HOW CASKS AFFECT MATURING WHISKY

 CASKS CAN MAKE OR BREAK AN EXPRESSION

What gives whisky its unique taste, colour, mouthfeel and aroma? The barrel — more specifically, the inside of the barrel. You can get all the steps right from the very beginning — the best grain, a pristine water source, a flawless distillation sequence — but it’s the maturation process that gives the spirit its character. And the barrel’s interior plays a big part in that. This article comes up again in a later post.

In Scotland, the spirit must mature in casks for a minimum of three years to be legally called whisky. In this time frame, the spirit is influenced by the cask it’s been matured in, which is why it is important to understand how these casks contribute. Obviously, the longer the time spent in the cask, the more the influence and hopefully, the better the quality and final taste of the whisky. The type of wood used, age, size and the previous liquid in the cask all matter. Different types of wood add different elements to the contents. The innate qualities of the new make are given the required finesse and final colour and taste by the cask.

Given the advancement in technology, it is possible to identify the different flavours using physicochemical methods. Even the smallest of variations can account for big differences in taste. That's how receptive our senses can be. Researchers measure the various substances like esters, tannins, furfural, congeners, lactones, etc., in ppm (parts per million) and are now reaching out to ppb (parts per billion) in a nanometric world.

THE WOOD

A wooden cask is a small masterpiece of craftsmanship. Although machines are increasingly used by coopers today, actual manufacturing is still done by hand. The planks for the staves aren't simply cut at right angles from a log, like construction timber. The grain direction of the wood must be taken into account so none of the radial vessels of the wood penetrates the side of the cask. Not only does wood contain annual rings but also vessels that lead from the core to the bark radially. The tree transports water and nutrients through these vessels. However, these vessels are inconvenient for whisky since they would make the staves leaky and allow excessive evaporation. Therefore the wood has to be cut in special patterns (star cut, mirror cut or rift cut) so the annual rings stand vertically. This is why far less usable wood can be cut from one log; a cask stave is thus much more expensive than a normal plank.

Yet another aspect has to be factored in- how the new make reacts with the staves of wood. Alcohol is insidious-it attacks from within, albeit at some micrometres per day. As it reaches into the wood, it meets both intractable and amenable wood. The alcohol slowly absorbs minute quantities of specific substances it can dissolve and integrates them with the new make. The more the reaction, the more the absorption. That said, the longer the stay in the barrel, the greater the evaporation as the Angel's share.

TIMBER AND STAR CUTS

These planks are then made into staves with trapezoidal vanes (according to the roundness of the planned cask). The newly made staves must then be dried until they reach a level of less than 10% residual moisture. This may be left to nature and solar heat or done quickly in modern drying chambers without affecting the quality of the cask.

If the casks were made from this wood, you would get a tight container, but the whisky cannot mature. From a maturation standpoint, the wood is still dead. Specific thermal treatment breathes life into the wood, as will be explained in depth later. The wood can be bent into the typical cask shape only with heat. The wood is heated up to 200°C in a big oven for approximately 30 minutes, and the firm wood structure is broken up. The cask begins to live in terms of maturation. Once the staves have been bent into shape, the cooper completes his job of assembling that cask.

                                      

A cut through a treated stave will reveal a red ring in the wood beside a charcoal layer of several millimetres depth, the so-called 'red layer'. This layer separates the 'activated' from the 'natural' wood. It is up to this layer that the heat has penetrated the wood and activated it for flavour extraction.

Only oak wood is suitable for cask production. Soft wood contains resin, which prevents the cask from breathing. Other types of wood have congeners that emit unpleasant flavours, making the whisky unpalatable. Oak wood from trunks with an age of 70 to 200 years is ideal. There are two fundamentally different species of oak: American white oak (Quercus alba) and the various European oak species (Quercus general). American white oak grows faster and has a mellower, softer and sweeter taste with notes of vanilla and caramel. In contrast, European oak is spicier with full, intense aromas and more tannins and has a stronger wood input that is prominent on the nose.

