The aging of whiskey in barrels is an essential step in the manufacture of whisky. During storage in oak casks, the new make spirit absorbs and creates the flavours and aromas that shape its later taste. In addition, the whisky gets its beautiful color from the wood. We have already devoted ourselves to the malting, mashing, fermentation and of course the distillation of whisky in our detailed articles on the production, history and science of whisky. This article will deal with the events and secrets of the whisky barrel itself and how and why these came to be used.
Today we want to deepen this overview and go even further into the chemical processes and taste nuances that take place and develop during the storage and maturation of whisky.
Why is whisky matured in wood?
Whisky maturation actually began as a legal requirement under the Immature Spirits (Restriction) Act of May 19, 1915, however this is now conducted as a refinement rather than a legal neccesity. Spirit flowing fresh from the still, called new make in Scotland, is not only completely colourless and high-proof (up to 85%), it also tastes, at least compared to whisky, far less complex and sometimes rough.
The introduction of the Immature Spirits (Restriction) Act 1915
In February 1908 when the the government announced that a Royal Commission on Whiskey and Other Potable Spirits was to be established to debate the legitimacy of the Coffey still, after 37 sittings the Commission concluded:
‘…that “Whiskey” is a spirit obtained by distillation from a wash saccharified by the diastase of malt, that “Scotch Whiskey” is whiskey, as above defined distilled in Scotland.’
Interestingly the commision made no mention of the use of wood for maturation, because prior to 1915 this was not a requirement. Although barrels had long been used to transport spirits and the maturing effectiveness of whisky was understood, and commonplace among reputable whisky producers, this additional step was not seen as neccesary by some for what was then a very working class drink*. Instead maturation became commonplace due to an attempt by the teetotal politician David Lloyd George to restrict the availability of whisky and other spirits. Having previously failed to introduce prohibition in Scotland Lloyd George attempted to inflict a damaging blow that would put the UK whisky industry out of business forever. The Act stipulated that Scotch whisky must mature in wooden barrels for at least 2 years before it can be sold, a year later this lower limit was raised to 3 years. To account for leap years an extra day was added.
The Act was initially successful, of over 130 Scotch whisky distilleries operating in 1914 only a handful survived the financial and logistical burdens by the end of the war. Ultiamtely however the attempt backfired, the distilleries that survived, and the enhanced reputation of mature whiskies spread and helped establish whisky as a premium product.
*Although whisky had become more popular in small part due to the shortages of brandy caused by Phylloxera in the preceeding 19th century the sufficient whisky was still sold with little aging to the poor to damage the drinks reputation.
As it was discovered that longer maturation in oak casks developed more interesting and sophisticated flavours what began in part as a legal requirement has now become the norm. Whiskies often mature for periods from 8 to 12 years as entry level offering, but there are also storage times of 15, 20, 25, 30 and 50 years, though older bottlings are understandably rarer due to loss to evaporation, the so called angels share. As casks are typically blended or vatted together the age statement on the front of the bottle must be that of the youngest cask. In part for this reason, and a lack of more mature stock distilleries are beginning to introduce more No Age Statement whiskies.
Barrels as storage
Barrels have long been used to transport and store all kinds of things, including food and liquids of all kinds. prior to the introduction of shipping containers the cask reigned suppreme as the most easily transportable way of storing and moving goods. For this reason the use of sherry transport casks from Spain, wine casks from France and beyond made their way to Britain’s ports and rather than being shipped home empty these were sold on to distilleries who discovered the casks imparted unique flavours of their own at a price far cheaper than that of sourcing a brand new barrel.
Whisky maturation in oak cask
It was recognized early on that oak is better suited to the manufacture of barrels than other types of wood. This is because, first of all, it is easy to process and not too porous, ie oak wood can actually hold liquid relatively undamaged for several years. Second, however - and this point is at least as important - we perceive the flavor substances given off by oak as comparatively pleasant and “suitable”, which in turn does not apply to many other types of wood.
After years of trials and experiments, two types of wood have become well established for storing and maturing whiskey; American white oak (Quercus Alba) and European oak (Quercus Robur). Japanese Mizunara Oak (Quercus Crispula, considered a variety of Quercus Mongolica by some) is also used albiet very infrequently as it is scarcer, far more porous and prone to leaking. While other cask types such as Chestnut have also been used in the past oak is the only kind currently allowed within Scotland, other countries such as Ireland where rules are less strictly defined have recently been experimenting with other casks.
