If we didn’t have casks, we wouldn’t have whisky. It is the interaction within oak casks which is so fundamental to the end result; a slow transformation of clear new-make spirit into Scotch whisky, creating a golden liquid with exceptional flavour.
As much as maturation matters, so does perfect timing and the perfect cask. Choosing the right casks to transform our specially selected clear new-make spirit into the best quality whisky is where our experience and expertise excel.
From the species of tree to the level of heat charring on the inside of a finished cask, it is imperative that we have control over every aspect. From branch to barrel, we aim to deliver a perfectly matured whisky time and time again.
Below we will explain the details we specify for our casks, what impact these decisions have on maturation, and how we maintain and manage our wood, for the benefit of our whisky.
Also known as Quercus Robur, European Oak is the wood type traditionally used to manufacture Sherry casks. Quercus Robur contains up to three times the tannin levels compared to Quercus Alba, giving a spirit a dry astringency in the early stages of maturation. Over time, lignin compounds break down slowly, releasing more intense notes of spice and toffee into the spirit. Quercus Robur is a slower growing species of oak, which leads to a wide grain. This wide grain results in a whisky with a highly developed complexity and character.
American White Oak
United States of America law decrees that every American bourbon barrel must be manufactured from Quercus Alba. Perhaps more surprisingly, the majority of newly produced Sherry casks are also made from imported American Oak. It is estimated that up to 97% of new casks used to mature Scotch whisky are currently made from Quercus Alba, whether ex-bourbon or ex-Sherry. Quercus Alba is high in compounds called oak lactones, which help to deliver a sweet, fragrant aroma, characteristically found in American Bourbon and often described as coconut, vanilla or citrus notes. Quercus Alba is a much faster growing variety of oak, which thrives in the US climate. It has a tighter grain than European Oak, which means a spirit extracts sweet and delicate flavour compounds.
The initial fill
American Bourbon whiskey is made primarily from maize, along with a mixture of other cereals such as barley, rye and wheat. Distilled in both large column stills and pot stills, the whiskey is matured in warehouses, which can be subjected to extremes in temperature.
At Gordon & MacPhail, we frequently use casks that have previously held American Bourbon whiskey. These casks impart specific flavours into our maturing Scotch and it is imperative to perfectly match the style of new make spirit with the individual attributes that each cask brings to the maturation process.
Bourbon whiskey is matured in fresh, unused or ‘virgin’ oak casks, which contain concentrated flavour compounds. The casks are heavily charred, which is a process that opens up the,wood grain to speed up maturation. This gives the spirit a highly distinctive, sweet, aromatic character along with a vibrant colour. As a result, Bourbon-matured Scotch whiskies carry flavours which can be derived from both the wood type and previous spirit matured in the cask. Typically, whisky matured in bourbon barrels will present a base of vanilla sweetness, vibrant and light aromas, with fresh tropical fruits and beautiful ethereal top notes on the palate.
Sherry is a Spanish white wine which has been fortified by the addition of a spirit distilled from grapes. The combination of the two is subsequently aged in oak casks. There are a number of different, charismatic styles of Sherry: from the light, bone dry freshness of a Fino, the robust dark and nutty character of Oloroso, through to the heavy, rich, sweet and spicy notes of a Pedro Ximénez. These varying styles will have different alcohol contents and enhanced levels of oxidation to create their unique flavour profiles.
The size of the cask has an effect on the overall maturation of the whisky, and the final flavour profile. The relative surface area of the wood to the spirit will influence the speed at which flavour compounds are extracted from the carefully coopered staves into the whisky itself.
The three most common cask sizes used in Scotch whisky maturation are butt, hogshead, and barrel.
500 litres Almost exclusively ex-Sherry Smallest surface area per litre Slowest maturation More time for alcohol to break down flavour compounds, more oxygen available to oxidise; less woody flavours, more complex tobacco, fig, and spice flavours.
250 litres There are two types of hogshead casks: Re-made hogshead is traditionally made by Scottish coopers by adding staves to barrels to increase the size resulting in sweet vanilla flavours Sherry hogshead is the smaller brother of the Sherry butt producing similar fig and spice flavours
200 Litres Generally ex-American Bourbon Largest surface area per litre Fastest maturation Likely to yield sweet, fruity flavours
Affects the speed and makeup of evaporation of volatile compounds from the spirit, influencing flavour profile.
Preparing the cask
During coopering the inside of the cask is heated. This serves two purposes: firstly it makes the wood more pliable for easy shaping; secondly the process begins to change the chemistry of the wood.
Heat transforms flavourless wood compounds into rich aromatic compounds that can be extracted during whisky maturation. The layer of carbon also extracts undesirable flavours and compounds.
The temperature level reached and length of time fired during this heating process make a big difference to the flavour profile of the wood. The heat levels cover a continuum from light toasting to heavy charring.
Gordon & MacPhail specifies these variables to an exacting degree based on the fate of the cask.
Request a booklet
Learn more about our philosophy, our good wood policy, and cask maturation with The wood makes the whisky booklet.