Cask Maturation

Cask maturation has the greatest influence on the future taste of a Whisky. During the time in the cask, the distillery character of the Whisky is supplemented and changed. Single Malt Whisky in particular is often stored in the cask for a long time and therefore has a lot of time to absorb aromas from the wood. But it is not only the duration of the maturing process that makes it so significant. What matters primarily is the type of cask, in which the Whisky matures. This determines to a large extent how the taste of Scotch Whisky, Bourbon and Irish Whiskey changes over the years. Various factors play an important role for this process: the size and age of the cask, the degree of charring, the type of wood and above all the previous content of the cask.

The preparation of the cask is more complex than one might think. Many steps are taken before a ready-to-use cask is crafted from a tree and can be filled with Whisky. First, the wood is cut into staves. To prevent them - in the truest sense of the word - from remaining stiff like a board, they are thermally activated. In this process, the staves are steamed for half an hour at around 200 degrees, so they can be bent into the right shape. This process, also known as toasting, simultaneously separates the cellulose in the stave into wood sugar, which caramelises at 150°C. Since the heating happens on the inside of the cask, the caramel is only produced up to about half the width of the stave. If you cut through a stave, you can see a red line (red layer) as the limit of the transformation. This caramel also gives the Whisky part of its colour.

The finished casks are then charred with full flame for three to five minutes and afterwards quenched with cold water, forming a layer of charcoal on the inner surface of the cask. As the Whisky matures, the harsh substances are deposited in this layer of charcoal, thus making the Whisky smoother. This process is similar to the way an activated carbon water filter works.

When stored in a larger cask, such as a sherry cask (500 litres), the Whisky takes more time to mature than in smaller casks. Whisky matures particularly quickly in quarter casks (125 litres).

The liquid that was previously stored in the cask has a very great influence on the taste of the Whisky. At the very beginning of the maturation, Whisky makers traditionally used former Sherry and Port Wine casks. They were available in sufficient quantities due to the import of these fortified wines to Great Britain. As the popularity of Scotland's Whisky increased, former Bourbon barrels were increasingly used, which established themselves as a sort of standard after World War II. In the meantime, in addition to the traditional Sherry and Port Wine casks, maturation or finishing in Wine casks of all kinds has also become more common. Sherry and Port Wine are still very popular with connoisseurs, but there are also more and more barrique casks available, as Wine sells better when matured in oak casks. In addition to the mostly European, spicy oak, the aroma of the Wine is still inside the staves of the cask and is very quickly absorbed by the Whisky.

The decisive factor is the type of wine previously stored in the cask. Rich fortified wines, such as Sherry, add more flavour and also change the colour of the Whisky more clearly than a white Wine such as Chardonnay. The results can vary greatly, depending on how long the Whisky is matured. Sometimes malts are matured exclusively in fresh Sherry casks and thus strongly influenced by Sherry and oak. Other times the Whisky is matured only for a few months and absorbs the Sherry aroma but less oak notes.

The two Sherry varieties Pedro Ximénez, or simply PX, and Oloroso are particularly popular for maturing and finishing. The former is very sweet and the Whisky maturing or finishing in the cask, such as the Lagavulin Distillers Edition, gains notes of dark fruit, raisins and syrup. The Oloroso Sherry gives the Whisky a deep, nutty aroma with notes of ripe dark fruit, which we find for example in the Dalwhinnie Distillers Edition.

Casks in which port or fortified Wines matured before are also popular in Whisky making. Whisky matured in ex-Tawny-Port-casks gains very fruity and nutty notes with caramel, chocolate and tobacco. Casks previously containing Ruby port give the Whisky a kind of dry sweetness with very fruity aromas, especially of dark fruits. One example is the Distillers Edition of Cragganmore, which matured for about three months in port casks. Fortified Wine casks are also being experimented with in Whisky maturation. Popular, among others, are Madeira, which brings spiciness, light fruitiness, sweetness and dryness to the Whisky, as well as Marsala, which makes the Whisky sweet, complex and spicy.

Even red and White Wine casks are sometimes used for the maturation of Whisky. Red Wines of course have a greater influence on the colour of the Whisky, which takes on a deep red colour depending on the time it is stored in the cask. Amarone and Barolo casks provide a slightly dry mouthfeel to the Whisky, especially due to the tannins contained in the Wine. Both make Whisky fruity with notes of dried fruit. Former Bordeaux casks influence Whisky particularly with aromas of dark fruits, such as berries or grapes. White Wine casks, on the other hand, have little influence on the colour of the Whisky due to the colour of the Wine. The aromas that Whisky acquires when matured in White Wine casks are mainly fresh, light and sweet. A Chardonnay cask gives the Whisky dry, light and fresh notes of tropical fruits - such as the 10-year-old Glen Moray, which matured entirely in Chardonnay casks - while a Muscatel cask gives the Whisky sweet floral notes with citrus and peach. An example for this is the 16-year-old Tyrconnell, which received its finish in both Oloroso Sherry and Moscatel casks. Sauternes casks also have a sweet and dry influence on Whisky with notes of lemon zest and light fruit (Arran Sauternes Cask Finish), while Tokaji casks bring notes of light fresh fruit such as lemon or mango.

If the Whisky is placed in a fresh Wine cask, it can absorb much of its aroma. Since such casks are very expensive, they are often used for storing Whisky several times. After the first Whisky has absorbed most of the aromas of the Wine, the second can be influenced much less by the cask. These casks are referred to in the industry as refill casks. If there is nothing on the label, the colour of the Whisky indicates the type of cask, which is not entirely reliable for Whiskies containing caramel for a darker colour.

Tip: In the future, pay attention to the flavours of Amarone, Barolo, Chardonnay, Tokaji and co. Some of them may also be found in your favourite Whisky!