CASKS USED FOR MATURING WHISKY
THE BASICS
Maturation in casks to provide taste, colour and provenance is one of the main reasons whisky is so widely popular across the globe. Interactions between the organic compounds in newly distilled spirit and the chemical compounds in wood produce a wide and complex range of flavours, and aging it in different types of wood can create incredible variations in taste. This is a crucial fact to pay regard to, since the raw make that enters the Spirits Safe has an obnoxious taste which has to be cautiously and continually nurtured, first to an acceptable and then to enticing taste. Evidently, whisky gets much of its flavour from the wood it’s aged in. If many different types are used, what do they add? The key points about whisky maturation are:
- The number of times the cask has been used–the more times, the less flavour will be imparted.
- The size of the cask–the greater the surface-area-to-volume ratio, the more flavour the whisky will get.
- The time spent in the cask–more time means more flavour extraction.
- The intensity of the spirit–a lighter style will get more influence from the wood than a heavier one.
CASK SIZES AND COMPONENTS
The bigger the cask, the longer it takes the liquid inside to mature.
Barrel, 180–200 litres, also known as an American Standard Barrel (ASB), is commonly used for bourbon. These are made from American oak.
Hogshead, 225–250 litres, is made by taking part ex-bourbon barrels to create one bigger cask. The Scotch whisky industry often prefers larger casks for ageing as this allows more whisky to be aged in the warehouse.
Madeira Drum, 600–650 litres, is made from French oak and used for Madeira wine.
Port Pipe, 550–650 litres, is made from European oak, and as the name suggests, is used to age port wine.
Sherry Butt, 475–600 litres, is made from American oak, although some are made from European oak. Butt is the most common size used for ageing sherry.
Barrique, 250–300 litres, is widely used in the maturation of wine and cognac. Barrique is mainly made from French Oak.
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THE AGEING PROCESS
- The temperature in the warehouse fluctuates day-by-day and during the changing seasons
- The resulting pressure change forces the whisky into the pores of the wood by way of the charred interior surface
- The whisky reacts with air/water and undergoes a chemical reaction that breaks down some of the organic compounds in the wood
- Some of the ethanol and water evaporates as part of the reaction and exposure to air causing the “Angel’s Share” to leave the barrel
- The wood constituents and whisky diffuse back into the barrel as the warehouse environment fluctuates
- Convection currents mix the contents of the barrel and the whisky turns darker brown as the cycle repeats
NUMBER OF FILLS
Bourbon must be aged in new oak, so when whisky is aged in a 1st fill ex-bourbon barrel, this means it is the first time that malt whisky has gone into that barrel after bourbon. In this case, the whisky will be heavily influenced by the wood, and therefore the spirit is only left in the barrel for a short period to avoid heavy wood influence. This can be around five years.
In Scotland, the same cask can be used for years and years, although it is uncommon for the cask to be refilled more than three times. So obviously, after the 1st fill you will have a 2nd fill, which averages around eight to twelve years, and, confusingly, the third and final fill is known as the refill. After the 3rd fill, the inside of the cask can be shaved to show new wood before re-toasting and charring.
Some distilleries will even put a cask back into service for a fourth filling, depending on the spirit and intended purpose. 5th and 6th fill casks definitely exist, but it’s considered uncool to talk about them. Most of them land up in the three-year grain whisky maturation process.
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. An example of Loch Lomond techniques regarding 'Charring' will follow. Casks or whisky releases noted as STR undergo this process, with STR standing for Shaved, Toasted, and Re-charred.
Long-term maturation in 2nd 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 2nd fill casks produce some of the finest results.
Glenfarclas prefers 2nd 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 1st 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 1st 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
MORE ABOUT OAK
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 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. Only oak may be used for the storage of Scotch whisky.