European oak grows in northern Spain and Portugal. French oak is used to age wine and cognac. An American oak tree can be cut down at 70-100 years, while the slower-growing European oak must grow for at least 150-200 years. Five major factors influence the Whisky in the cask: 

                                                 TYPE OF PREDECESSOR FLUID

Predecessor	Prior Spirit	Alteration of Taste	Colour Change
Bourbon	Whiskey	Vanilla, sweetness, caramel, creamy	Golden
Burgundy	Wine	Very fruity, slightly sweet, slightly dry	Dark red
Madeira	Fortified Wine	Spiciness, light fruitiness, sweetness, dryness	Dark, amber
Port (sweet)	Fortified Wine	Sweet, dried fruit, spiciness	Red
Port (semi-dry)	Fortified Wine	Lightly sweet, dried fruit,  spiciness	Red
Port (dry)	Fortified Wine	Dry, dried fruit, spiciness	Red
Oloroso	Sherry	Deep, dark, nutty, dark ripe fruits	Red, amber
Pedro Ximenez	Sherry	Very sweet, dark fruits, raisins, syrup	Amber
Fino	Sherry	Light fruits, sweetness, dryness, light wood	Bright
Manzanilla	Sherry	Salty, dryness, sea flavours, fresh, some fruit	Bright
Amontillado	Sherry	Sweetness, nutty, dry, fresh, acid	Amber
Palo Cortado	Sherry	Rich, sweet, dry, sweet spices, fruits	Brown
Sauternes	Wine	Sweetness, zest, acidity, light fruits	Bright to amber
Bordeaux (red)	Wine	Strong red fruits, grapes (wine), berries	Red
Tokaji	Wine	Light fresh fruits (citrus, mango), very sweet	Bright to amber
Ruby Port	Fortified Wine	Very fruity, dark fruits, berries	Red
Barolo	Wine	Fruits, tannins (bitter), dry fruits, heavy aromas	Red
Chardonnay	Wine	Lean, crisp, acidic, tropical fruits	Bright
Muscat	Wine	Floral , sweet, citrus, peach	Bright/red
Muscat	Fortified Wine	Very sweet, dark fruits, raisins, syrup	Amber
Rum (white)	Spirit	Sweet, molasses, vanilla, tropical fruit, almond	Bright
Rum (dark)	Spirit	Sweet, syrup, dark fruits, oak, caramel, vanilla	Amber
Amarone	Wine	Tannins (bitter), dry, raisins, ripe fruits	Red
Marsala	Fortified Wine	Sweet, complex, spices	Dark red
Virgin Oak	-	Vanilla, cloves, caramel	Dark brown

                                                   SIZE OF THE CASKS

The sizes of casks are awkward to define because there is no ISO standard regulating the volume of a standard cask. In the interim, a cask type taken as the baseline is the American Standard Barrel (ASB), which holds approximately 200 litres. ASBs are also the 'raw material' for the production of Scottish hogsheads, which hold approximately 250 litres. If you disassemble an ASB and use slightly bigger rings (hoops) for reassembly, you can produce casks with a bigger diameter from the same staves. Out of four to five ASBs, you can make three hogsheads. Since the trapezoidal shape of these staves is designed to contain 200 litres, you can’t make casks with even bigger diameters from them. The casks would eventually become leaky at the rifts.

There is another problem with the volume of the casks as the cask sizes are also a unit of measurement. Take the Butt for example. The normal butts come in sizes of 500 litres (132 US gallons). But there is also a measurement unit called a butt, which is 1/2 a tun and is 122 US gallons (477 litres).

Besides the type of wood and the thermal treatment, viz., the size of the cask influences the maturation process. Whisky matures faster in small casks since more of the content is in contact with the wood, as compared to large barrels. The exchange of substance between wood and whisky takes place faster.

In Spain and Portugal, European oak is made into casks holding 500 to 600 litres, which are ideal for the maturation of Sherry and Port. The Scots call these casks 'butts' or 'Sherry butts' and 'Port pipes'. Today more and more butts are made from American oak for financial reasons.

The following table shows the actual sizes the most barrels come in, not the sizes of the measurement units.

