Distilleries in Scotland have been exploring the effects of wooden casks since the 1800s. The chemistry involved is now beginning to be understood, but managing the maturation process remains as much an art as a science. Our Whisky and the Barrel blind tasting experience, which you can order as a gift set or sharing box, aims to demonstrate some of what this art can achieve. Below we explore some of fascinating aspects of maturation you may be able to pick up in your next dram.

• THE HISTORY • THE SCIENCE • TYPES OF OAK • CASK SIZE • TOASTING AND CHARRING OAK CASKS • CLIMATE AND GEOGRAPHICAL FACTORS

It is important to remember that maturation is but one stage in an immensely complex process involving hundreds of flavour compounds. Malting, milling, mashing, fermentation, distillation, maturation and blending (from multiple casks from the same distillery) all play their part in the making of single malt Scotch whisky.

All this and more contributes to the character of the distillate, but it is when the spirit is put into the cask and begins its ‘conversation’ with the wood, that the real magic begins.

Hidden in the cask, the clear pungent new make spirit mellows and darkens into a whisky that can, after at least three year but usually much longer, be blended and bottled for sale. According to Difford’s Guide to Single Malt Scotch Whisky Production, at least 60% of a whisky’s flavour (and much or all of its colour) comes from the cask.

THE HISTORY

Maturation in wooden casks is a practice that emerged during the nineteenth century to exploit the readily availability of the tens of thousands of casks used to transport rum, wine and sherry to the UK. This meant it was no longer necessary to add herbs and spices to the spirit to make it palatable. The cask would do that—and much more besides.

It was soon recognised that prior use mattered, and casks that had been used to transport sherry, often many times before being turned over to distillers, quickly became the preferred option. However, a decline in popularity and, from 1986, a shift to bottling sherry in Spain before it was transported abroad, meant that sherry casks became less available and more expensive. Some distillers responded by commissioning Spanish cooperages to recreate the transportation casks and fill them with sherry foe a year or two to ‘season’ them before using them to mature their whisky.

Other distilleries turned to second-hand barrels coming out of the expanding bourbon industry. By tradition and law bourbon, must be matured in charred virgin oak barrels. As they could only be used once, these barrels soon became plentiful, and today around 97% of all Scotch whisky is maturing in American oak casks which previously held bourbon.

THE SCIENCE

During its time in the cask the taste, aroma and feel of the spirit is transformed through what are described as additive, subtractive and interactive maturation.

Additive maturation, as the name implies, refers to the manner in which components in the wood break down and combine with the spirit in the cask.

Wood primarily comprises cellulose, hemicellulose and lignin. Cellulose is a natural polymer (a long-chain molecule of repeating units) which, although it gives wood its structural strength, does not impart much by way of flavour. Hemicellulose and lignin, on the other hand, contribute a range of aromatic compounds to the maturating spirit.

Hemicellulose is a so-called ‘heteropolymer’ with a random amorphous structure and little strength which, in the presence of ethanol, readily gives up its component sugars (such as xylose, mannose, galactose, rhamose, arabine and glucos). These all have a sweetness which can influence the taste of a spirit, but xylose is also said to impart something of the mouthfeel and roundness often associated of beer.

Hemicelluslose also breaks apart when heated, for instance through the toasting or charring of barrels. This can result in a wide array of compounds being released into the whisky, such as furfural (described as almond-like and grainy), hydroxymethylfurfural (bringing musty of waxy hints of butter and caramel), maltol (malty, sweet notes), cyclotene (maple, caramel, liquorice, and 2,3-dimethylprazine (which is associated with roasted hazelnuts, cocoa and toast).

Lignin, the third major component of wood, is another large organic polymer which, like cellulose, is important in providing structural stability and strength. However its chemical structure means that, as with hemicellulose, the oresence of ethanol alcohol and, in peaticular, heat and fire, will result in the release of a variety of aromatic compounds. These include guaiacol (a smoky phenolic compound), syringaldehyde (which imparts a spicy, smouldering aroma), 4-vinylguaiacol (a spicy, pungent phenol with clove-like notes), phenylethanol (floral, rose) and vanillin.

