Whisky Magazine Issue 55
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Ian Wisniewski looks at evaporation and how it varies
Losing around two per cent of a cask's contents through evaporation can seem a depressing annual statistic for distillers (and even more so for accountants). But this type of regret is a pointless emotion, as evaporation is an essential enabler of the aging process.
Often summarised as ‘concentrating' the spirit, evaporation of water and alcohol is also about developing complexity. But it's also about losing certain undesirable elements such as sulphur compounds (although they are mainly absorbed by the layer of char in the cask).
The process of evaporation effectively begins as soon as the cask is filled, although the initial rate may be insignificant. Moreover, the evaporation rate is variable and hardly monitored on an annual basis. Checking casks focuses on the quality of the spirit as it develops, rather than the quantity of evaporation losses (although older casks need careful monitoring to ensure they don't go below the minimum bottling strength of 40% ABV).
Exactly how varying evaporation rates affect the way malts mature is still not fully understood, though quality is not considered to be commensurate with higher rates. Acask experiencing two per cent evaporation per annum for example, won't automatically be superior to a cask showing lower losses.
Various factors influence the evaporation rate, including the type of aging warehouse and a cask's position within it, the ambient temperature and humidity levels, as well as the filling strength.
Although 63.5% AB...