Name	Litres	US Gallon	Imperial Gallon
American Standard Barrel /Bourbon Barrel	200	53	44
Butt	500	132	109
Quarter Cask	125	33	27
Standard Hogshead	238	63	52
Puncheon	320	85	70
Madeira Drum	650	172	143
Port Pipe (tall)	500	132	109
Sherry Hogshead	245	65	54
Cognac type	300	79	66
Bordeaux type	225	59	49
Barrique cask	225	59	49
Bloodtub	50	13	11

Cask Sizes Displayed at Edradour Distillery

Casks are also grouped by volume. The Scotch Whisky Association has ruled that no wooden cask in excess of 700 litres may be used. The breakdown is listed below:

Size of Casks		Type of Casks
Big	> 400 Litres (>132 US gallons)	Butt, Port Pipe, Puncheon, Madeira Drum
Medium	200 - 400 Litres (53 - 106 US. Gallons)	ASB, Bourbon barrel, any Hogshead, Barrique cask, Cognac cask, Bordeaux cask
Small	<200 Liters (53 US. Gallons)	Quarter cask, Bloodtub

TYPE OF WOOD

Type of Wood	Effect on Taste
American white oak (Quercus alba)	mellow, soft, vanilla, caramel
European oak (Quercus robur and petraea)	spicy, bitter, strong on the wood
Mizunara Oak (Quercus crispula)	sandalwood, coconut, oriental spices
Maple (Acer)	sweet, maple syrup

There are major differences between the two primary types of oak. American white oak grows in the east of the United States of America and a few parts of Canada. The tree grows rather fast for an oak tree and is therefore a bit less expensive than the European counterpart. Its wood is very dense (770 kg/m³) and contains a lot of monogalloyl glucose. This is later evinced as the typical Bourbon vanilla taste.

European oak grows all over the European continent far into Russia and Turkey. It grows slower than its American counterpart and is less dense (720kg/m³). It contains Gallic acid which is considered a pseudo-tannin. This acid in combination with water gives the whisky a slightly bitter note. The European oak has also a lot of other components that also add to the spiciness of the whisky.

Mizunara oak is very common in the forests of northeast Asia, where it is used primarily for high-quality furniture. Due to its high density and thin fibres, the staves must be cut along the fibres, which never grow perfectly straight. The Oak also lacks waterproofing oil enzymes, so much more whisky evaporates during ageing than when stored in American or European oak. Despite these characteristics, which make it much more difficult to use for whisky barrels, Mizunara oak gained popularity for whisky ageing in the early 20th century, because of the special aromas it gives off to whisky, reminiscent of sandalwood, coconut and oriental spices, for example.

Maple grows in many parts of the world, including Eurasia, North Africa and Central & North America. With its relatively low density of 653 kg/m3, it is used primarily in the production of Tennessee Whiskey and is not used elsewhere.

CHARRING AND TOASTING

The barrel itself must be prepared beforehand. This is done by burning the insides. While this process may relatively straightforward, there are distinct two types of burning procedures that whisky makers use when crafting their barrels: toasting, and charring.

Heat causes hemicellulose within the wood to break down into natural sugars, resulting in toasty caramel notes, colour and aromas, while oak lactones add woody, coconut-like notes.

When exposed to heat, tannins become less astringent while oak lignin (a fibre within the wood) breaks down into flavour molecules like eugenol (spiced/cloves) and, more famously, vanillin (vanilla).

Charred Barrel

When comparing the two methods, the fundamental variable is the degree to which the wood is burned. Charred barrels, for example, are heavily burned and resemble the remains of a campfire after it has been extinguished. The interior of charred barrels is black and has much more ash residue, resulting in a much darker colour for the whisky. As far as flavour goes, charred wood imparts sweeter flavours like caramel and honey. The reason for this is that the wood sugars are caramelised when heavily burned, and thus they leach into the whisky.

Moreover, the carbon in the ash acts as a filter for the harsher elements of the liquor. Ageing whiskey (like bourbon) in charred barrels results in a smoother, mellower flavour.