There are two main types of oak used for barrel making and another two used for variety, i.e., less often. Type of Oak Flavour Influence American white oak (Quercus alba) Vanilla and caramel, soft and mild European oak (Quercus robur & petraea) Spice and bitter notes Mizunara Oak (Quercus crispula) Sandalwood, coconut and oriental spices Maple (Acer) Sweet, maple syrup American white oak is found mainly in the eastern USA
and parts of Canada and is used to make the barrels for bourbon and Tenneessee
whiskey. Its growth is relatively strong for oaks. The wood has a high density
of 770 kg/m3and 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
contained is a plant tannin and belongs to the gallotanins. It causes the
bitter notes in whisky. European oak gives off much stronger and spicier aromas
than American white oak. It is mainly used to make barrels for European wines
and strong wines such as sherry. Mizunara oak is very widespread in the forests of
Northeast 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. In addition, oak lacks waterproofing oil
enzymes, so much more whisky evaporates during maturation than when matured in
American or European oak. Despite these characteristics, which make it much
more difficult to use for whisky casks, Mizunara oak gained popularity for
whisky maturation in the early 20th century, not least because of the special
aromas it gives off to the whisky, which are reminiscent of sandalwood, coconut
and oriental spices, for example. Maple grows in large 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 brandy in maple barrels to give the whiskeys the sweet
notes of maple and maple syrup. While only oak may be used in the most important
whisky-producing countries of Scotland and the USA, other types of wood, such
as chestnut, are also used in other countries. COMMON TYPES OF WHISKY CASK Bourbon Bourbon barrels are made from American white oak. They
go through both toasting and charring before the bourbon is added. Also, the
ABV level of the bourbon will impact the flavours of the barrel – the higher
the ABV, the bigger the impact on the wood. Basically, the higher alcohol level
will strip the barrel of the benefits of toasting and charring, leaving less
flavour for the next spirit. Bourbon barrels are the most common cask used for
Scotch whisky. Why? It’s all down to supply and demand. By law bourbon must be
aged in new casks, meaning that after their first use they are generally
surplus to requirements. They are also cheap compared with other types of cask,
which leads to high demand from Scotland (and other distilleries around the
world). For example, the exclusive Balblair 1997 from
Gordon & MacPhail shows the classic sweet, fruity style that
bourbon casks impart along with the trademark notes of coconut and vanilla. The charcoal in the cask softens the bold flavours from the distillation process during the charring process. Depending on its tastes, each distillery uses varied toasting and charring. Used bourbon barrels are typically burned for 40 seconds, though this can occasionally increase to 3 minutes. The more char there is, the easier it is for the spirit to enter the pores of the oak.
The charcoal in the cask softens the bold flavours from the distillation process during the charring process. Depending on its tastes, each distillery uses varied toasting and charring. Used bourbon barrels are typically burned for 40 seconds, though this can occasionally increase to 3 minutes. The more char there is, the easier it is for the spirit to enter the pores of the oak. Sherry The sherry industry avoids using new oak, and most of
the casks used in Solera are at least ten years old. These casks have been
seasoned with wine before being used for sherry. Different varieties of sherry
get their unique flavour through a range of ageing methods. For example,
oloroso has contact with air, which contributes to the fruity and nutty
flavours, while fino is protected from the air by a layer of yeast, also known
as flor, and this keeps the sherry light and crisp. Pedro Ximenez (PX) gets its
sweetness from the raisins; the grapes are left to dry in the sun before being
fermented. It is not uncommon for a whisky distillery to buy
European oak (mainly Spanish), have the casks coopered in Jerez and filled with
any sherry until they have enough sherry influence to be used in whisky making. There may be up to ten litres of sherry left in the
cask. It simply soaks into the wood, which will have a huge impact on the
flavour and colour of the whisky. The inside of the cask is usually kept
slightly wet while it is being transferred. Other contributing factors are the
number of fills and the type of oak used. The more fills, the less of those
