The complete guide to oak, casks, & whisky maturation

Whisky maturation is both a simple and, yet, incredibly complex process.  Let’s look at the subject in detail; explore what the distilleries are doing; address a few myths; and explain all the beautiful things that happen when we age whisky in oak casks…

One of the quintessential images associated with whisky these days is the classic “action shot” inside the warehouse.  Think of those old-school, dunnage style, dimly-lit warehouses with earth floors and casks racked just two or three tiers high.  It’s an evocative image, showcasing the majestic oak casks quietly slumbering, doing their part to mature the spirit that will one day give us delicious whisky. 

But it wasn’t always this way.  Whisky – uisge beatha, or the water of life – was originally an unaged spirit, taken off the still and flavoured with additives such as honey or heather to quell the drink’s aggressive nature and to make it more palatable.   Tradition, or perhaps just a convenient story for the distillery tour guides, tells us that the maturation and aging of whisky was a happy accident.  Variations on the story abound, but the crux of it is that some unfortunate person had more uisge beatha than he could immediately consume, and so he put it in an oak cask to store it until he was ready for it.  The cask was left alone or forgotten about for some time, and when it was finally retrieved, he made the happy discovery that the spirit had mellowed, improved, and taken on new characteristics.  Maturation, as we know it today, was born.

Of course, the reality of it – certainly in today’s commercial industry – is that whisky production is more than just sticking the spirit into oak casks for a few years and waiting until it’s ready.  Today’s producers forge and shape the final character and flavour of their whisky by deliberately and strategically manipulating several different parameters of maturation.  These parameters include:

• Time – that is, how long the spirit spends in the wood. We’ll look at this in more detail in a moment.

• Oak species – for example, American oak, Spanish oak (aka European oak), French oak, and so on. The different oaks have different porosity, permeability, cellular structure, wood sugars, and chemical make-ups that all impact how the spirit, wood, and oxygen interact with one another to shape the development and character of the resulting whisky.

• Freshness of the cask – that is, is the cask being used for the first time, or is it a refill cask that’s previously already matured whisky for a stint? (The terminology will be 1^st fill, 2^nd fill, 3^rd fill, etc, or simply just refill).  Clearly, a 1^st fill cask will have a faster and more pronounced influence on the spirit than a 3^rd fill cask.  However, this needs to be carefully monitored – we don’t want the wood influence to dominate or overpower the spirit to the point that it’s too dry and woody. 

• Cask provenance and history – the vast majority of casks used for maturing whisky (particularly Scotch whisky) will have been seasoned by maturing some other liquid previously, e.g. bourbon, sherry, etc.  Obviously, the cask’s previous filling soaks into the wood, changing its nature and also absorbing the more “up front” woody flavours.  The previous filling might also leave behind a residual flavour footprint.  Hence, a cask that previously held bourbon will impart very different flavours to the spirit than a cask that previously held, say, chardonnay or port.  The use of new “virgin oak” was traditionally shunned in the Scotch industry but, more recently, is finding favour when used as a finishing cask, or for short maturation periods with newer distilleries looking to bring youthful spirit to market sooner.

• Cask size – the influence of the wood is obviously a function of how much of it there is, relative to the amount of spirit. In the case of smaller casks, the ratio of the contact surface area of wood relative to the volume of spirit contained is larger, and so smaller casks will influence the spirit at a relatively faster rate.  Larger casks such as 500 litre sherry butts might therefore need longer time periods to mature their whisky than, say, a 250 litre sherry hogshead.   

