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, while European oak tends to be more porous with a looser structure, and a higher evaporation rate, but as there are so many other parameters that come into play this is only a generalisation.
“Ultimately temperature is more important than the type of wood,” says Dennis Watson, director of technical and scientific affairs, Chivas Brothers.
Casks are usually filled with spirit that has an alcoholic strength of 63.5% ABV, which means that the balance (approximately one third) is water.
Consequently, it’s important to look at the evaporation of water and alcohol separately.
A key influence on the evaporation rate of alcohol is temperature, as when the temperature rises so does the evaporation rate of alcohol. Meanwhile, a key factor in the evaporation rate of water is the level of humidity in an aging warehouse. This is due to the laws of physics, and the fact that the concentration of water in the cask is constantly trying to reach a balance with the concentration of water in the environment outside the cask (ie. the level of humidity in the aging warehouse). Consequently, the greater the humidity in a warehouse, the lower the level of water that evaporates from the cask, as the surrounding atmosphere is equally ‘saturated.’ Correspondingly, a drier environment promotes greater evaporation of water, in an attempt to balance conditions between the interior and exterior of the cask.
Temperature and humidity levels are in turn influenced by the type of aging warehouse.
Humidity levels are usually higher in dunnage warehouses than racked warehouses. Earthen floors in dunnage warehouses, compared to concrete floors in racked warehouses, are one reason for this.
Some distillers believe higher humidity promotes a slower, and more desirable rate of maturation, though exactly how humidity levels affect maturation is still being researched.
Moreover, racked warehouses are much taller, have thinner walls and metal roofs, which helps to transmit temperature changes more readily than the thick stone walls and slate roofs of a dunnage warehouse.
Consequently, temperature fluctuations can be more frequent in a racked warehouse, compared to more gradual changes within a dunnage warehouse.
However, average annual temperatures tend to be similar in dunnage and racked warehouses. So, where does that leave us?Some distillers believe that experiencing changes in temperature on a more frequent basis can affect the character rather than the quality of a malt.
Meanwhile, the exact effects of more rapid, or more leisurely, temperature changes during aging are still being researched.
In addition to alcohol and water evaporating from the cask, some other compounds also follow suit.
“Some of the most volatile esters, which provide pear drop notes for example, can evaporate from the cask, but this type of ester is present in such large numbers that it’s not a significant loss.
“Some of the most volatile sulphur compounds also evaporate from the cask, which means loosing rubbery, cabbage-type notes from the spirit,” says Rachel Barrie, whisky creator and master blender, Glenmorangie.
Another consequence of evaporation is declining alcoholic strength, which has significant implications.
“You’re loosing alcoholic strength at a rate of 0.5% abv per annum. The rate and extent of various reactions in the cask are, to a certain degree determined by the alcoholic strength of the spirit, and as this reduces the dynamic of various reactions changes, and what the whisky has to offer also changes,” adds Barrie.
For example, as alcoholic strength declines this in turn increases the proportion of water in the spirit.
This can subsequently result in a higher level of water-soluble compounds being extracted from the cask, such as tannins, which contribute to structure and balance in the spirit.
While the benefits of evaporation are fully appreciated, the process is still not fully understood. This means that the exact influence of different evaporation rates is still uncertain, though it’s clear that they change during maturation.