The way in which we describe the taste, aroma and feel of these aromatic compounds reflects, of course, their dominance in particular natural products. Vanillin for instance, is the primary component of the vanilla bean, but in lower concentrations it can leave hints of vanilla in other food and drinks, including whisky.

Subtractive maturation, meanwhile, describes how interaction with the wood removes undesirable and raw flavours from the distilled spirit. For instance, although copper stills strip most of the sulphur dioxide and hydrogen sulphide from the spirit during distillation, other sulphur compounds will usually remain in a new-make spirit. Dimethyl disulphide, has a garlic like smell of rotten vegetables and is definitely something distillers wish to avoid.

Subtractive maturation tends to have removed the unpleasant tangy, metallic taste from an immature whisky within 5 - 8 years—a process encouraged by the presence of a heavily charred barrel. Thereafter the processes of additive and interactive maturation tend to dominate.

Interactive maturation refers to reactions taking place within the spirit itself, largely due to oxidation—perhaps the least well understood part of the maturation process.

As soon as a cask is filled and stopperd the oak begins to absorb some of the spirit. This creates a headspace with negative pressure which draws in air through pores in the wood and seams into the cask. Thereafter the cask begins to breathe as temperatures rise and fall through the seasons.

In the summer months the contents within the barrel will expand and, with nowhere else to go, some of the air and alcohol vapour in the headspace is ‘exhaled’ from the cask. As temperatures cool, and the contents contract, so the cask ‘inhales’ fresh air. This process contributes to the loss of about 2% of spirit in Scotland each year—the so-called ‘Angels Share’.

The oxygen in the air is also an extremely reactive element. It oxidises ethanol to produce carboxylic acids which, in turn, react with the ethanol to produce esters with fruity and floral overtones. Hydrogen peroxide, formed from tannins, also acts as an oxidising agent, transforming ethanol into acetaldehyde diethyl acetal and imparting delicate fragrant citrusy top notes.

Interactive maturation is a slow process, but one which eventually contributes to the elegance and complexity of mature whiskies.

Although barley considered prior to the nineteenth century, today maturation plays a vital part in the making of Scotch whisky. There are so many parameters to consider; what type and size of cask, what type of toasting or charring should be used to prepare the cask, what previous liquids should it have contained, how many times can it be used, and where and how should it be stored; and would a second maturation in a different barrel add desirable qualities? It all matters!

OAK - THE WOOD OF CHOICE

Casks of one sort or another have been used for thousands of years and, because of its unique mechanical, chemical and physical properties, oak has long been the wood of choice for storing and aging products. With few knots and tight grain, and easily coopered because it begins sort and malleable before drying hard and durable, many species of oak are perfect for making watertight casks.

Large scale cooperages survive to serve wine makers and distillers in Europe and America, but whisky distilleries have never been large consumers of newly made casks. Thry have always exploited the availability of casks that had been previously used to store, age or transport other spirits and wines.

Sherry producers—from whom Scottish distilleries traditionally sourced their barrels— mostly used new American oak for the casks used in their ‘solera’ system of maturation. Some of the Spanish-coopered oak casks used over the last two centuries may thus have been made of American oak but, as the Scottish whisky industry tended to re-use cheaper transportation casks, it is likely he majority were made of locally-sourced European oak.

There are important differences between the two. American white oak (predominantly Quercus alba) is denser than its European cousin (Q/robur and Q.pyrenaica) and therefore tends to interact less with the spirit, producing a lighter flavour with vanilla, coconut and caramel tones. European oak, on the other hand, contains more tannin—which contributes to colour and astringency—and is associated with spicier and peppery flavours.

Today, the vast majority of casks once held bourbon and are made of charred American white oak, but many distilleries order sherry-seasoned casks of European oak, albeit usually only to impart a ‘finish’ to a malt predominately matured in ex-bourbon casks.

This practice of ‘finishing’ has lead to the use of some more exotic oaks, such as the rare and fantastically expensive Mizunara (Q. mongolica). This is not an altogether satisfactory material as it is difficult to cooper, relatively porous and prone to leaking but, being native to Japan, it was widely used when the local whisky industry expanded after the Second World War and was found to impart tropical notes of sandalwood and coconut.

