A key aspect of ageing is the changing ratio between the 'distillery character' (ie. profile of the new make spirit, which can include fruit, cereal and peated notes) and the 'maturation character' (ie. influence of ageing, including the cask, evaporation and oxidation). The initial ratio of 100/0 (distillery character / maturation character) soon changes, as discernible differences are evident in the spirit within weeks of filling a cask. However, exactly where the balance lies between distillery character and maturation character after three years is hardly a straightforward calculation, as it's influenced by various factors.
For example, a lighter new make spirit shows the maturation influence sooner than a richer new make spirit. The same applies to unpeated compared to peated new make spirit (and the higher the peating level the longer it takes).
Correspondingly, a cask's credentials determine the range of flavours it contributes to the maturing spirit. The usual choices are: Bourbon barrels, which contribute for example vanilla, honey and fruit notes, while sherry casks add vanilla and dried fruit notes including raisins.
Vanilla notes from the cask can be evident in the spirit within a few weeks, and continue increasing at the greatest rate in the first three years, then the rate slows down. A significant benefit is that vanilla notes also make other flavours, such as fruit, seem richer and rounder.
Another important influence is tannins. "The cask contributes tannins at the greatest rate in the first three years, then the rate slows down dramatically although never stops. Tannins add body, structure and dryness, which balances for example the sweetness of vanilla. Tannins also contribute to the mouthfeel, promoting a richer, mouth-coating texture," says Brian Kinsman, master blender, William Grant & Sons.
The degree of influence a cask has depends on the 'fill.' A 'first fill' indicates a cask used to age malt whisky for the first time, 'second fill' is a second usage, with a 'third fill' or 'fourth fill' generally the limit. Each successive fill sees the cask's influence decreasing (the extent depends on various factors, including the length of each fill).
"After three years, a first fill Bourbon barrel gives intensely creamy, sweet vanilla and runny caramel flavours. Each subsequent fill contributes fairly similar vanilla notes, though progressively less intense. But this also means, for example, that The Glenlivet aged in a second fill Bourbon barrel has more distillery character coming through, particularly pear notes, compared to a first fill," says Sandy Hyslop, director of blending at Chivas.
Consequently, each fill has its own value.
"Second or third fill casks are very useful for putting together a recipe, as they give a different expression, augmented by first fill. Recipes can be far more varied and complex by using different fills," says Sandy Hyslop.
Cask size is also significant. Sherry casks can be either a hogshead of 250 litres capacity or a butt of 500 litres, while Bourbon barrels are 200 litres.
"The influence of a Bourbon barrel shows sooner than a butt, as the smaller size means a larger surface area of alcohol being in contact with the oak, from which the spirit extracts flavour compounds. Similarly, a hogshead gives more intensity than an equivalent butt. I consider hogsheads and butts completely separately, and they are different components in any recipe," says Sandy.
Another development is that the levels of cereal notes and sulphur compounds present in the new make spirit start reducing within months. This is due to the cask 'absorbing' these characteristics, in conjunction with evaporation (accounting for a loss of around two per cent of the cask's contents per annum) and oxidation (air passing in and out of the cask). As cereal notes and sulphur compounds (including vegetal, meaty, rubbery notes) are assertive, a reduction in the level enables lighter notes including esters (fruitiness) and sweetness to show through, significantly altering the profile of the maturing spirit.
"The first three years are hugely important in the development of a malt whisky, as this is when various characteristics develop at the fastest rate. However, this also means the relationship between distillery character and maturation character is initially slightly disjointed, with an odd balance at around 6-18 months. It needs three years to harmonise and reach a new equilibrium," says Brian Kinsman.
New-make spirit is colourless which means the colour of a mature malt is gained from the cask. However, exactly which aspect of a cask contributes colour is uncertain, and still being researched.
The first sign of colour generally appears after a few weeks, with the greatest colour development in the first three years, after which the rate is much slower.
The cask type plays a significant role in the range and rate of colour development. An initial distinction is that Bourbon barrels contribute a lighter range of colour, such as straw, gold and amber, compared to the darker colours gained from a sherry cask, which include mahogany and even reddish notes.
Additionally, a first fill cask contributes more colour, more readily, than a second fill, which in turn contributes more than a third fill. Similarly, a hogshead provides more colour, more rapidly than an equivalent butt.