European oak maturation of any whisky.

Also the wine producers learned to appreciate the extreme taste of this European wood. More and more Barrique casks are used for a short maturation time with French red wine.


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letzte Änderung: 20. Januar 2011