Cask Influence

Claxton's single malt scotch whisky cask head with motif

There are many factors that influence the character of a Single Malt Scotch Whisky, but one of the most important of all is the cask maturation. In fact, many recent studies have concluded that this accounts for more than 50% of the final flavour and aroma of a whisky.

 The Scotch Whisky industry offers an incredibly dynamic and varied world of cask maturation, making it diverse and endlessly intriguing. Many Scotch Whisky producers have their own cask management systems and each cooperage (cask maker) produces and rebuilds casks slightly differently. Therefore the information below provides a general guide to the different aspects of oak cask maturation as there are many variations within the industry.

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The Wood

The casks in which whisky matures are made from a wide variety of oak species. Often the previous liquid held in a cask is cited when referring to a Single Malt Scotch Whisky’s maturation, e.g. matured in an Oloroso Sherry Cask, but the species of oak used to make the cask also has a profound effect on a Scotch Whisky’s final character.

When discussing the type of oak used in cask making, it can be misleading, as the wood is often referred to as European, American, French Oak, etc. These are used as a general geographic names, as opposed to the specific species of the wood. This is sometimes done for a number of reasons; not least many oak trees are prone to hybridisation. However, there are some species that are used most commonly in the production of whisky casks, which we have been outlined here. Each species will have its own effect on the flavour of a whisky.

 

American Oak

The most common species of American White Oak used in the production of whisky casks is Quercus Alba, so called due to its paler appearance compared to European Oaks. American White Oak is by far the most commonly used in cask production, due to its abundance. It is invariably used to create barrels which mature American Bourbon Whiskey, that are then shipped to Scotland to mature Scotch Whisky.

Quercus Alba has a typically much softer and more mellow influence on whisky than many of the European Oaks and can impart a taste and aroma of caramel, coconut, honey and vanilla.

Quercus Alba

 The above shows the grain and pale characteristics of Quercus Alba (American White Oak).

 

European Oak

The most common species of European Oak used are Quercus Robur and Quercus Petraea. However, far fewer casks are made from European Oak than from American White Oak, as the wood takes longer to grow and prepare for cooperage. Quercus Robur grows more commonly in the forest regions of Northern Spain and Portugal and is most frequently used in the production of Sherry casks. Quercus Petraea is more prolific in the forest regions of France and is generally preferred for the construction of Wine Barrique Casks.

Quercus Robur has a strong character and can impart a very dark colour to whisky. It will often have strong, punchy influences, such as spices (nutmeg, cinnamon), dried fruits (sultanas, prunes) and citrus peel.

Quercus Petrea has a more subtle influence compared to Quercus Robur, with notes of vanilla, pepper, and more delicate spices and dried fruits.

Quercus Robur cropped web final

Quercus Robur is distinctively darker than the more commonly used Quercus Alba (American White Oak) and imparts a much darker appearance to whisky.

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Varieties of Cask

The different types of casks used in the Scotch Whisky industry vary in size, shape and previous use. All of these aspects can have a profound effect on the character of the whisky maturing in the cask and the rate at which it matures. Recent years have seen an increase in the use of all manner of different casks to mature or finish whisky, which has created some very interesting whiskies and an even greater variety of options.

The shape and size of a cask has a significant influence on the maturation of whisky, because it will dictate the ratio of the wood coming into contact with the spirit, e.g. a smaller cask will have a higher ratio of wood contact to spirit than a larger cask. Therefore, the wood will interact with the spirit more rapidly or slowly, depending on the size of the cask. This not only influences the rate at which the whisky matures, but also the final characteristics of the whisky.

The infographic and table below outline the typical features of the most frequently used types of casks, although these can vary between cooperages and whisky producers.

CASK TYPES AND SIZES FINAL

Cask Type

Approximate Volume (litres)

Common Wood Type

Typical Predecessor Liquid

Blood Tub
40 Variable

Beer

Quarter Cask
125 Variable

Variable

ASB (Barrel)
200 American Oak

Bourbon Whiskey

Barrique
225 American & French Oak

Wine

Hogshead
250 American Oak

Bourbon Whiskey

Cognac Cask
350 French Oak

Cognac

Puncheon
500 American & Spanish Oak

Rum & Sherry

Butt
500 American & Spanish Oak

Sherry

Port Pipe
550 American & European Oak

Port Fortified Wine

Madeira Drum
650 French Oak

Madeira Fortified Wine

Gorda

700

American Oak

Variable

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Previous Liquids in Cask

Scotch Whisky, unlike American Bourbon Whiskey, is very rarely matured in virgin oak casks.  This means that nearly always the casks will have contained another alcoholic drink prior to maturing whisky. This is done for a number of reasons, not least as using virgin oak would result in a very strong wood influence, which is not typically associated with the more delicate, smooth style of Scotch Whisky. In addition, the previous liquid held in the cask can also impart some very desirable characteristics and add variety to the industry.