dried fruit and Christmas spice flavours you’ll get. The whisky is often ‘finished’ in a sherry butt to
allow just enough influence without making the sherry flavour overpowering. Sherry casks (usually around 500 litres) are very
popular, but are very expensive (up to 10 times the cost of a bourbon barrel),
so many distilleries choose not to (or cannot afford to) use them on a regular
basis. Casks used for all the main styles of sherry are used for ageing whisky,
with the sweet Pedro Ximénez and rich oloroso the most common. Consider two
sherried whiskies: a refill butt from Signatory of Clynelish 1995
and Lagavulin 1995 Distillers Edition, which is finished in Pedro Ximénez (PX)
sherry casks. These add notes of chocolate, orange and dried fruits, with
the smokiness of the Lagavulin adding an extra dimension. Port There are several types of port available and each has
its unique style. Tawny port is golden in colour and is mainly aged in older
wooden casks to allow gentle oxidation. While tawny port can be aged in oak for
up to 40 years, vintage port only spends two to three years in the cask. Ruby
port is mainly matured in steel tanks to avoid oxidation, therefore ex-ruby
port wooden casks aren’t widely available. And when ruby port is aged in wood,
the maturation time is kept short. Port can be aged in large port pipes or smaller
port barriques. The most common port casks used for whisky are those used to hold
ruby port, although tawny or white are also used, the latter adding both
richness and a refreshing dryness. Casks used for the red styles of port add
notes of strawberry and raspberry. The Benriach 21 Year
Old Tawny Port Finish shows
wonderful balance between dry spicy flavours – clove and nutmeg – and red
fruits. Rum The Glenfiddich 21 Year Old, finished for around four months, has proved a very popular release since its relaunch at the start of the
decade, and with exceptional complexity and enticing notes of green apple,
pear, mango, fudge and crème brûlée, it’s easy to see why. Wine Ex-wine casks are not generally as commercial as bourbon or sherry casks – Maturing whisky in wine casks is still a process that is being refined and developed. The market for it is still in its infancy, and while it may expand with time, the current market generally values ex-sherry and ex-bourbon casks much more highly – there are a few exceptions for specific distilleries. Some distilleries have a more established history of working with wine casks – Distilleries such as Bruichladdich and Glenmorangie have an established history of working with wine casks. Any distilleries with a proven history of working well with wine casks can command a competitive market value. Wine casks vary from dry to sweet, with both red and
white used. Sauternes casks add sweetness and notes honeyed fruit, whereas dry
white wine, especially Chardonnay casks, will add butteriness and a
mouth-coating texture. Due to the tannin in red wine, these casks impart
dryness and fruity notes. The Glen Garioch 1998, aged for 15 years in
red-Bordeaux casks, displays lovely integration between the red fruit
(cherry and strawberry) and dry, earthy, smoke, with an underlying notes of
grape. Madeira Whiskies matured in Madeira casks usually pick up the
spice and fruitiness of the wine remaining in the staves. The effect on the
taste also depends very much on the type of Madeira previously contained. The
Malvasia grape is grown almost exclusively for Madeira production. It is very
sweet and provides the special taste of Madeira. The Estufagem barrel ageing production process is what
makes Madeira Wine so special. It is a special heat and humidity treatment
designed to simulate the climate during the voyage of the Madeira barrels on
the ships of the 15th century. Both stainless steel tanks and oak barrels are
used for ageing, and the law requires that the wine be kept at 55°C for at
least 90 days. Through this heat treatment, the wine undergoes many reductive
and oxidative phases and acquires its typical colour. After this ageing
process, the alcohol content is raised by adding brandy. A fortified wine from the eponymous island, these casks
add extra spice and dark-fruit notes, along with dryness or sweetness depending
on the style of Madeira. This year has seen a prominent release with the eighth
edition of Glenmorangie’s Private Edition series: Bacalta, which is finished
in sweet Malmsey Madeira casks. Marsala Marsala is an Italian fortified wine that comes from the town of Marsala in Sicily. The climate on the Mediterranean island is mild, with temperatures between 13° C and 29° C. The European Community has protected the name 'Marsala' for wine produced in Italy.
