Various factors influence the character of the new make spirit during maturation, and determine a malt's final flavour profile, as Ian Wisniewski finds out.
As soon as a cask is filled various reactions are initiated and maturation begins. It’s a process that can be divided into three elements: subtractive, additive and interactive maturation.
Subtractive maturation deals with the loss of immaturity. The level of sulphur character for example, which is variously manifested as struck match, rubbery or meaty notes, reduces as it’s absorbed by the charred surface of the cask. This is essentially an amelioration rather than removal, with sulphur compounds also oxidising into subtler, less flavour-active forms. As sulphur character masks other notes, reducing this can significantly alter the spirit’s profile, by allowing ester notes to show.
Additive maturation sees the spirit endowed with colour and character from the oak. Bourbon barrels contribute notes such as vanilla, coconut and honey, compared to Sherry casks providing fruit cake and raisin notes, among others, together with a richer sweetness.
Interactive maturation refers to the complex reactions occurring between the spirit and the oak, including oxidation and evaporation. This yields an additional range of characteristics that neither the spirit, or the oak, possess individually.
Oxidation is a consequence of casks breathing in ‘fresh’ air,and exhaling ‘saturated’ air. This ‘breathing’ is initiated by the headspace (ie. the area above the level of liquid in the cask) contracting and expanding.
When the temperature rises the liquid within the cask expands, though the gases (air being composed of various gases) in the headspace expand at a greater rate. As the headspace can’t expand into the liquid, it expands by ‘exhaling’ through the cask.
Correspondingly, a fall in temperature sees the liquid and headspace contract. This draws air from outside the cask into the headspace, from where air dissolves into the spirit (oxygen being the key element). This sees esters, aldehydes and acids within the spirit continually reaching a new equilibrium. The result is that the spirit tends to become fruitier and more balanced, while also gaining complexity.
Evaporation of alcohol and water from the cask is another essential enabler of the aging process, as this entails loosing certain undesirable elements such as sulphur compounds, while also developing complexity.
The evaporation rate, typically around two per cent per annum, is influenced by factors including the type of aging warehouse and a cask’s position within it, the ambient temperature, humidity levels, and the filling strength.
Exactly how varying evaporation rates influence maturation is still not fully understood,though higher rates aren’t considered to promote a finer result.
Evaporation actually becomes the key influence during the latter stages of longer maturation. This is because a broad range of reactions, including oxidation and extractives from the cask, tend to calm down and reach an equilibrium after approximately 25 to 30 years of aging. And it’s principally evaporation that continues to alter the balance of character within the cask.
Trying to quantify the three principal aspects of maturation (ideally as a percentage) is hardly straightforward. One theory is that extractive maturation is the key influence, with subtractive and interactive maturation ranked second and third respectively.
However, the real significance is the essential team work and balance between them, as this is what creates a particular style of whisky.
A classic characteristic of mature malt whisky is vanilla, with bourbon barrels getting the credit for contributing this. However, vanilla character can already be present in the new make spirit.
“There’s a hint of vanilla in the Balvenie’s new make spirit,and when it’s put into first fill bourbon barrels we notice an increase in the vanilla character within the first two to three years, vanilla is one of the first characteristics that the cask contributes. The spirit continues to gain vanilla, but at a slower rate at up to 10 years of age, and after 12 years oak is beginning to come through combined still with a vanilla sweetness,”says David Stewart of Wm Grant & Sons.
How vanilla notes in the new make spirit develop, and interact with the vanilla character contributed by the cask is another consideration.
“Vanillin is in the new make spirit as a compound,and probably represents 10 to 20 per cent of the level found in Glenmorangie Original. The new make spirit vanillin is more interwoven with pastry, butter and butterscotch character,and evolving during 10 years in conjunction with vanillin from the casks this becomes more like a vanilla pod, and increasingly more like a dessert combined with coconut notes and creme brulee sweetness,”says Rachel Barrie of Glenmorangie.
So,what part of the cask does this come from?
Charring the interior of a bourbon barrel creates a surface layer of char typically two mm deep. Beneath the char is an underlying two to three mm of oak which is also altered by heat during charring.
“Consequently, it is this underlying layer of oak that releases various flavour compounds. Toasting a sherry cask has a similar influence, but not to a comparable degree.
“From our sherry casks,made from American oak, there is some vanilla influence, but it’s not as prominent as from a bourbon cask, and consequently this plays more of a supporting role rather than a starring role,”says Ewen Macintosh of Gordon & MacPhail.
The level of vanilla notes that can be extracted from a cask eventually reaches a maximum (when the concentration of vanilla notes is the same in the liquid as in the oak, no more is extracted).
The time span during which this happens is obviously variable, depending on factors such as the filling strength, the headspace, the volume of spirit in contact with the surface area of oak, and the temperature at which the spirit matures in the aging warehouse.
Fruit notes in the new make spirit also evolve and interact with fruit flavours extracted from the cask. It’s just a question of time.
“Benromach’s new make spirit has a clean, estery character of green apples and ripe bananas with a distinctive peat smoke note.
“After five to six years aging in a bourbon barrel the complexity of the fruit element increases, with subtle notes from the cask including vanilla sweetness, together with a hint of oak and earthy spicyness tempering the fruit,and moving the fruit into a more cooked, baked fruit flavour.
“After 10 years in bourbon barrels the spirit has got to the point where there’s more integration, with maltyness and fruityness laced together,and individual flavours are well balanced,”adds Ewen Macintosh.
The development of fruit flavours, including citrus notes, also of course reflects cask selection.
“It’s very difficult to pin down the chemistry of the citrus notes.
They are very zesty in the new make spirit, and depending on the cask, the citrus goes through a series of different stages. With less oak activity it might remain as lemon zest, while a first fill bourbon cask releases sweetness, and there’s an element of citrus notes that develops into a mouth filling luscious taste,”says Barrie.
More detailed understanding of maturation dates from the 1970s and 80s, following the development of technology that enabled characteristics in new make spirit and mature malt whisky to be analysed.
With the knowledge of flavour creation now at an advanced stage, one question that still hasn’t been fully answered is how a malt develops its ‘quality credentials,’ such as roundness,smoothness, and mouth feel.
For that answer we’ll just have to wait a bit longer.
FACT BOX Amalt whiskycan develop up to 70 per cent of its eventual character during maturation. One of the fascinations of aging malt whisky is thateven the same batch of spirit,filled into the same type of casks, and aged next toeach other in the same aging warehouse,will not provide an identical result. The differences may be subtle,or significant,because so many factors can play a role. The degree of evaporation and oxidation,for example,reflect a cask’s particular position within an aging warehouse,as each warehouse contains a series of ‘micro-climates.’