• Finishing the whisky in a different cask to the one it was originally filled into – also sometimes referred to as “ace-ing”, the classic example is whisky that might spend, say, 8-10 years in an ex-bourbon cask, then get finished in an ex-wine cask for 6-12 months. The finishing cask will add a new nuance or texture to the whisky, ideally adding also to its complexity.  A variation on finishing is wholesale re-racking – a not uncommon practice whereby whisky sitting in tired, old, casks is rejuvenated by transferring it into new casks to freshen and liven up the spirit.  Both Bruichladdich and Benriach were keen exponents of this when those brands were re-activated in the 2000’s with stock that had previously been filled into poor quality or tired casks; Tamnavulin undertook a similar program in 2017 with stock that was originally ear-marked for blending but instead re-purposed for bottling as a single malt, and thus needed to be “polished up” by re-racking into 1^st fill ex-bourbon casks.    

• Climate – Admittedly, this is a parameter that distilleries can’t particularly “control” or manipulate; they simply inherit the environment and climate of the region they build their warehouse in. Not surprisingly, Scotland’s consistently cool and damp climate has a very different impact on maturation to the hot and humid conditions of, say, India, which is different again to the hot and dry conditions of Australia.  Compare this with, say, the variance experienced in Kentucky, USA, where the maturing whisk(e)y is subjected to warm summers and freezing winters.   Distilleries – particularly the bourbon producers in Kentucky – will even utilise micro-climate to their advantage, rotating casks “vertically” through their warehouses to capitalise on the warmer temperatures at the top of the warehouse, compared to the lower temperatures at the lower levels.

When we speak about all the factors that contribute to a whisky’s flavour, there are many variables:  The barley grain variety, the use of peat, the strain of yeast used for fermentation, the duration and style of that fermentation, the shape and size of the stills, how the stills are run and where the spirit cuts are made, the type of condenser used, the amount of contact with copper, the local climate/ environment, and so forth.  The nuances, combinations, and permutations of all these are near infinite.  Notwithstanding that, the accepted wisdom is that, of the final flavour of the whisky that we enjoy in our glass, 60-70% of that flavour may be attributed to the influence of the oak cask and the maturation the whisky received.   On that basis, oak and maturation is king.  So what’s actually happening during maturation, and why should whisky drinkers be cognisant of these things?  Let’s explore some of the abovementioned parameters in more detail….

Time

Maturation of whisky in oak is a three-pronged action that requires three different processes to occur.   These three processes may be summarised as being additive, subtractive, and interactive.  They are worth elaborating on:

• Additive: The cask will add flavour and character to the spirit.  Wood sugars, oaky notes, tannins, the influence of the previous filling (e.g. sherry), and colour will all be added to the spirit.

• Subtractive: The cask will subtract certain volatiles and compounds from the spirit, making it more mellow, softer, and approachable.  Casks are typically charred before being employed in the drinks industry; the charred wood acts as a natural filter that removes undesirable compounds such as sulphur.  (As an aside, this is why the distilleries that produce a more sulphury new-make often benefit from longer years in the wood).  Other volatiles in the spirit, some of them undesirable, will be lost through evaporation.  The “angels’ share” is the lighthearted term given to evaporation, but it’s a serious matter for whisky producers in warmer climate countries:  Filling a 200 litre cask with your precious spirit and only having 100 litres of it left in the cask after five years is a challenging issue.   

• Interactive: This is where the magic happens.  Certain chemical and molecular reactions occur between the wood, the spirit, the residuals left by previous fillings, and – most importantly – oxygen, to create new flavours/aromas.  The casks breathe; the spirit oxidises; and the oak and spirit transform one another.  This interactive process can be where the different oak species produce different results, as they vary in the nature of their chemical and biological make-up:  The differing characteristics of the wood’s density, permeability, hemi-cellulose, lignins, tannins, oils, lipids, and so on, will all impact the spirit differently.  Despite the modern Scotch whisky industry being more than 250 years old, commentators and industry professionals such as Dr Bill Lumsden of Glenmorangie believe that this third interactive process is only just now starting to be understood from a scientific perspective.

