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Issue 92 - The angel's share

Whisky Magazine Issue 92
December 2010


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The angel's share

Ian Wisniewski asks how does the process of evaporation affect the ageing process of a malt whisky

The evaporation of alcohol and water from a cask is a vital element of the aging process, as this instigates a series of complex reactions which help the spirit to mature and develop finesse.

The process of evaporation begins as soon as a cask is filled with new make spirit. Casks are rarely filled to the brim which leaves a small ‘unoccupied' area between the surface of the spirit and the top of the cask, known as the headspace.

Moreover, the process of indrink sees the staves of the cask absorbing around two per cent of the total volume of spirit, within 48 hours of filling a cask.

As indrink reduces the amount of liquid in the body of the cask, the level also falls. This in turn increases the headspace, which plays a significant role in the evaporation process.

“As vapours rise from the spirit they enter the headspace and look for a way out of the cask.

“The vapours can permeate through tiny spaces between the staves, as well as tiny spaces between joins in the cask, and also permeate through pores in the oak staves,” says Jane Millar, technical support team leader, William Grant & Sons.

The density of pores in bourbon barrels, which are made from American oak, differ to European oak from which sherry casks are typically made.

Most of the casks used to age malt whisky are American rather than European oak, raising the question of whether the different types of oak used affect evaporation rates.

“On average American oak is less porous, tighter and denser, ...

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