Malt whisky develops up to 70 per cent of its character during the aging process, due to a number of influences including oxidation (ie. the result of air passing in and out of the cask). This instigates various reactions in the spirit that develop a range of characteristics, as well as the colour of the resulting malt whisky. Consequently, oxidation is a vital process, but it's also one of the least understood elements of the maturation process, and is difficult to monitor and quantify.
Air is able to enter the cask through pores in the oak and through joins in the cask. These joins include the croze, where the heads at either end of the cask slot into staves that form the body of the cask, and also the bung, the block of wood that seals the opening through which a cask is filled and emptied.
Air entering the cask collects in the 'headspace,' an area between the surface of the spirit and the top of the cask. This is initially formed (within 48 hours of filling a cask) by the process of 'indrink,' which sees the staves of the cask absorbing around two per cent or more, of the total volume of spirit. This reduces the amount of liquid remaining in the body of the cask, which in turn creates a headspace.
The volume of liquid in the cask also decreases due to evaporation, which begins as soon as a cask is filled. This sees alcohol and water vapours rising from the surface of the spirit, and exiting the cask through the pores and joins. The evaporation rate is typically two per cent of the volume of the cask per annum, which continually increases the headspace and consequently the amount of air it contains.
The crucial element within the air is oxygen, which dissolves in the spirit and instigates various reactions.
"Oxidation is a continual process which sees flavour compounds reacting with oxygen, and oxygen then becomes part of those compounds, modifying them in the process and creating something new, including floral aromas and exotic fruit notes," says Gordon Motion, Edrington's master blender.
Fresh air continually enters the cask replenishing the oxygen, and so perpetuating the process of oxidation.
This influx of air is also influenced by the headspace, which is subject to a seasonal influence. When the temperature rises during the summer the spirit in the cask expands.
Meanwhile, the headspace also expands at an even greater rate than the spirit.
As the headspace can't expand into the liquid, it can only expand by leaving the cask through the pores and joins.
Correspondingly, when the temperature falls during the autumn and winter, the spirit and the headspace contract.
When the headspace expands it effectively 'exhales' air from the cask.
This air is also saturated with alcohol and water vapours that rise from the surface of the spirit. When the headspace contracts it effectively 'inhales,' drawing fresh air (and crucially oxygen) into the cask.
In addition to air entering the headspace above the surface of the liquid, air also passes through the pores of the cask below the surface of the liquid, and so comes directly into contact with the spirit.
"I think air in the headspace and air passing through the staves into the spirit has the same influence, but the headspace has the greatest impact on the maturing spirit," says Stuart Harvey, master blender, Inver House Distillers.
The extent to which oxidation can influence the resulting malt whisky partly depends on the character of the new make spirit. The more complex the spirit the greater the range of characteristics that oxidation can influence, and consequently the greater the transformative effect. But it's also a process that requires patience, as the results of oxidation only begin to show in the maturing spirit after several years of aging.
"Oxidation is a slow process but it results in a more elegant, more complex and more mature character," says Gordon Motion. Rachel Barrie, Morrison Bowmore's master blender, adds, "It's like the metamorphosis of a caterpillar into a butterfly as it lifts off and flies into the air."
Tannins, which are extracted by the spirit from the cask, play a vital role in the development of a malt whisky.
Although the exact contribution that tannins make is still being researched, tannins get the credit for adding body, structure and colour. Tannins are extracted at the greatest rate in the first few years of the aging process, then the rate decreases significantly.
Tannin levels in European oak (ie.
sherry casks) can be up to several times higher than American oak (ie.
bourbon barrels). Consequently, malts aged in European oak generally acquire higher tannin levels. However, above a certain level tannins can add astringency and bitterness.
"The large tannin molecules react with oxygen, instigating a process that breaks down alcohol and develops oxidation-promoted flavour characteristics, particularly fresh herbal and fragrant top-notes.
"Other oxidative reactions also see tannins forming compounds responsible for a brownish colour, although tannins are only one source of colour," says Rachel Barrie.