The cask ageing of Whisky

The cask maturation of the whisky has one of the greatest influences on its later flavour. It complements and changes the distillery character that the whisky has acquired during distillation. Single malt whisky in particular is stored in casks for a long time and therefore has a lot of time to absorb flavours from the wood. However, it is not just the length of the maturation process that makes it so influential. Above all, it depends on the type of cask. It plays a key role in determining how the flavour of Scotch whisky, bourbon and Irish whiskey changes over the years.

What was in the barrel before?

Ex-sherry - what does that mean? This is about the previous contents of the cask; what was the cask previously filled with?

The liquid that was previously stored in the cask has a major influence on the flavour of the whisky. Ex-sherry and ex-port wine casks were traditionally used at the historical beginning of whisky maturation. They were available in sufficient quantities due to the transport of these strong wines to Great Britain. With the rise in popularity of Scotch whisky, ex-bourbon casks were increasingly used and became the quasi-standard after the Second World War.

In addition to the traditional sherry and port wine casks, maturation or post-maturation in barrique casks of all kinds has now also become common. Rum casks and beer barrels have also arrived in whisky finishing.

This video is about finishing, the post-maturing of single malt whisky in different casks, as well as the question of why whisky is actually post-matured and whether a longer post-maturing period is really better.

Only oak may be used for the storage of Scotch whisky and Kentucky Straight Bourbon. There are two main types of oak used for the production of casks.

The American white oak is mainly found in the east of the USA and in parts of Canada and is used for the production of barrels for bourbon and Tenneessee whiskey. Its growth is relatively vigorous for oaks. The wood has a high density of 770 kg/m3^and contains a lot of monogalloyl glucose, which is responsible for the typical vanilla notes in bourbon.

European oak, on the other hand, grows more slowly and has a lower density (720 ^kg/m3) than its American relative. The gallic acid it contains is a vegetable tannin and belongs to the gallotanins. It causes the bitter notes in the whisky. European oak gives off significantly stronger and spicier flavours than American white oak. It is mainly used for the production of barrels for European wines and strong wines such as sherry.

Mizunara oak is very widespread in the forests of North East Asia, where it is mainly used for high-quality furniture. Due to its high density and thin fibres, the staves have to be cut along the fibres, which never grow completely straight. The oak also lacks waterproofing oil enzymes, which means that considerably more whisky evaporates during the maturing process than when maturing in American or European oak. Despite these characteristics, which make it much more difficult to use for whisky casks, Mizunara oak became increasingly popular for maturing whisky at the beginning of the 20th century. Not least because of the special flavours it gives off to the whisky, which are reminiscent of sandalwood, coconut and oriental spices, for example.

Maple grows in many parts of the world, including Eurasia, North Africa, Central and North America. With its relatively low density of 653 ^kg/m3, it is mainly used in the production of Tennessee whiskey: this is filtered through a charcoal layer of sugar maple before being stored in barrels, which makes it particularly smooth. In addition, some American whiskey producers mature their spirits in maple barrels to give the whiskeys the sweet notes of maple and maple syrup.

While only oak may be used in the main whisky-producing countries of Scotland and the USA, other types of wood such as chestnut are used in other countries.

The age of the cask and its reuse!

Oak casks are very durable and can hold whisky for many decades. However, the flavour that the whisky can absorb from the wood decreases over the years. The cask becomes leached out. Whisky stored in fresh casks therefore absorbs the most flavours.

Bourbon is stored in fresh casks and is already flavoured enough to be bottled after 2-5 years. Scotch whisky matures in used casks that have previously been used for bourbon or sherry, for example. The cask has already released a large proportion of the flavours. Scottish malts therefore only become really good after a longer period of storage. However, this also gives them more time to break down the alcoholic flavour.

Casks are expensive these days. It is therefore common practice in the whisky industry to mature whisky in casks more than once. After 10 years of malt whisky maturation, a cask still contains a lot of flavours and is therefore reused for the next malt. In the industry, they are called refill casks. Refill casks are reused for up to around 30 years.

