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The magic of maturation
Having produced malt spirit destined to become whisky, there is then the little matter of having to wait three years for it to achieve legal Scotch whisky status. This is the part of the process that distillery accountants dislike most. However, the process of maturation is not there merely to frustrate the bean-counters – it also tames and transforms the raw spirit, imbuing the drams we ultimately drink with what some say could be up to 70 per cent of their overall character. In the final two instalments of his series on the production of Scotch whisky, Gavin D Smith turns his attention to the last act – maturation
PHOTOS: PETER SANDGROUND / MIKE WILKINSON
Nobody knows when it was discovered that spirit improved in quality if allowed to age in wooden casks, but we can speculate that it came about as a result of illicit distillers hiding casks and forgetting where they had been secreted, only to retrieve them at a later date.
Once opened, they discovered an apparently miraculous change in the character of the contents, compared to how it was usually consumed, fresh, or nearly fresh, from the still. The discovery probably occurred on many occasions and in many locations in isolation, but the phenomenon has been widely acknowledged since at least the 17th century.
Nowadays modern distillers employ rigorous wood management programmes, designed to produce the optimum whisky, recognising the true importance of the cask. But not too many years ago, the prevailing philosophy was more like ‘if it doesn’t leak, fill it!’
In the world of whisky, the cask has a dual role, acting both as a container and as an agent for maturation. Such is the importance of the latter function that by law Scotch whisky must be matured for a minimum of three years in casks with a capacity of no more than 700 litres. The restriction on size underlines the need for optimum contact between spirit and wood. As a general rule, the smaller the cask, the faster maturation occurs, because in a small cask the surface of the wood interacts with a comparatively modest amount of spirit.
Although virgin oak is sometimes used for maturation or part-maturation, the vast majority of casks used by the Scotch whisky industry have previously contained either bourbon or, in much more limited quantities, sherry. Wine casks are also employed for maturation, usually for a secondary or ‘finishing’ period of maturation to imbue the whisky with different aroma and flavour characteristics.
By law, all Scotch whisky must be aged in oak, but a relaxation of the rules in 2019 allowed whisky makers a greater degree of flexibility in terms of the type of oak casks that could be used for maturation in terms of previous contents, since when we have seen the use of calvados, mezcal and tequila casks.
Until the years following the Second World War, most casks used for Scotch whisky had formerly housed sherry, but gradually producers switched to ex-bourbon casks, which were comparatively cheap and plentiful, due to the law which states that bourbon must always be matured in new, charred oak barrels.
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ABOVE: Speyside Cooperage has been home to the ancient art of coopering since 1947 and is now a visitor attraction on the Malt Whisky Trail
A sherry butt may cost the whisky distiller 10 times that of a bourbon barrel, and some 90 per cent of all casks now used by the Scotch whisky industry formerly contained bourbon.
Bourbon casks are made from American oak (Quercus alba) and will have been ‘charred’ or fired internally for anything from 30 seconds to four minutes. This process creates a layer of carbon which aids the removal of undesirable sulphur compounds from the spirit.
Bourbon barrels contain approximately 200 litres of liquid and are the smallest casks regularly employed by the Scotch whisky industry. A hogshead holds some 250 litres of spirit, and is constructed by adding extra staves to a barrel. Together, these two are the most commonly used cask types for whisky maturation.
The largest cask in regular use for Scotch whisky is the butt, which holds around 500 litres. Butts are made from either European oak (Quercus robur) or American oak, and previously contained sherry, most commonly oloroso, which interacts particularly well with more robust Scotch whiskies.
When a cask has held liquid prior to its acquisition by Scotch whisky distillers, it is referred to as a ‘first-fill’ cask. A ‘second-fill’ cask has been used once to mature Scotch whisky, while a ‘refill’ cask has had a minimum of two previous whisky fillings.
A first-fill cask has the most overt influence on the spirit within, as each filling of whisky diminishes the impact and influence of its original contents, whether bourbon, sherry or wine.
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ABOVE: A Society cask maturing in our warehouse with a #4+ level of charring
A cask may have a lifespan of half a century, depending on how many times it is filled and for how long the spirit remains in it on each occasion. ‘Exhausted’ American oak casks which have been filled several times may be rejuvenated by being reamed out and re-burnt, in a process known as ‘de-char/re-char’.
Another cask type that has achieved popularity in recent years, especially with start-up distillers, is the ‘STR’. The initials stand for ‘shaved, toasted, re-charred,’ and the STR was developed by the late whisky consultant Dr Jim Swan, using red wine casks.
A layer is shaved off the inside of the staves, which are toasted using oak chips to caramelise the sugars and vanillin, and the vessel is then re-charred. The result is a cask that has the characteristics of a first-fill red wine cask, along with a degree of virgin oak influence.
Having filled casks with new-make spirit – usually diluted to around 63.5% abv for optimum maturation – they are then stowed in warehouses to allow the interplay between time, wood and the external atmosphere to work their magic, resulting in the myriad colour, aroma and flavour permutations we enjoy when we pour ourselves an aged dram of single malt.