Time is therefore of the essence, and due time is needed for all three of these processes to play out.  Three years is the minimum period of maturation stipulated by law in Scotland, which was set as a minimum benchmark for quality….although the climate and production regimens in Scotland generally dictate that most whisky will take 8-15 years to peak.  Climate and environmental conditions (namely heat and humidity) obviously have a large influence on maturation – particularly evaporation – and so different countries factor this into their production methodologies and timescales.  There’s a truism that whisky matures faster in hotter countries, but it’s important to understand the difference between true, three-pronged maturation and simply base evaporation and taking on colour.

Evaporation over time is a complex problem for distillers, and it differs the world over, subject to the local climate – particularly the local humidity.  The conditions in Scotland lead to the alcohol evaporating faster than the water within the spirit, meaning that the alcohol content of the whisky decreases with time.  In contrast to this, the conditions in Kentucky, USA, lead to the water evaporating faster than the alcohol, meaning that the alcohol content of the spirit actually increases over time!  

One of the challenges for the so-called new world whisky producing countries is to find the balance or sweet spot with their maturation programs whilst juggling the many forces, demands, and financial factors at play for start-up operations.  For new distilleries wanting to bring aged whisky to market sooner and bring in much-needed cash flow, there is temptation and a growing tendency to adopt small-cask maturation in active casks to supposedly fast-track the process.  However, as many distilleries in Australia have found to their detriment, aging spirit in 20L or 50L casks for just over two years in ex-wine casks might produce a dark whisky that initially seems ready to some drinkers, but the reality is that only two of the three maturation processes discussed above have partially occurred:  The additive and interactive processes have occurred to some extent, but there simply hasn’t been sufficient time for the subtractive process to fully play out.  The result is often hot and dry whisky that is over-oaked yet under-matured.   The situation is exacerbated at distilleries who insist on using ex-wine casks that have only been toasted, rather than heavily charred, meaning that their casks are less capable of removing (subtracting!) the volatiles and other aggressive compounds that maturation seeks to address.   (Hence leading to whisky that is described as “hot”, regardless of the actual ABV percentage.)

Fans of peated whiskies should also be aware that peatiness decreases over time.  The phenol levels in the spirit reduce with years in the wood (there’s that subtractive process again!) and so those wanting to experience the real “smoke bombs” should be seeking younger whiskies and not necessarily older releases.

Oak species

It’s no accident that oak is used for making barrels.  Coopers have been using it since the middle ages.  As natural timbers go, it’s strong, durable, and well suited to storing liquids.  Coopers could certainly make barrels and casks out of different species of timber but in both the American and Scotch whisk(e)y industries, oak must be used by law.

When the current single malt boom had its genesis in the early 1990’s, the situation in the Scotch whisky industry – and thus the lore that was written in the books of that era – was pretty much that whisky was matured in either American oak casks (quercus alba) that previously held bourbon, or European oak casks (quercus robur) that previously held sherry.  As a gross generalisation, that remains a not-unreasonable statement, but it conveniently ignores a few details, and certainly overlooks some current practices.

The first ignored detail is that a lot of ex-sherry casks used in the industry are actually American oak.  For more information on sherry and how sherry casks are used in the Scotch whisky industry, you can read Whisky & Wisdom’s The whisky lover’s complete guide to sherry.    The other overlooked fact that is an increasing amount of whisky is being either fully-matured or partially finished in French oak barriques – and certainly so in other whisky-producing countries.  French oak (quercus petraea) is less dense than its American oak counterpart, and is favoured in the wine industry for being more subtle in the flavours it can impart to wine (the additive and interactive processes).  That being said, it also imparts firmer tannins to the wine.  The presence of these tannins needs to be carefully monitored and kept in check when French oak ex-wine casks are subsequently used in the whisky industry.  A “double whammy” can occur if the tannins in the French oak combine with residual tannin soak left from a previous wine filling, resulting in an overdose of tannins that overwhelms the whisky and destroys the balance.  This is, again, a growing issue for the Australian whisky industry, which favours French oak ex-wine casks due to their abundance and cheaper affordability, courtesy of that country’s enormous wine industry.  The issue of overly-tannic whisky in Australia is discussed in detail here.