At times, a barrel is given a number reflecting the degree it has been charred to on a scale of 1-4, i.e., a new American white oak barrel with a #4 char. This “barrel char level,” as it is called, basically means the higher the number, the deeper the burning char into the wood. There’s a lot of science around this; in essence, the darker the char, the more different the flavour profile and the more colourful the whisky will likely be.

Toasted Barrel

Unlike their charred brethren, toasted barrels are heated much more gently, resulting in a dark brown toast rather than a blackened char. Toasted barrels add a bit more vanilla flavour to the liquor, as well as spicy accents. Because the wood hasn’t been heavily burned, the sugars haven’t had time to caramelise, making the whisky a bit sharper on the tongue. Toasted barrels also do not impart much colour to the spirit, resulting in a lighter shade.

Toasting is a slower process. It involves gently heating the barrel’s interior over an open flame so that the heat can penetrate deep into the wood, which mellows and takes on a dark brown colour.

REUSE OF THE CASK (FIRST FILL vs REFILL)

Oak casks are very durable and can contain Whisky for many decades. However, over the years the aroma that the Whisky can absorb from the wood decreases. The cask is leached out. Whisky that is stored in new casks, therefore, absorbs the most aromas.

In the first (1st) fill casks, the term ‘1st fill’ doesn't mean the original filling of the cask with Bourbon, Sherry or Port, but the first Scotch Malt Whisky that's filled into a cask. No work is done on that cask either. This way, the 1st fill extracts the strongest flavours from the wood.

Bourbon is mandatorily matured in fresh casks and is aromatic enough to be bottled after only two years, leaving a still well-endowed cask. Scotch Whisky is matured in used casks, which have already been used, e.g., Bourbon or Sherry. The cask has already released a large part of the aromas. Therefore the Scottish Malts only become really good after longer maturation. However, this also gives the Whisky more time to reduce the alcoholic spiciness. Nowadays, this historically developed system is an essential distinguishing feature between the Whisky types.

Casks are an expensive commodity. Therefore it is common in the whisky industry to mature whisky in casks more than once. A cask still contains a lot of aromas after ten years of Malt Whisky maturation and is therefore reused for the next Malt. In the industry, they are called 'refill casks'. Refill casks are reused for up to about 30 years.

It is natural that the influence of the cask on the taste weakens progressively. It is no surprise if a whisky from a refill cask has absorbed very little flavour - and also colour. On the labels of some bottlers, you sometimes find indications like 'Refill' or 'First Fill', which reflects the degree of use of the casks. This gives you as a customer an indication of the approximate intensity of the cask aroma. However, it is also common to add colour to whisky with tasteless caramel. In this case, the influence of the cask is hidden.

It is more and more common to refurbish casks, as this is cheaper than buying new ones. This brings out the oak aroma again. Sherry, Port or Wine aromas cannot be reproduced in this manner. These 'rejuvenated' casks make whisky very spicy within a short time. They are most often used to produce NAS whiskies or single malts for blending. 

Data, videos and images courtesy whisky.com

SHELF-LIFE OF WHISKY

 Does Whisky Go Bad?

There’s this lone bottle of whisky sitting in the pantry for as long as you can remember. It was probably bought in the `90s and sits there since. And sooner or later, you start to wonder: does whisky go bad? Or maybe you have a half-open bottle left from your birthday party last year, and you’re not sure if it’s still any good. It has been in the cupboard for over half a year already, so you expect that it might not taste good. And since you’re definitely not a whisky connoisseur, you’re not quite sure if high-proof liquors go bad or not. You heard they last pretty much forever, but you’d prefer to know for sure before pouring yourself a glass. What you need to remember is the shelf life of whisky depends on whether the bottle is opened or unopened.

Creating a Whisky Display: One of the joys of owning a collection of fine whiskies is the pleasure in showing it off, selecting a special bottle that you have been saving, and sharing its story with like-minded connoisseurs and friends. Too many bottles are unceremoniously shoved into cupboards or abandoned on top of bookcases. Invest in deep shelving or glass display cabinets, or build a home bar to show off the best aspects of your collection. Choose museum-quality lighting that will bring out the gorgeous colours of the liquid without affecting the inks on the labels or warming up the contents.

