Whisky Magazine Issue 114
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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...