As you’d expect, the different oaks impart different characteristics and flavours to the wines and spirits they mature.  American oak, which is rich is vanillin, typically imparts sweeter notes, with flavours falling into the vanilla, coconut, and tropical fruit families.  European oak (often referred to as Spanish oak in Scotch whisky circles) imparts characteristics that are typically drier and spicier, including dried fruits, cinnamon, and citrus. French oak combines an unusual mix of both savoury and sweet, with common descriptors including tobacco, cedar, nuttiness, coffee and chocolate.   

You can experience these differences first hand, and very effectively.  The visitor centres at some distilleries (Highland Park being a good example) have sample casks of both oak types (that is, American oak and Spanish oak) that both held the same seasoning sherry.  The differences in the aroma of the oak and the flavour of spirit produced is surprisingly stark!  Similarly, try a whisky that’s been matured in an ex-oloroso sherry cask that’s made of French oak….the contrast to the more familiar Spanish oak aromas and flavours of, say, a Glenfarclas or an Aberlour a’Bunadah will be very apparent, even though the oloroso sherry gave them a common thread.

There’s often a lot of excitement and chatter in whisky circles around the use of Mizunara oak – an oak native to Japan.  However, such excitement is often more hype than reality.  Commercially, Mizunara is difficult to work with:  For starters, the tree generally needs to be 200 years old, and the wood is unusually porous, meaning leaks and evaporative stability are constant problems.  Quality Mizunara casks are rare, making them a particularly expensive product.  Whiskies matured or finished in Mizunara oak tend to carry high price tags accordingly, but most consumers probably don’t fully understand certain realities:  The general consensus amongst the experts is that Japanese and Scotch whisky needs around 15 years minimum in Mizunara casks for the true qualities, properties, and flavours of the oak to be realised.  Whiskies that are younger in age, or were merely finished in Mizunara oak for short periods, are not benefiting from or capable of displaying the very characteristics that Mizunara oak can impart. 

Cask history – 1^st fills and refills

It’s a long-used cliché, but casks can be likened to tea bags and brewing a cup of tea.  If you make a cup of tea with a fresh, new tea bag, you’re going to get maximum flavour and extraction.  If you elected to make a second cup of tea using the same tea bag, then it follows that the leaves’ influence will be significantly diminished, and you’ll need to steep the leaves for significantly longer to get a satisfactory result. 

And so it is with casks.  1^st fill casks can give strong, assertive results in a shorter time frame; 2^nd fill casks need longer – at least for the additive process to play out.  It’s not uncommon for casks to be used a third time (3^rd fills), and some distilleries will even put a cask back into service for a fourth filling, depending on the spirit and intended purpose.  5^th and 6^th fill casks are a bit like the Star Wars prequel films:  They definitely exist, but it’s considered uncool to talk about them.

With the massive boom in the whisky industry and demand for casks at an all-time high – plus the cost of casks increasing accordingly – tired casks that might once have been discarded are now being rejuvenated to get a second lease on life.  The internal surface of the cask is scraped back to fresh wood, then re-charred to re-instate the all-important charcoal filter.  “De-char, re-char” is the common phrase and process in Scotland, and many casks will also receive a level of toasting prior to re-charring, in order to better break down the wood, release the vanillins, and soften the tannins.  Casks or whisky releases noted as STR undergo this process, with STR standing for Shaved, Toasted, and Re-charred.