However, it is clear that the influence of the cask on the flavour is becoming weaker and weaker. It is not surprising if a whisky from a refill cask has absorbed very little flavour - and therefore colour. On the labels of some bottlers you will sometimes find information such as Refill or First Fill, which stands for the degree of utilisation of the casks. This gives you, the customer, an indication of the approximate intensity of the cask flavour. However, it is also common to colour whisky with flavourless caramel. In this case, the cask flavour is disguised.

Nowadays, it is becoming increasingly common to recondition casks, as this is cheaper than buying new casks. This does bring out the oak flavour again. New sherry, port or wine flavours cannot be produced in this way. These 'rejouvenated' casks make whisky very flavourful in a short time.

Size also plays a significant role in the maturation process. A whisky matures faster in a smaller wooden cask, as there is relatively more surface area available per litre of whisky enclosed. The exchange of substances between the wood and whisky is therefore faster in smaller casks.

There are many cask sizes. However, there is no uniform ISO standard for certain types. Sometimes the cask sizes that have developed historically are also a unit of measurement. The hogshead, for example, is an old English measure of volume.

The most important barrel in whisky production is the American Standard Barrel (ASB). It holds around 200 litres. ASBs are also the 'raw material' for the production of Scottish hogsheads. These are barrels with a capacity of around 250 litres. If an ASB is broken down into individual staves and slightly larger rings (hoops) are used to assemble it, the same staves can be used to assemble barrels with a larger diameter. From four to five ASBs you get about 3 hogsheads. Even larger barrels cannot be assembled from the staves. The trapezoidal shape of the staves, originally intended for 200 litres, prevents a larger diameter. Sooner or later, the barrels would leak at the gaps.

European oak is processed in Spain and Portugal into barrels of 500 to 600 litres. These barrels are ideal for maturing sherry and port wine. The Scots call these barrels 'butt or sherry butts' and 'port pipes'. Nowadays, more and more butts are being made from American oak for cost reasons.

shaved - toasted - recharred

The so-called 'STR Cask' is a special type of cask. STR stands for 'shaved, toasted and re-charred', i.e. 'shaved, toasted and re-charred'. The casks are produced using a special process developed by the well-known, now deceased whisky expert Dr Jim Swan. The production process for STR Casks is as follows: fresh red wine hogsheads are taken and their bottoms are first removed. The outer layer of wine-saturated oak is thenshaved away to expose the combination of the lightly saturated oak and the fresh virgin oak. The shaving has removed some of the stronger flavours from the surface of the oak, creating a more subtle red wine influence. The exposed inner layer is thentoasted again to caramelise the sugars and vanillins from the fresh white oak. Oak chips from staves already used for maturing whisky are used to heat the cask during toasting. This brings out the spicy and sweet notes from the casks particularly well. Finally, the casks arere-charred to extract as many flavours and aromas as possible from the cask. Toasting and re-charring also ensures that undesirable chemical compounds such as lignin, cellulose or hemicellulose are broken down and the bitter tannins are removed.

The STR process therefore produces a combination of virgin oak (fresh white oak) and first fill red wine barrels. Whisky matures faster in fresh white oak barrels than in barrels that were previously filled with bourbon, sherry or others. This elaborate process ensures that the spirit matures quickly and intensively, as well as giving the whisky subtle fruity red wine flavours.

Where is the cask with the fresh distillate stored for the next few years?

There are different types of warehouses. The construction and the way in which the casks are stacked in the warehouse influence the finished whisky.

The biggest difference between the two main types of warehouse lies in temperature regulation. Dunnage warehouses offer a more stable temperature and humidity, while racked warehouses have a more varied climate due to their thin walls and tin roofs. This affects whisky maturation as temperature fluctuations cause the whisky to expand in the cask and draw more flavours from the wood. In an environment with higher humidity, whisky generally matures more slowly.

There is no clear preference for either type of cask, as the choice of cask is made by the Master Distillers and Master Blenders who aim to produce the best whisky possible!

The cask is crucial!

Wood type, cask size, cask finish, age of the cask and cask storage are all factors that influence the final whisky product. The whisky cask therefore cannot be given enough attention.