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Apart from the largely experimental or exotica bottlings of a small minority of producers, some of whom have spotted a so-called white whisky gap in the market, the vast majority of whisky distillate is laid down in wood.  Indeed, for a whisky to be a whisky, it is required by law to be matured in wood, and usually in oak. Accordingly, aging contributes to 50-70% of the final article. Exactly how will depend on a variety of factors, including the type of oak used, how it’s seasoned, and the size and the treatment of the cask. Let’s jump in, but before we do, please: knowing or not knowing the below has not the slightest impact on how much you enjoy the glass of whisky I hope you have poured yourself to help get through it.

The tree and the trees

Oak is valued for its strength, its structure, the tightness and run of its grain, and especially for the flavours it gives – and helps facilitate in – aging whisky. Of the world’s 600 species, the industry favours two varieties of white oak: American white oak, usually Quercus alba, Q. bicolor and Q.macrocarpa; and European oak, mainly Q. robur, sometimes Q. petraea. American oak is by-and-far the most prevalent, it being a quicker grower, cheaper, and the oak of choice in the bourbon industry, which by law must use virgin casks, and whose castoffs are reused by just about everyone else. Other varieties include French oak – either Q. petraea and Q. robur, sourced from forests traditionally used by wine and brandy makers – and several species used largely in Japan, including mizunara (Q. crispula), Japanese emperor oak (Q.dentata) and to a lesser extent Mongolian (Q.mongolica).

This said, whatever the species, and wherever it is sourced, all oak is composed of a small amount of so-called oak wood extracts and of three key polymers: cellulose, hemicellulose and lignin. For cellulose and hemicellulose, imagine huge molecular structures repeating themselves, a bit like those massive models we used to meet in chemistry class. Lignin’s the same, only without the repetition. Flavour-wise, cellulose is something of a dead duck, producing a very limited number of carbohydrates under extreme heat. When broken down, oak extracts release – in different degrees, and depending on the type of oak – coconut, vanilla, spice and incense flavour compounds. When heated, hemicellulose breaks down into sugars and tannins, and then on into new aromatic compounds, variously giving the spirit the taste of almond, grain, butter, must, caramel and maple. Heat and subsequent reactions sees lignin give aged spirit more vanillas, those barbecued aromas, layers of flower, and the wanted sharp and bitter notes.

While the original reason for having it will have more to do with the economics of supply than anything else, producers’ use of a certain type of oak over another makes for the creation of specific flavour profiles. Very generally speaking: American oak gives its whiskies the taste of vanilla and coconut, of honey and toffee, of red fruits, ginger, almonds, and eventually, leather and tobacco. If European oak, normally ex-sherry, then there’s loads of tannin, of fruit cake, of cinnamon, clove, caramelised orange, the rich smells of a kitchen come Christmas pudding time. If French oak, then add in its capacity for extra spice making. For the much rarer Japanese oak, as well as inhabiting a cross-section of the above, there’s a whole extra bunch of fruits, a prevalence of coconut, and an incense-like spiciness born of its high levels of whisky lactones. Please, take a sip of your whisky.

A brief science of maturation

We have known of oak’s influence from at least as early as 100 AD, and certainly by 500 AD, when its use as a means for the storage and transport of wine became ubiquitous. However, it’s not until relatively recently that we’ve begun to understand anything close to what occurs once a given cask is filled with a distillate and laid down to mature. Briefly, it’s a necessarily long and drawn out affair, and one that consists of three key processes: extraction, evaporation, and reaction:

Extraction
The spirit in a barrel is never still. Rising ambient temperatures cause the liquid to expand, pushing into the wood, variously dissolving and decomposing its sugars, tannins and simple flavour compounds. As ambient temperatures decrease, so the spirit turns in on itself, contracts, and draws or extracts the wood’s caramels, colouring and flavour compounds into itself. Expanding and contracting – year in, year out – the spirit rhythmically feasts on the barrel, though most fiercely in the first year.

Evaporation
At the same time, changes brought on by this interaction with the wood are compounded and aided by the process of evaporation, a phenomena popularly known as the Angel’s Share, with sharp and bitter tasting sulphides expelled suddenly, in the first month, and then gradually over time. The exiting gas is replaced by fresh oxygen, which acts as vital catalyst for the oxidation of ethanol into further flavour making chemical reactions. In this way, barrels can be said to breathe –through gaps in the joins and the bung hole.

Reaction
Finally, as time passes, facilitated by oxygen, water and ethanol, a complex chain of reactions will occur between various wood chemicals in themselves, between the certain wood and spirit chemicals, and between specific components of the spirit itself, all of which eventually amounts to sour, off-smelling aldehydes turning into more pleasant acids, which in turn are converted into a range of flavoursome fruity esters, the final outcome being, hopefully, a deep and finely balanced package of flavours.