The biggest unique selling point of Marsala is the special ageing method, called 'perpetuum' by locals. This name comes from the (non-sparkling) wine that was previously produced in Sicily. In the perpetuum method, the barrels are never completely emptied. The barrels are filled with the latest harvest, left to mature and then bottled according to demand. When the next harvest is ready, the barrels are filled again. Thus, each Marsala wine is a combination of different vintages.
The difference between the local Perpetuum wine and Marsala is that Marsala is fortified with brandy - originally to make it durable for long sea voyages. Marsala has a sweet, complex and sometimes nutty taste. This also comes into play in whisky that has been aged in Marsala casks. The different Marsala wines have different colours, ranging from dark amber to light brown tones. The barrels used for whisky production previously contained mostly sweet, dark Marsala, so Marsala maturations or post-maturations are usually darker than normal whiskies. Ongoing releases of Marsala-matured whiskies are rare. The limited-edition Ledaig 19 Year Old is a rich, fortified wine from Sicily which can be dry or sweet, in both cases adding
extra complexity and extra spice. New (Virgin) Oak The purchase of a fresh Virgin Oak cask is a larger
investment for a distillery than that of a used Bourbon, Sherry or wine cask. Very seldom used for full maturation, as it imparts
powerful, spicy flavours which can overwhelm the spirit, it is useful for
finishing, where the extra toast can add a layer of complexity, especially to
longer-aged whiskies. An example of this is the newly release Bladnoch Talia 25
Year Old. Long-term maturation in 2nd 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 2nd fill casks produce some of the finest results. New charred oak imparts a flood of colour and flavour
that can give the initial impression that a spirit is older than it really is.
Benromach Organic is aged in new oak and bottled between 5 and 6 years of age,
but the whisky's colour resembles that of scotch aged in sherry casks for 12
years or more. The casks are highly active and the spirit matures quickly. They
use virgin American oak casks, and as such you can pick out strong vanilla and
toffee aromas. A spicy, peppery note develops through the freshness of the
wood. The caramel, which is produced by toasting and burning
out the barrel, gives the whisky its dark colour. We know this from American
whiskey, which is very dark although it is usually only matured in the barrels
for two to four years. The dark colour comes to the whiskeys from the cask
alone. This is because a fresh barrel not only has more aromas to give off, but
also more colour than a refilled one. For this reason, Scotch whisky usually turns out much
lighter when ex-bourbon casks are used. Much of the colour has already been
transferred from the oak to the bourbon. Virgin oak casks, even when
re-matured, result in a darker colour for Scotch whisky. Before the whisky is matured in the cask, the cask is
toasted and burnt out. During toasting, the cask is heated to at least 150°C.
