Malt whisky is generally aged using three different sizes of cask. These 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 per cent of its flavour during aging, every aspect that influences this process, including cask size, is 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 here 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.
"You see more rapid development in Issue 112 | Whisky Magazine 39 Production Casks hogsheads, and what you get after 18 months in a hogshead takes two years to develop in a butt," says Stuart Harvey, master blender, Inver House Distillers.
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).
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.
"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 Years Old.
Another comparison is that the flavours of the 10 Years Old show on the palate all at once, whereas quarter cask maturation separates the flavours and they each show individually," adds John Campbell.
So, cask size is clearly an important factor. But how does this compare to all the other influences during maturation?
"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," says Dr Bill Lumsden, head of distilling and whisky creation, Glenmorangie.
Moreover, some significant reactions that take place in the cask are not influenced by cask size.
"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," says Brian Kinsman, master blender, William Grant & Sons.
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 third of a 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.
"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," says Gordon Motion, Edrington's master blender.
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.
"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," says Stuart Harvey, master blender, Inver House Distillers.
Aging warehouses in Scotland traditionally contained a variety of casks in a range of sizes.
This was essentially a case of practicality, as imported wines, port and Sherry were (historically) shipped to the UK in casks, with a key destination being the port of Leith near Edinburgh.
Casks were emptied and the contents bottled locally, which left importers with numerous empty casks. An ideal solution was selling the casks to Scotch whisky distillers, who could re-use them for aging.
However, from the mid-20th century there was a growing focus on using Bourbon barrels and Sherry casks, which became the choice for aging.
Various bottlings use a 'recipe' of malts that have been aged in Bourbon and Sherry casks.
However, some bottlings consist entirely of malt whisky aged in Bourbon barrels, such as The Balvenie 12 Years Old Single Barrel. Similarly, some malts are aged entirely in Sherry casks, such as Berry's Own Selection Bunnahabhain 1990 (aged in a sherry butt).
Meanwhile, the last 20 years has seen a huge growth in the range of casks used, in particular for special finishes and secondary maturation (ie. an additional aging period applied).
The choice of casks for secondary maturation includes barrels previously used to age Sauternes wine (wine barrels typically have a capacity of 225 litres), which are part of the aging regime for Glenmorangie Nectar D'Or.
Similarly, port casks (with a capacity of around 600 litres), play a role in aging Glenmorangie Quinta Ruban.
While the focus of such casks is inevitably on the previous occupant, such as port, the size of the cask also plays a role in delivering the final result.
The Balvenie Single Barrel 12 Years Old, 47.8% ABV
William Grant & Sons
Nose: Rich and elegant, honey and vanilla, then fresh, ripe apples, hint of underlying digestive biscuits and toffee.
Palate: Indulgent texture. Elegant honey and apple notes, which intensify with a ripe-dry balance that includes a hint of oak and digestive biscuits.
Finish: Subtle dryness leads, oak follows with a hint of honey, then these notes merge and linger.
Comments: Elegance, intensity and evolution. With water the apple aromas come to the fore.
Nose: Honey and toffee, very clean and fresh. Candied pear and green toffee apples. Shades of malt loaf and warm fresh bread.
Palate: Very fruity, jammy with those pear drop travel sweets. The honey comes through with an oak grip and a malty edge.
Finish: Fairly short with more spices loading in. Vanilla and oak too.
Comments: You might expect the cask to dominate being first fill American oak, but the balance here is impressive. Will be interesting to see how each batch differs.
Glenmorangie Nectar D'Or, 46% ABV
The Glenmorangie Company
Nose: A generous helping of tarte tatin, followed by a slice of lemon tart, rich and rounded with a hint of underlying oak.
Palate: Elegant texture. Flavours gradually reveal themselves: cooked apples, lemon tart and light creamyness, with mellow spice and subtle dryness providing a counter-point. Fresh apples follow, then oak.
Finish: Light dryness leads, then apples and lemons emerge with a hint of oak.
Comments: A Scottish-French rendezvous that can be celebrated by the glass. With water the aromas are fruitier, and the palate delivers more apples.
Nose: Earth and fresh wood that then develops into passion fruit, kiwis and apricots with a lemon zip.
Palate: Cream and fruits coat the mouth.
This is sweet and gentle. Barley sugar sweets too.
Finish: Short, drying and morish.
Comments: Takes its time to open but when it does that Sauternes cask has infused a white chocolate sweetness and citrus edge to the dram.