By far the most common casks used in the Scotch Whisky industry are Bourbon Whisky and Sherry casks (typically known as ASBs and Butts), but other types of cask are being used increasingly frequently to either mature or finish whisky, with some very interesting results (e.g. wine barriques and cognac casks). The table below offers an overview of the influence that some of the most common predecessor liquids can have on the final character of a whisky.

Predecessor Liquid
Influence on Character of Whisky
Bourbon (Whiskey)
vanilla, caramel, toffee, coconut, fresh fruit
Oloroso (Sherry)
ripe fruits, dried fruits, woody, nutty, spicy, sweet
Pedro Ximinez (Sherry)
dark fruits, treacle, dates, syrup, very sweet, nutty
Manzanilla (Sherry)
dry, costal/salty, dried fruit, dry, citrus zest
Cognac (Brandy)
dark, rich fruits, spices, nutty, caramel, vanilla
Port (Fortified Wine)
sweet, citrus, red berries, dried fruits, dark chocolate, spiciness
Madeira (Fortified Wine)
sweet, tropical fruit, cherries, floral, spice
Marsala (Fortified Wine)
sweet, nutty, spicy, brown sugar, apricot
Light  Rum (Rum)
sweet, vanilla, spices, pepper, citrus, honey
Dark Rum (Rum)
molasses, spices, caramel, dried fruit, vanilla, oak
Chardonnay (White Wine)
buttery, tropical fruit, honey, green apple, fresh pear
Bordeaux (Red Wine)
dark fruits, red berries, peppery, nutmeg, honey
Sauternes (Sweet Wine)
sweet, apricots, peaches, zesty, honey, nutty

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Other Factors

In addition to the species of wood, the size of the cask and the previous liquid held in it, there are many other factors that influence cask maturation and the character of a whisky greatly. Outlined below is an overview of just some of those other factors.

Age

Perhaps one of the most widely recognised and well advertised influences on a whisky’s character is its age, meaning the length of time it has spent maturing in the cask. This is widely recognised for a very good reason – it’s very important. By law, to be classed as Single Malt Scotch Whisky the spirit must have matured in an oak cask for a minimum of three years, but the longer a whisky spends maturing in a cask, the more significant the influence on the whisky.

Older age does not necessarily equal better whisky, however. Maturation of whisky in casks is a fine balancing act. If a whisky is matured in the cask for too long, the wood flavours can overpower the character of the spirit; but if matured for too short a time, the spirit will not have mellowed sufficiently. Depending on the type of cask and the character of the spirit inside it, this time can vary hugely. However, generally speaking, Single Malt Scotch Whiskies that have aged well over a longer length of time tend to be more complex and mellow than whiskies that have aged over a shorter length of time – these tend to be a bit more vigorous and punchy.

butts web

 

Number of Cask Refills

Another significant factor affecting the maturation of whisky is the number of times a cask has been filled. It is very rare within the Scotch Whisky industry to use virgin oak casks, as new wood has a very powerful impact on the spirit. A more subtle influence from the wood is generally preferred for Scotch Whisky, so casks that have been used to mature other alcoholic drinks first are the most common, e.g. Bourbon Whiskey and Oloroso Sherry casks. However, the casks are not simply used once more and then discarded; they can be refilled with young spirit multiple times before the wood becomes exhausted. Each time a cask is refilled, it can have a different effect on the whisky maturing inside and each whisky producer will have a preferred number of cask refills to achieve a desired effect.

inside cask web

 

Environment

The environment a cask is kept in can affect the pace of maturation, the alcoholic content, rate of evaporation of liquid from the cask (known as the Angels’ Share) and even the flavour and aroma of a whisky. For example, the hotter the climate, the faster the whisky will mature, due to more aggressive evaporation and interaction with the wood. Or, for example, if a whisky is matured in a warehouse near the coast, it may pick up some marine-like, briney characteristics from the atmosphere.

The kind of warehouse in which a cask is stored (is also significant. There are three main categories: Dunnage; Racked; and Palletised. Dunnage warehouses are the most traditional in style and generally have earthen floors and stone walls. Typically casks are stacked no more than three high and the average temperature is cooler and more stable than the other styles of warehouse. This causes the casks to ‘breathe’ more slowly and produces different results in the final character of the whisky. Racked warehouses are built in more modern materials, such as concrete blocks. Casks are stacked much higher than in Dunnage warehouses (up to eight high) and are much more practical for maturing large numbers of cask. Due to the construction and layout of these warehouses, casks can experience a more varied range of environmental changes than in Dunnage warehouses.  In Palletised warehouses the casks are stacked closely together on end, as opposed to on their side in the other warehouse. As the casks are stored closely together, this can influence airflow between the casks, again impacting on maturation.

cliffs web

 

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