This causes the cellulose to split and break down into wood sugar, which begins
to caramelise at these high temperatures. Only then does the cask release the
familiar caramel, vanilla and coconut aromas into the whisky. During the
subsequent burnout, you go directly into the cask with a large flame, which
causes a dark layer of activated charcoal to form on the inner cask wall. This
helps to filter out the pungent tastes and aromas from the whisky like an
activated charcoal filter during cask maturation. During toasting and burning
out, the distillery can vary the duration and thus depth of activation
depending on the desired result. On Islay, Bruichladdich Distillery first experimented with full virgin
oak maturation in 2008 by filling 300-litre, medium-toasted new French oak
casks with heavily peated new-make spirit, released in 2016 as Octomore 7.4. They got fantastic flavours developing
very quickly in the spirit, although, because the casks have so much to give,
they need to be watched very closely as they mature. The downside is that if one
take his eyes off the casks for too long, they can end up with a whisky where
the cask has dominated the spirit and it becomes overly oaked. While not for
everyone, virgin oak casks are adding new dimension to Irish, Scotch, and even
Indian whiskies, imparting wood-forward flavours that beckon like an old flame
to fans of American whiskey. Cognac One of the rarer cask types, Cognac casks are made from
French Limouisin oak and add aromatics and perfumed fruit, richness and subtle fruitiness
to the whisky. Cognac casks tends to be very old as cognac can be aged up to 50
years. One example of this
is Bruichladdich’s excellent Cuvée 640 Eroica. Cognac barrels initially use new highly active French
oak, which makes oak selection a critical aspect of each producer’s approach to
the finished spirit. The most abundant oaks are the Pedunculate Oak (Quercus
robur) and the Sessile Oak (Qeurcus pentraea). Hennessey extols Limousin oak
for the “rich and creamy vanilla character” it imparts in their spirit, but
Limousin is a forested region where both oaks grow, so that does not indicate a
preference for either species. Barrels are toasted and charred by naked flame at the
cooperage. Barrels are reused, much like in Scotch, to provide blenders with
various flavour profiles. “Futs-roux” is the term for refills. Cognac can be
filled into casks which have had almost all the flavour removed from the wood,
to allow for much greater ageing. As with a whisky cask, the alcohol will act as a
solvent to extract wood components such as tannins and vanillin from the
staves. Cognac casks have been used occasionally in Scotch maturation over the
last 50 years, including a Glenfarclas 43 year old, some interesting releases
from Bruichladdich where Cognac casks have been used for peated and unpeated
spirit, and Arran releases. It is becoming much more of a trendy proposition
these days as the stuffy old world of Scotch begins to open up to a bit more
experimentation. Cognac casks seem entirely pedestrian in the face of tequila
and mezcal casks. Many other cask types are used for ageing whisky. Some
of them work – the recently launched Glenfiddich Experimental Series IPA Cask
was finished in beer casks, resulting in a floral and fruity whisky – and some
should never be tried again.Caveat Emptor! Original
Content Category Influence
on Taste Bourbon Whiskey Vanilla, sweetness, caramel Amontillado Sherry Sweetness, nutty, dry, fresh, acidity Fino Sherry Light fruit, sweetness, dryness, light wood Manzanilla Sherry Salty, dryness, sea aromas, fresh, some fruit Moscatel Sherry Very sweet, dark fruits, sultanas, syrup Oloroso Sherry Deep, dark, nutty, dark ripe fruits Palo Cortado Sherry Rich, sweet, dry, sweet spices, fruits Pedro Ximénez (PX) Sherry Very sweet, dark fruits, sultanas, syrup Port (sweet) Port wine Sweet, dried fruits, spiciness Port (semi-dry) Port wine Slightly sweet, dried fruits, spiciness Port (dry) Port wine Dry, dried fruits, spiciness Ruby Port Port wine Very fruity, dark fruits, sweetness, dryness Tawny Port Port wine Very fruity, nut, caramel, chocolate and tobacco Madeira Liqueur wine Spiciness, light fruitiness, sweetness, dryness Marsala Liqueur wine Sweet, complex, spices Amarone Red wine Tannins (bitter), dry, sultanas, ripe fruits Barolo Red wine Fruits, tannins (bitter), dried fruits, heavy aromas Bordeaux Red wine Strong red fruits, grapes, berries Chardonnay White wine Light, fresh, sour, tropical fruits Muscat White wine Floral, sweet, citrus, peach Sauternes White wine Sweet, lemon peel, acidity, light fruit Tokay White wine Light fresh fruits (lemon, mango), very sweet Burgundy Wine Very fruity, slightly sweet, slightly dry Rum (white) Spirit Sweet, molasses, vanilla, tropical fruits, almond Rum (dark) Spirit Sweet, syrup, dark fruits, oak caramel, vanilla Beer Beer Hops, malt Virgin Oak Vanilla, cloves, caramel or tannins Mizunara Oak Ex Japan Fruity, sweet
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