I say hopefully because nothing’s certain in life, and especially when it comes to aging whisky. Poor wood can taint a whisky. An exhausted cask will result in nearly nothing. An overactive one can over-wood the spirit. Age is no guarantee of quality. This being the case, any producer worth his or her salt exercises control over anything oak-related that will bend to it, going so far as to obsessively track the oak to final cask, insisting on wood from a particular part of the tree, directing its seasoning, the coopering, and the type and profile of the cask, and making decisions as whether to use either virgin or first, second or even third fill casks. Let it be known: cask is king.

Cask is king

All oak needs drying, whatever its use. When destined for the life of a cask, this will mean a moisture content of approximately 12%, a task achieved artificially, via a kiln, or naturally, outside, or a combination of both. The advantages of kiln drying are speed and control. A much longer process (between two and three years), the advantages of air or yard drying are slightly more complicated, and interesting. Exposed to the vagaries of season and weather, rot sets in, the various and successive colonies of fungi that make the wood their home taking root, and with it the formation of hydrogen peroxide, which as your hairdresser will tell you is a catalyst for chemical change, the sum of which, in the case of oak, serves to soften the tannins and unlock the hemicelluloses’ sugars. The result: a softer, sweeter oak. It’s an old and natural practice, practised in Europe, and intermittently in America.

The oak dried, the rudiments of the whisky cask are the same whatever the type, whatever the size. Tight (liquid holding), and yet showing nothing as much as a nail or tack, it’s a real craft, and one that has hardly changed since the time of the Celts, generally agreed to be its inventors. The standard two casks for the maturation of whisky are the American Standard Barrel (ASB – bourbon) and the slightly larger hogshead (Irish and Scottish). Others include the 500 litre sherry butt, whisky octaves, small gallon barrels, quarter casks, barriques, port pipes, madeira and the puncheon. This is not an exhaustive list.

Notwithstanding tradition, rules and core product restrictions, producers will think carefully about how best to match the cask to their new make. Virgin oak casks make for an immediate and powerful wood influence, so diluting the spirit will ensure that the oak isn’t overworked. Meanwhile, a used cask – ex-bourbon, sherry, cognac or wine – is valued for how the original liquid’s years of interaction with wood will influence the taste of the new make, and even more so for the fact that the strength of influence of the used wood is less than that of a virgin oak cask, an influence that diminishes significantly with every reuse.

Further, the size of the cask itself impacts on flavour. The greater the ratio of surface (wood) to volume (liquid), the greater the rates of wood extraction and evaporation. Fans of exclusive small cask maturation champion speed over time, while its detractors argue that the spirit is too rapidly wooded, the cask too easily exhausted, and that not enough time is given over to the flavour changing reactions that occur later on down the line. The argument continues to rage, though it’s more teacup than storm. A tiny percentage of the world’s whisky is aged exclusively small casks, most small-cask advocates operating a mixed-size approach, the smaller casks used as a means of accelerating extraction, the larger for depth of character. Please, another sip, and let’s move on. We’re nearly there.

Fire as catalyst

The above apart, a final and crucial piece to the cask-as-flavour-giver puzzle. Historically, we will have disinfected our general storage casks by quite literally burning the insides clean, leaving a charred layer. At the same time, the toasting or ‘firing the barrels’ of new casks would have been – and continues in some places to be – the method by which coopers will have bent the dried staves and so tightened the casks. Both were found to improve the taste of the stored alcohol, so much so that today – as well as being concerned with oak type and size and history of cask – a producer will place his or her thermal heating order: toast or char, or both.

Toast-wise, much has changed over the years. Whereas previously wood chips set alight in a grate inside the cask would have sufficed, today toasting is much more technologically advanced, the heat source a probe positioned down the centre of the cask. Designed to penetrate the wood evenly, for anything between 15 and 45 minutes, at temperatures of between 100 and 200° C, the heat’s enough to partially destroy tannins, caramelise the wood’s sugars, and release vanillin, all of which is the biochemical cocktail acted on during maturation. On account of the change in wood colour, this caramelised area is known as the red zone. Depending on what a producer might require, so a cask will get a light, medium or heavy toast.

Charring, meanwhile, is toasting gone spectacularly wrong, the barrels innards actually set alight, and only for a matter of seconds, the result a charred surface, and less penetration than a toast – think of the difference between burning and toasting bread. The charred layer has the added effect of filtering impurities during maturation. Charring begins at 250° C plus, the seconds spent being fired generally divided into 5 standardised grades, with a producer usually making do with one or two standard chars. More experimental whisky producers will investigate the flavour enhancing properties of deeper chars, though at this level there is a risk of destroying the cask. The extra heat is enough to generate the appearance of various phenols, especially clove-like aromas and notes of smoke.

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On which final note, I salute you. You’re still here. Favoured species of oak, oak-type flavour profiles, the processes of maturation, cask production. We’ve covered a lot of ground, and rather quickly, and we’re yet to explore the flavour-making wonders of the warehouse itself, a task, you will be delighted to hear, I’ll reserve for another time. In the meantime, I encourage you to finish that glass of whisky – and to pour yourself another.