“Younger whiskies can have great enthusiasm, then after about 10 years you start to get a real mellowing out and balance; for me the balance is complete within 15 to 20 years,” says Jim McEwan, Bruichladdich’s malt maestro. “At 20 years the oak starts to really exert its influence, and at 25 years has a different, really good character, being mellower, rounder, slightly drier, and the heat drops into the chest, not the palate. Between 25 to 30 years the oak, and where the malt is matured, start to play a very important role. Thirty years and onwards there is a huge concentration of oak in the spirit.”Maturation can be divided into three essential elements. Subtractive maturation, like a ‘rites of passage’ for the ingénu new make spirit, entails the loss of immaturity. Additive maturation sees the oak endowing the spirit with colour, aromas and flavours, while interactive maturation refers to reactions between the spirit and the oak. This is something of a ‘mystical union’ that is not fully understood, yielding an additional range of characteristics that neither the spirit or oak possess individually.It would be very convenient if subtractive, additive and interactive maturation followed each other in an ordered, chronological sequence. However, these separate elements effectively occur simultaneously throughout maturation, albeit at differing rates. Oak-derived flavours, for example, can be evident within the spirit from around six months, though losing immaturity may take a couple of years, or even extend over several. Similarly, filling a barrel with spirit can be said to initiate a form of interactive maturation. This reflects the fact that 2 to 3% of the new make spirit is ‘drunk in’ by the staves, possibly even within 48 hours, and subsequently begins to mingle with the ‘wood extractive liquid’ of the cask’s previous contents (although the full effect will, of course, take years to complete). In fact, specific flavours can be attributed to interactive maturation, due for example to the formation of esters. “While The Macallan’s citrus and floral notes originate from the new make spirit, the dried fruit notes derive from esters that have been created during interaction with the cask, and also from oxidisation,” says David Robertson, The Macallan’s Master Distiller.An initial consideration is the difference between using bourbon barrels, fashioned from American oak, and sherry casks, which are usually produced from European oak, though American oak casks can also be seen in action at sherry bodegas. European oak, typically harvested from 60 to 150-year-old trees, comprises a looser, more open and porous grain than American oak. This enables the spirit to penetrate the oak more readily than American oak, usually harvested from 40 to 100-year-old trees, which comprises a straighter, tighter grain. The level of tannins, promoting astringency, balance and structure, is also far higher in European oak than in American oak. However, charring bourbon barrels on the inside, while sherry casks are merely toasted, makes the interior of a bourbon barrel more accessible to the spirit.These technical differences also promote a varying range of characteristics. In terms of colour co-ordination, sherry casks lend an orangey, amber hue, which is distinct from the lighter, golden, straw tint of bourbon barrels. The flavour profile also varies significantly with sherry casks contributing rich fruit (such as raisins, prunes, dates, figs, apricots), fruitcake, fortified wine, almond and walnut notes, spices such as nutmeg, ginger and cloves, not to mention crème caramel, chocolate, and a (positive) sulphurous note, delivered within a rich sweetness. Bourbon barrels lend a lighter, drier sweetness, with a palate thriving on a medley of flavours: vanilla, honey, various fruits, almonds, hazelnuts, coconut, crème brûlée, sherbet, spices such as cinnamon, as well as mint and eucalyptus notes. The influence of bourbon barrels can be further sub-divided according to the degree of charring. The scale ranges from one to four, beginning with a ‘burnt toast’ effect, and culminating in an ‘alligator char,’ the popular term for a number four, as the surface resembles alligator hide. Various malt distilleries have an inventory of bourbon barrels with a range of char levels, with a heavier char typically giving greater amounts of vanilla, crème caramel, toastiness and hint of smoke, not to mention more intense colour. Alternatively, a milder char promotes greater sweetness, honey and vanilla, while also endowing the spirit with more body. The ‘fill’ (referring to the number of times the cask has been filled with spirit) is another important consideration, with many distillers using a ‘recipe’ of different fills to help achieve consistency, or a particular flavour profile. Each fill of a cask results in its degree of influence on the spirit diminishing, until the Master Distiller deems it to be exhausted. If a first-fill cask is said to contribute 100% of its available characteristics, a second-fill will contribute around 60%, with a third-fill (when relevant) dropping to around 35%. Consequently, a second- and third-fill require a longer time-frame than a first-fill to complete subtractive maturation. For example, spirit maturing in a second- or third-fill could require about 10 and 15 years respectively, in order to reach the same degree of maturity as a spirit aged for seven to eight years in a first-fill cask. However, this does not mean that each successive fill also delivers the same flavour profile, in a progressively milder format. It’s actually a case of obtaining varying, rather than subtler flavours from different fills. Then again, it’s also a case of imparting an ‘appropriate’ influence to the particular house style of the spirit. A lighter, unpeated malt, for example, may principally be aged in second-fill sherry casks, to ensure that the oak influence doesn’t predominate. “The first-fill of Spanish oak sherry casks gives a dark mahogany whisky, with rich, woody spices such as cloves and ginger, toffee sweetness and dried fruits,” says David Robertson. “When refilled there is a less robust oak influence and more new-make spirity character coming through, together with more apples, esters and floral notes, while the tannins manifest themselves as light spices (such as cinnamon) rather than oak.” Similarly, a first-fill bourbon barrel offers a different influence to products further down the line. “A first-fill bourbon barrel has more natural sweetness and body, the second-fill allows more products of oxidation to shine through and gives more of a floral top note, though greater oxidation also reflects the damper ageing conditions of a traditional warehouse,” says Dr Bill Lumsden, Glenmorangie’s Head of Distilleries and Maturation.Spirit within a first-fill bourbon barrel also has optimum contact with the charred layer, though the char’s active influence does diminish during maturation, not to mention physically breaking down within the cask. (Consequently, when the spirit is emptied after the first-fill, some charcoal also leaves the barrel, reducing the level of char available during the second-fill, which is also less active anyway). This gives a lower level of vanilla and ‘burnt heather’ notes which stem from the char, while simultaneously increasing the level of oak influence. Another principal difference between successive fills is the level of wood extractive liquid remaining within the staves of the cask. Not simply a case of residue bourbon or sherry, this liquid also incorporates additional wood-derived compounds, which could total around
75 cl in a 200-litre bourbon barrel, or 10 litres in a 500-litre sherry butt. The largest amount of wood extractive liquid is present within a first-fill, and released during the first few years, with some wood extractive liquid remaining in the second, but only a minimal amount is released from a third-fill. Traditionally this residue was considered an important element, though the nature of the oak, European or American, is now established as the primary influence during maturation, and the wood extractive liquid a supplementary influence.