Display bottles will accumulate dust, so rotate your presentation frequently. Perhaps you’ll want to put up every single bottle from a prized collectable series, an A to Z of scotch or bourbon, or a super-fan shrine to your all-time favourite distillery. If you have space, arrange the whole collection by name, or stratify it by whisky style and country of origin.

How to Store Unopened Whisky: Whisky is stored upright. In contrast to Wine, however, Whisky in unopened bottles doesn't become better (or worse) during storage. Whisky only matures in the cask when it has contact with the oak wood. The bottles must be stored upright since the cork doesn't close as tightly as a Wine cork. Whisky corks are designed for multiple use, whereas Wine corks are disposable.

Whisky mustn't be exposed to direct sunlight, since the ultraviolet rays bleach out the colour pigments over time. A bar cabinet or a package protect the whisky from light. That way it's possible to store whisky for more than ten years. However, since tiny amounts of liquid evaporate through the cork, the filling level will decrease over time. You can monitor the liquid level of a bottle by marking the neck on the back with a felt pen.

If you're a collector and want to store whisky for a longer period of time, you have to give more thought to it. First of all, the temperature should be lower than room temperature. The lower the temperature, the less the quantity of whisky that evaporates. An additional cap on top of the cork can also help, but take care that the cap doesn't soil or damage the original sleeve. This would reduce the value of the bottle, as would sealing wax.

Bottles with screw-tops must be retightened by hand regularly since they always loosen on their own, which leads to increased evaporation.

When you store bottles in the basement, you should also protect them from humidity. In a damp basement, the label starts to mould easily. A thin plastic bag is ideal to pack the whisky airtight. But beware! The plastic bag mustn't contain any plasticisers or other chemicals. They could bleach out the label and affect the bottle contents through the cork.

In effect, you should keep the bottle in a dark and cool place, away from sunlight and any source of heat. When it comes to where exactly should the bottle sit, the pantry or a wine cellar are both perfect choices, but the kitchen works too, but on the side opposite to the cooking range. A liquor cabinet is another popular option, but make sure you do not keep the bottle on display, where sunlight can reach it. In wine cellars, make sure you keep the whisky in an upright position, especially if it’s a corked bottle. If the bottle has a plastic cork, then it doesn’t matter which way you place the bottle.

Why Whisky in an Open Bottle Changes Its Taste: An opened bottle sees the whisky behave differently. The taste changes. However, it takes much longer than wine, which must be consumed within a couple of days. Opened whisky has a shelf life of between six months and two years. What happens in the bottle during that time?

First, the alcohol evaporates and the taste of the whisky becomes smoother. Furthermore, the air in the bottle contains active gases that slowly react with the flavour substances of the whisky. How the taste changes can't be predicted. Sometimes it becomes better, but, more often than not, it turns worse.

To prevent these changes, reduce the air contact of the whisky by filling it into a smaller bottle. DO NOT use a decanter. We don't know of any high-quality decanter that is truly tight over a longer period of time. The decanter must have a plastic seal or a ground glass joint. That said, it’s best not to have too many whisky bottles open and to drink those you open within a few months.

How to Seal a Bottle of Whisky: To prevent these changes, reduce the air contact of the whisky by filling it into a smaller bottle. DO NOT use a decanter. We don't know of any high-quality decanter that is truly tight over a longer period of time. The decanter must have a plastic seal or a ground glass joint. That said, it’s best not have too many whisky bottles open and to drink those you open within a few months.

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Kill Them Quickly: Nearly-empty bottles take up precious space; do not get overly sentimental about the last pour rattling around the bottom. You are never going to get that whisky back, so rediscover storage space for new bottles by de-cluttering. Have fun with it: host a dregs party, where everyone commits to helping you finish the last pours, or try your hand at home blending. Tip your last drams into a Mason or Kilner jar, let the liquid marry for a few weeks, and then try the results. You may find you want to keep adding to the mix, creating your own “Infinity Bottle.”

Storing and organising your whisky correctly involves an investment in your time, but once you get in the habit, maintaining your system will be a breeze—giving you much more time to enjoy your whisky. Slainte Mhath!