WHY CASK SIZE MATTERS

According to UK law, a single malt whisky must be made exclusively from malted barley distilled using pot stills at a single distillery and aged for at least three years in oak casks that are no larger than 700 litres. Beyond this there is no constraint on the size of the cask within which the whisky matures.

A whisky bottle will seldom say anything about the size of the cask used, but it is something the distiller will be thinking about as it affects the amount of contact between the spirit and the wood.

In theory, there are plenty of casks with wonderfully evocative names from which to choose—such as pins, bloodtubs, firkins, octaves, kilderkins, hogsheads, puncheons, butts, pipes, drums and tuns—but traditionally many of the smaller casks were used for transportation and what really mattered is what was readily available.

The one-time mainstay, the sherry butt, is a large cask of as much as 500 litres. With a low wood-to-spirit ratio, maturation tends to be slow. The American Standard Barrel of about 200 litres used in the bourbon industry, ismu ch smaller and has a higher wood-to-spirit ratio. This promotes greater contact with the wood and a somewhat faster maturation.

Many argue, however, that a whisky matures more gracefully when it matures more slowly. Some distillers thus take the ex-bourbon casks that have been dismantled for transport to Scotland and, adding extra staves, reassemble them into bigger hogsheads of about 250 litres.

TOASTING AND CHARRING OAK CASKS

European oak casks originally made for the wine industry are typically toasted—either over an open flame or in an oven. This is used to mellow the tannins and help breakdown the hemicellulose and lignin in the wood to release their aromatic compounds (above) more readily during maturation.

There are varying degrees of toasting, and studies measuring its effect on whisky suggests that a medium toast will impart soft caramel and toasted bread flavours, whereas heavy toasting is associated with chocolate and darker fruity tones.

It’s worth remembering, however, that the effect of toasting, and indeed of the wood itself, is strongly mediated by whether the cask is a ‘first-gill’ or ‘re-fill’. The use to which the cask had been put prior to being filled with the new-make spirit is also important. For instance, genuine ‘solera casks’ used to mature sherry for up to a hundred years have little ‘wood impact’ left to contribute to the whisky.

In contrast to the toasting typical of European oak casks, ex-bourbon barrels made of American oak are invariably charred. Blazed with fire, the barrels are usually charred to a depth of about an eighth of an inch and look black on the inside. A heavy char can crack the top layer of the barrel and encourages the penetration of the spirit into the wood.

Maturing bourbon whisky in charred barrels became commonplace towards the end of the 1800s. However it first originated, bourbon distilleries came to recognise the positive impact of charring. It seems to speed up the maturation process, helped remove sulphurous overtones, added colour and imparted vanilla, coconut and caramel sweetness now associated with the spirit.

As bourbon barrels are only ever used once before being passed on, these influences carry through to when they are reused to mature whisky—although if, as tends to be the case, the barrels are reused more than once they have to be rejuvenated by charring.

CLIMATE AND GEOGRAPHICAL FACTORS

Maturation is obviously an enormously complex phenomenon in which innumerable processes play off against one another. Indeed, interactions between the contributing factors mean it can be difficult to pin down the precise origins of many aspects of the taste, aroma and feel of a finished whisky.

Perhaps it doesn’t matter too much as you sip your final dram, but you can be sure the distiller of that dram worried long and hard about the effects of the cask and, of course, how and where it is kept.

Higher temperatures tend to speed up additive and interactive maturation, the latter because higher rates of evaporation draw in more new air to feed processes of oxidation. But there is a penalty to be paid. In hotter climates, such as India, as much as 10% of the spirit can be lost each year to evaporation. In Scotland, maturation is more leisurely (allowing subtractive the time it needs), but at least the ‘Angel’s Share’ only amounts to about 2% per annum.

But it’s not just temperature that matters. In a cool and damp Scottish warehouse alcohol evaporates at a faster rate than water which, over time, lowers the alcohol by volume. In drier hotter climates more water is lost through evaporation than alcohol, and the ABV of the spirit increases over time.

It’s all so marvellously complex!

Order our Whisky and the Barrel blind tasting set and see what you pick up within the six carefully selected single malts. With a blind tasting guide, six information cards and the wax-sealed envelope for ‘the big reveal’, explore the effects of maturation with six diverse single malts. What better way to explore this fascinating subject?