Malt whisky is generally aged using three different sizes of cask. This includes barrels previously used to age bourbon, with a capacity of around 200 litres; and two types of casks previously used to age sherry, which are hogsheads with a 250 litre capacity and butts at 500 litres. As malt whisky develops up to 70% of its flavour during aging, every aspect that influences this process, including cask size, is obviously a significant factor.
How influential the cask size can be on the maturing spirit is based on a straightforward formula: the volume of liquid in the cask in relation to the surface area of the cask. The smaller the cask, the greater the amount of liquid (proportionately) is in contact with the oak surface, and it’s from the oak surface that the spirit extracts various flavour compounds during the aging process. Consequently, the smaller the volume of liquid in the cask, the greater the concentration and impact of the flavours which are extracted from the oak. Correspondingly, the larger the volume of liquid the more ‘diluted’ the extracted flavours.
The range of flavours extracted depends on the type of cask. Bourbon barrels, for example, contribute vanilla, honey, and various fruit flavours, with a light, dry sweetness. Sherry casks add a richer sweetness with dried fruit notes including raisins. These flavour differences are principally due to the species of oak, with bourbon barrels made from American oak (quercus alba) while sherry casks are European oak (quercus robur).
Comparing the influence of each size of sherry cask (which has previously been used to age the same style of sherry) is an obvious starting point.
Stuart Harvey, master blender, Inver House Distillers says: “You see more rapid development in hogsheads, and what you get after 18 months in a hogshead takes 2 years to develop in a butt.”
John Campbell, Laphroaig’s distillery manager, adds: “You get the same range of fruit notes from a sherry butt as a sherry hogshead, but the flavours are more intense from a hogshead. This also means the distillery character (ie the original character of the new make spirit) can remain more prominent in a butt than a hogshead. So, with a peated malt such as Laphroaig, the larger the cask the more intense the phenolic character in the mature malt.”
A similar comparison is between a bourbon barrel and a quarter cask, which has half the capacity of a bourbon barrel at 125 litres (the term ‘quarter cask’ stems from this type of cask being a quarter of the size of a butt at 500 litres).
Laphroaig’s quarter casks are made by breaking down bourbon barrels and re-using a proportion of the staves. These quarter casks are used for secondary maturation, ie. a subsequent aging period for malt whisky already matured in bourbon barrels.
Campbell adds: “Laphroaig Quarter Cask shows a more intense bourbon barrel influence, particularly toffee and caramel notes, together with more dry oak and spice. This also masks some of the peaty, smokey notes, which are less intense in Laphroaig Quarter Cask compared to Laphroaig 10 year old."
"Another comparison is that the flavours of the 10 year old show on the palate all at once, whereas quarter cask maturation separates the flavours and they each show individually.”
So, cask size is clearly an important factor. But how does this compare to all the other influences during maturation ?
Dr. Bill Lumsden, head of distilling and whisky creation, Glenmorangie says: “The size of the cask is a contributory factor rather than a primary factor in the aging process. There are so many other considerations, for example whether the cask is American or European oak, and what was previously aged in the cask, which are more important than the size.”
Moreover, some significant reactions that take place in the cask are not influenced by cask size.
Brian Kinsman, master blender, William Grant and Sons suggests: “Maturation is essentially a two stage process, additive maturation, in which the spirit gains flavours from the oak cask, and subtractive maturation, in which the spirit looses certain notes such as cereal character, for example, through evaporation from the cask. While smaller casks enable additive maturation to occur more quickly, subtractive maturation happens at a similar rate regardless of the cask size.”
Two other traditional cask types are remade hogsheads and puncheons.
Remade hogsheads (250 litres) are remade using the staves of a bourbon barrel, the usual calculation being one and a third bourbon barrels makes a hogshead. This approach was typical when bourbon barrels were shipped to Scotland in the form of staves (rather than actual barrels). Remade hogsheads have a similar though less intense influence than bourbon barrels.
Gordon Motion, Edrington's master blender says: “Creating remade hogsheads at a cooperage in Scotland was a practical way of gaining extra storage space. However, its standard to ship bourbon barrels rather than staves, so the level of remade hogsheads is declining."
A puncheon (500-550 litres) has a similar capacity to a butt, though at around 1.2 metres in height it’s shorter and rounder than a butt at around 1.4 metres.
Stuart Harvey says: “When oak is sawn to make butts there are lengths of timber left-over which are insufficient for another butt, so using them to make puncheons instead gets the best use out of the oak. Puncheons and butts give the same result when aging malt whisky.”
23 October 2016 - Ian Wisniewski The Drinks Report, editorial assistant