A very complex issue and surely the most important factor in the production of whisky. New-make spirit, almost vodka-like, becomes rounded and enjoyable after having spent a number of years in an oak cask. Much is still a mystery, but the fog is not as thick as a few years ago. Components in the spirit go through chemical alterations at the same time as compounds are extracted from the wood and volatile components evaporates through the wood. The course of events are influenced by a number of factors, such as:

– the alcohol concentration
– the kind of wood used for the cask
– the previous use of the cask
– the size of the cask
– the length of the maturation
– the environment inside and outside the warehouse

FILLING

The distilled spirit is diluted with water to 63.4% ABV. Higher concentration results in longer maturation time, lower concentration results in a greater need for casks, so this has for long been considered as a good compromise. The spirit is then transferred to the vessel where it is going to spend many years.

THE OAK CASK

Maturation of scotch whisky is always made on oak casks. Oak is an ideal sort of wood being strong yet flexible, with the ability of holding back spirit while allowing air in an out of the cask. It consists of cellulose, lignin, hemicellulose and different extractable substances. The ability to release compounds to the content is vital for the end result.

Scotch malt whisky is rarely filled on new casks, it is said that this results in a heavy, woody character that disguises the whisky itself. Instead casks that have previously been used for the production of other beverages are used. Oddities like old port pipes or rum casks may be found, but two types are dominating:

- More than 95% are old bourbon barrels. They are made of American white oak, a hard and tight-grained wood with low interaction with the spirit and little permeation through the wood. It is charred heavily on the inside with an open flame to open up the structure and make the wood more accessible for the sprit. This treatment also results in a degradation of oak wood components where, among other things, different aldehydes are formed. It is also likely that charring results in a carbon layer which absorbs different compounds in the spirit. After 4-8 years in Kentucky, the cask is shipped to Scotland.

- Old sherry butts were formerly widely used in Scottish whisky production. They are still used but to a much lesser extent. Sherry butts are made of both American and European oak. The latter is more open in its structure allowing the cask to breathe easily, as well as making it easy for the spirit to extract compounds. Butts are also heat treated, but more roasted than charred.

The casks ability to mature whisky are diminishing for each filling. After being used two or three times the casks are exhausted and get retired from malt whisky production. These are called plain wood and are mostly used for the production of grain whisky and marrying whisky blends before bottling.

THE IMPORTANCE OF SIZE

The ratio between the spirit and the internal surface of the cask plays an important part. Alcohol matures faster in a smaller cask since there are more wood for the spirit to interact with, relatively speaking. But the use of smaller casks is also more expensive and requires more space in the warehouse.

The standard size for a sherry butt is approximately 500 L. An american barrel is about 180 L but has more than 30% greater surface to liquid ratio, consequently the maturation time is shorter.

THE LONG "SLEEP".

The spirit will stay on oak casks for at least 3 years, this is the law. After this time it can finally be called whisky, but for single malts the stay is longer, more likely to be 10-12 years or even longer. The maturation time is determined by the character of the spirit, the condition of the oak cask, the storage conditions but most often the marketing department.

To call it sleep is definitely an understatement. The chemical reactions that once started in the fermentation continues. The spirit extracts components from the wood such as sugars, acids, oils, tannins, vanillin and colour pigments. New reactions take place between the congeners and the extractables. All this result in the flowery and grassy aldehydes, fruity esters, sweet sugars and bitter tannins that shapes the character of the final product.

The former inhabitant of the cask will also leave its mark. This is especially noticeable for a first fill sherry butt which gives the whisky a rich, round and nutty character.

WAREHOUSES

Most distilleries store the casks on their own grounds but the larger companies also use centralised warehouses. Several still use old buildings made of stone with no heating and an earth floor. The latter has the advantage of releasing moisture in dry weather and absorbing moisture when raining, a simple but practical way of controlling the humidity.

Modern warehouses are much larger with the casks stacked high, and has often got temperature and humidity control.

THE ANGELS’ SHARE

Oak is porous and volatile substances in the cask will evaporate. The loss is 1-2 % every year, more alcohol than water resulting in a decrease in strength, usually 3-5 % over a period of 10 years. This is called the angels’ share and is considered a fair deal by the producers, maybe because it’s tax deductible.

But air is also drawn into the cask and with it part of the surrounding environment. There are warehouses located close to the shore and it may well be so that sea air may be responsible for the saltiness recognised in some whiskies.

BLENDING AND BOTTLING

Only two distilleries have their own bottling plant, Springbank and Glenfiddich (where also its neighbouring sister Balvenie is bottled). For the rest the whisky is transported to centralised bottling plants.

In order to get a consistent product, the content of several different casks are blended together. The blender, or the head nose as he is also called - a very important person indeed, chooses the casks mainly using his sensitive nose. Sometimes the only difference is that they are from different production runs, but they may be of different age or be matured in different ways. For instance, even if ex-bourbon barrels are most common some sherry butts are often a part of each bottling.
(Note that this practice is not to be confused with neither blended whisky which is a mix of grain whisky and malt whisky from several different distilleries, nor vatted malt which is a blend of malt whisky from several different distilleries).

In most cases the whisky is diluted to 40 or 43% ABV before bottling. Thus, for most whiskies a great part of the water is not clear, spring water taken from the distillery’s own well, but instead treated municipal water from Glasgow or Perth.

Since different casks renders different colour to the content, the final whisky has a different appearance from time to time. A great part of the industry sees this as a problem which has to be disguised. So colouring (E150) may be added to ensure an even colour.

Another thing that is also looked upon as a problem, is that whisky taken direct from the cask gets hazy when diluted with water below 46% ABV. To solve this problem chill-filtering is introduced. This is done by cooling the whisky, sometimes below 0ºC, and passing it through a filter system which reduces oils and other components that precipitates. The disadvantage is that this treatment reduces flavour at the same time. Probably due to increasing customer awareness, it is getting more common to find whisky which is neither chillfiltered nor coloured.

The diluted spirit is filled on oak casks. The casks are never new, they are mainly old american bourbon barrels but old sherry butts are also used.

The type of oak used, the degree of charring on the inside and the size are important factors that have influences on the character of the final whisky.

After 3 years the spirit has finally earned the right to be called whisky, but maturation time for a single malt whisky is more likely to be 10-12 years and even longer. During this time many chemical reactions occurs which will give the whisky its final flavour and appearance.

The casks are still often stored in traditional warehouses made of stone with earth floors and no heating. The environment may influence the character of the whisky since the casks are breathing in the surrounding air.

The breathing also results in a loss of whisky. 1-2 % evaporates every year, this is called the Angels' Share.

After the maturation is completed, the content of several different casks are blended together before being filled on bottles. The head nose chooses what to include in order to get a consistent product.

Only a few distilleries have their own bottling plant. The other send the casks to centralised bottling plants.


The whisky is mostly diluted to 40 or 43 % ABV before bottling. It is also often chillfiltered and colouring may be added.