(As a secondary comment here, the amount or level of charring is a further parameter that can be tweaked to influence whisky maturation.  As discussed earlier, charring is critical to whisky maturation – it promotes fast lignin breakdown and caramellisation of the hemicellulose, thus extracting considerably more guaiacols, isoeugenol and vanillin into the spirit. Courtesy of the bourbon industry, four levels of charring are recognised, depending on the amount of time that the wood is fired for – it varies from 15 up to 55 seconds.  Obviously, the amount/extent of charring will have an impact on both the additive, subtractive and interactive processes of maturation.  Level 4 char – also known colloquially as “alligator char”, owing to the pattern it imparts to the wood’s surface – has been used by marketers to good effect, as demonstrated by Ardbeg with their “Alligator” release).  

Whilst most people don’t fancy the idea of a cup of tea made from tea bags that were already previously used, the Scotch whisky industry, in particular, benefits tremendously from 2^nd fill casks.  Long-term maturation in 2^nd fill casks allows the magical interactive process to really play out without the additive process becoming over-bearing or upsetting the balance.  Single malts matured from 15 to 25 years in 2^nd fill casks produce some of the finest results.

John Grant, current Chairman of Glenfarclas, is a fan of 2^nd fill casks, and believes that the best Glenfarclas whiskies come from refill casks.  Glenfarclas, of course, is famed for maturing its whisky in ex-sherry casks, and the distillery is careful to ensure that 1^st fill casks aren’t left for too long.  The danger is that the sherry-influence would over-power and dominate the spirit.  Most releases in the Glenfarclas portfolio (e.g. the 10yo, 12yo, 15yo, 17yo, etc, etc) are thus a careful vatting of casks that combine 1^st fills and re-fills; American oak and European oak.  Macallan goes down a similar route with its Double Cask range, further introducing the influence of ex-bourbon casks in its Triple Cask Matured range.

 

Cask size

It is risky making such generalisations these days but, for the most part – and ignoring puncheons and port pipes, etc – Scotch whisky is chiefly matured in three different cask sizes:  Barrels (180-200 litres), Hogsheads (250 litres), and Butts (500 litres).  Butts are unique to the sherry industry, hence it can be safely assumed that a butt being used in the whisky industry will previously have held sherry at some stage in its life.  And if you’re wondering about Quarter casks, we’ll address them in a moment.

In the decades between World War II and the early 2000’s, it was common (if not de rigueur) for 200 litre ex-bourbon barrels to be broken down into their individual staves and shipped across to Scotland where they were subsequently re-assembled into larger hogsheads (250 litres).  Increasing the number of staves would create the larger cask; larger ends or heads were then fitted to close off the larger diameter.  Five barrels would subsequently produce four hogsheads, and it was thus possible to fit more casks and a greater volume of whisky inside each warehouse.  Thus, whilst hogsheads are still regularly coopered for the aging of sherry and subsequently find their way into the Scotch industry, the vast, vast majority of hogsheads in Scotland are actually ex-bourbon – a fact that’s often under-stated and not widely appreciated.  

However, with coopering being a dying trade and the cost of labour so high, the practice of re-coopering to enlarge the casks has fallen out of favour, and ex-bourbon barrels are now generally shipped whole across the Atlantic.   Whilst there are still ample hogsheads in the industry (mostly now refills), the industry’s demand for casks and their high costs have kept distillery accountants awake at night.  The response from many distilleries has been to now fill their casks at higher ABV filling strengths.  Whilst it was once standard practice to water the spirit down to a common filling strength of 63.5% ABV before laying the casks down for their long slumber, distilleries realised it was getting increasingly expensive to mature water.  Accordingly, at many distilleries, casks are now being filled at higher strengths of 65-69% ABV to reduce the number of casks needed.  This is a key reason why many independent bottlings of single cask releases are now coming out with ABV’s at 59-62%, whereas up until 10-15 years ago, ABV’s of 55-59% or so were more the norm.   (Higher filling strengths are nothing new, however.  For example, it’s a higher filling strength that allows Glenfarclas to produce its “105” expression – even with age statements of 22 and 40 years old!)

