Malt whisky is aged in casks which can be used more than once. A cask used for the first time is referred to as a first fill, which could for example mean a 12 year aging period, before the cask is emptied and filled with new make spirit for a second time, when it’s referred to as a second fill. Casks may be filled a third or even fourth time. Each fill has a different influence on the flavour of the resulting malt whisky, which means the fill is a significant factor.
Casks have previously been used to age either bourbon or sherry, with each type of cask contributing a different range of flavours to the maturing malt whisky. Bourbon barrels add, for example, vanilla and a light sweetness, while sherry casks lend richer sweetness with a range of fruit cake and dried fruit notes.
Each time a cask is filled the influence of the oak on the maturing malt whisky is reduced. Let’s look at a first fill bourbon barrel used to age malt whisky for 12 years. When filled for a second time, and used for a further 12 years, the influence of this barrel is typically around 25 to 30 per cent compared to the first fill. The influence of a third fill, another 12 years, would be around 10 per cent of the first fill.
By comparison, the second fill of a sherry cask typically has an influence of around 50 per cent compared to the first fill, with a third fill around 15 to 20 per cent, and a fourth fill around 10 per cent. The influence of a sherry cask doesn’t reduce as rapidly as a bourbon barrel, because they have a higher level of flavour compounds than bourbon barrels when they arrive in Scotland. One reason for this is that sherry casks are originally used to age sherry, which has a strength of around 17% ABV, compared to bourbon barrels originally being filled with a spirit, up to a maximum strength of 62.5% ABV, in order to produce bourbon. A higher strength spirit is more ‘active’ while aging in a cask compared to sherry, and extracts a higher level of flavour compounds from the oak.
Successive fills aren’t simply a ‘milder’ or ‘diluted’ version of the previous fill, as each fill also delivers a slightly different version of the same flavours. Vanilla, which is a hall mark of aging in bourbon barrels, is a prime example.
“The level of vanilla gained from 12 years of aging in a second fill would be lower, and different to that gained from 12 years aging in a first fill. The vanilla notes from a second fill would have a more caramel character and not be as rounded or honeyed as the vanilla notes from a first fill,” says Jane Millar, technical support team leader, William Grant & Sons.
Similarly, the archetypal fruit notes contributed by a sherry cask change from one fill to another.
“A first fill gives really rich fruitcake and fruit notes like raisins. A second fill delivers fruitcake notes which are distinctive but not as dominant as from a first fill, while the fruits are more prunes, dates and stewed fruit,” says Ian MacMillan, master blender, Burn Stewart Distillers.
In addition to individual flavours evolving in each fill, the overall balance of flavours also changes. The character of the new make spirit aging in the casks, for example, can be more prominent in subsequent fills. During the first fill the character of the new make spirit is to a certain extent ‘masked’ by the intensity of flavour compounds contributed by the cask. However, as second and third fill casks deliver a lower level of flavour compounds, this allows the original character of the spirit to show through more clearly. As the new make spirit of various distilleries contains fruit notes, for example, a second fill ‘showcases’ this fruitiness more than a first fill.
Similarly, the effect of oxidation can show more clearly in a second and third fill cask compared to a first fill, due to the lower level of flavour compounds coming from the cask. Oxidation is the result of air passing in and out of a cask. During this process air dissolves into the spirit (oxygen the key element) which initiates various reactions, and typically results in the spirit becoming fruitier, more balanced and gaining complexity.
Various malts are based on a ‘recipe’ of different fills from bourbon and sherry casks, which are all blended together. Consequently, different fills provide master blenders with a broader range of characteristics to draw upon when composing the ‘recipe’ for a particular malt whisky.
“For Glenmorangie Original second fill casks are critically important, as these casks give more fruit flavour, particularly a citrus bite, compared to a first fill. These fruit flavours come through partly from the character of the new make spirit, and also from the effects of oxidation, as second fill casks allow the spirit character and effects of oxidation to show through more than they would in a first fill cask,” says Dr Bill Lumsden, head of distilling and whisky creation, Glenmorangie.
Meanwhile, how many times an oak cask can be filled, and the result that each fill will give, is hardly guaranteed. After all, oak is a natural product and casks can show significant variations. The only way of knowing how a cask has performed, and whether the cask is capable of another fill, is to analyse the mature malt whisky when a cask is emptied.
A worst case scenario is having to remove a cask from the inventory after just one fill. But fortunately that’s rare.