Today’s standards of barrel storage for high-proof spirits developed independently in Scotland and the USA. While there was no shortage of available oak in the vast area of North America in the 19th century, the situation was different in the Highlands in northern Great Britain. In this respect, it is understandable that discarded wine barrels were used in Scotland, while fresh oak barrels were used overseas. The distinct taste notes of different barrel storage traditions can therefore be traced back to the chance of different availability.
Today most of the casks for the basic maturation of whiskey come from the USA; American white oak is the international standard. However, single malt still most often gets the finish in old wine barrels. The most popular are sherry casks from Spain (mostly in barrel sizes of 500 to 600 liters). Whether new barrels made of American white oak for basic maturation or European oak in the form of used wine or brandy barrels, oak is now the standard for whisky, grappa, cognac & other matured spirits.
Processes during the maturation of whiskey in the barrel
The years of maturation and storage in the barrel are still not perfectly understood however the processes that happen to the distillate prior to bottling can be described roughly. The maturation of a whisky takes place in three very essential steps through a complex chemical reactions described as additive, subtractive and interactive maturation.
During additive maturation, the distillate begins to draw flavors and colours from the wood. They are distributed throughout the liquid and a large number of chemical compounds are formed, these include tannins, vanilla, syringaldehyde or eugenol. These aromas come from the wood and especially the toasted inner surface of the barrel. In addition, during additive maturation, the whiskey absorbs compounds from the liquid that was previously stored in the barrel such as bourbon, sherry & wine etc.
After the whisky has built up a broad spectrum of aromas in additive maturation, subtractive maturation is now about breaking down unwanted flavours. Metallic and sulfur notes are reduced and the process of masking occurs, masking refers to the molecules released from the whisky barrel that overlay existing aromas and flavours. Another aspect of subtractive maturation is to be found in the form of oxidisation which can strip away and reduce pungent aromas.
The third area of maturation of whisky is the least fully understood, so called interactive maturation. This is where the peculiarity of the oak wood and the influence of the climate come into play. During interactive maturation, gases enter and escape the cask. There is a lively exchange between the contents of the barrel and the environment through the barrel wall. What enters or leaves is determined by temperature fluctuations and humidity, as well as the peculiarities of the warehouses and their location. In the process of interactive maturation, the distillate character of the distillate is combined with the character of the wood of the barrel to create new compounds flavours and esthers resulting in a unique and complex product.
Until the last few decades whisky was largely matured according to the processes above then diluted with water and bottled typically as part fo a blend or vatting. For some years now another feature has become a prevelant part of the whisky production process in some distilleries, namely finishing. Finishing refers to an additional maturation process which takes place within a second, or even third cask. Primarily these are casks in which wines such as Sherry, Madeira, Port, Bordeaux, Syrah, Chardonnay, Burgundy or Marsala have been held, however other barrels, such as rum, cognac or calvados can and are increasingly being used for this second maturation.
This allows for the creation of whiskies with completely new flavour profiles, and allows distilleries to salvage or refine casks which have not matured as expected. These finishings may not convince every purist but have nevertheless significantly expand the range of whiskies available to try. Interesting examples of whiskies with finishing are the Madeira Finish from Glenmorangie, the Port Wood from Balvenie or the Dusk from Bowmore. The finishing goes so far that for the second maturation of malt whiskey, for example, you also use barrels in which a malt has already matured.
Why are whiskies only matured in oak?
By law whiskies can only be matured in oak casks in Scotland, though this was not always the case, Chestnut casks were once quite common as well. Other countries such as Ireland, Sweden and Germany are far more flexible on cask type. However for a number of chemical and technical reasons oak and other large growing, not overly porous, and easy to work with woods are preferred.
What does oak do to whisky?
The longer a whisky matures in oak barrels, the stronger the wood aromas, and the flavours left behind by its previous content become within the spirit. Young whiskies primarily derive flavour from the grain and production process, older whiskies from maturation. The intervening years are a eb and flow of flavours and aromas due to a combination of additive, subtractive and interactive maturation.
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