As established earlier, maturation – or at least the influence of wood – can be faster in smaller-sized casks, owing to the ratio of the surface area of the wood to the contained liquid.  (For an excellent mathematical explanation and demonstration of this, check out the article by my colleague, Matthew Fergusson-Stewart, on his Whisky Molecules’ Facebook page here and scroll down to the article posted 18/05/2018.)  Notwithstanding the shortage of aged whisky that led to the NAS explosion, the higher proportion of faster-maturing 200 litre barrels to 250 litre hogsheads in the industry these days may be another reason why distilleries are willing and able to put out younger yet still impressive releases of late – note the recent releases of numerous 8yo age-statement whiskies by several well-known brands.

It was the old Allied distilleries – chiefly Laphroaig and Ardmore, both now part of Beam/Suntory – that made a virtue of quarter casks.  Quarter casks are 125 litres and hold roughly half that of a Hogshead, leading to faster maturation (all other things being equal!).  Laphroaig used this to great effect with their original Quarter Cask release in 2004.  The whisky was aged in regular Bourbon barrels for around five to six years, and then finished in Quarter casks for around six months.  The result was a seemingly fully-matured whisky that had an altogether different and pronounced flavour profile to its older 10 year old stablemate.   (Noting also, as discussed earlier, that the phenols in whisky decrease over time, hence the younger Quarter Cask release was also perceived as being peatier than its older sibling). 

 

The cask’s previous filling

We’ve covered the basics of this already in the sections above, so we needn’t go into too much more detail here about the influence on flavour from a cask’s previous filling (e.g. bourbon, sherry, port, wine, etc). 

However, you may be wondering at this point why ex-bourbon casks from America are so prevalent in the Scotch whisky industry?  It is a function of legal definitions and simple economics:  One of the requirements and laws of bourbon production is that the oak casks must be made from American oak (quercus alba); they must be  virgin casks; and the casks can be used only once.  If a distillery elects to re-fill a cask a second time, the spirit is still American whisk(e)y, but it cannot legally be called bourbon.  Since the majority of casks cannot effectively be re-used by the American distilleries, they are sold as unwanted discards to the Scottish distillers (and other distilleries around the world) who happily buy them up.   In very rough terms, an ex-bourbon barrel is roughly one tenth of the cost of an ex-sherry butt, so ex-bourbon barrels are thus an economically attractive prospect.  (The exact costs of ex-bourbon and ex-sherry casks are listed in our article, The whisky lover’s complete guide to sherry, which you can read here)

Finally, no article on the use of ex-sherry casks in the whisky industry is complete unless the issue of sulphur is discussed.  There’s nothing worse than paying good money for a rich, dark, delicious-looking, sherry-matured whisky, only to pour a dram and find that it smells of rotten eggs, boiled cabbage, brimstone, and rubber tubes.  The culprit here is the use of sulphur candles that are burned inside a sherry cask to kill off bacteria.  Whilst the practice has supposedly stopped, the sulphur contamination stays in the casks permanently, and will thus still be present in the industry for decades to come as casks are re-filled and used a second, third, or fourth time.   For more information on sulphur from sherry casks, see Whisky & Wisdom’s article, “The stink about sulphur“, which received industry acclaim on the subject and is oft-quoted and referenced.

– Conclusion –

Like a Level 1 char, we’ve barely touched the surface here, but the above discussion hopefully gives you some insights into the many complex variables and parameters of whisky maturation.  As stated above, the common wisdom (rightly or wrongly) is that the oak cask and maturation contribute roughly 60-70% to the final flavour we experience in whisky.   That means that 30-40% is coming from other sources and influences.  And that, dear reader, is an article for another day….

Cheers,
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PS…if you enjoyed this article, you might also like our other “explainer” articles:

The complete guide to peat and peated whisky

The whisky lover’s complete guide to sherry

The complete guide to non chill-filtered whisky

The 1980’s heavy metal guide to whisky

Oxidation – does whisky go off in the bottle?

Corked whisky – fact or myth?

The stink about sulphur