Craft whisky has witnessed an exponential growth over the past few decades. New World whisky has taken a significant portion of the limelight, but innovative ideas around maturation are now part and parcel of the category at all levels. Chief amongst that – the choice of cask type and combination they are used in.
Understanding the nuances of cask selection — including size, oak type, and previous occupants — becomes a big part of the learning curve for those looking to master maturation.
Not only does it help you comprehend what modern production is like, but it also showcases the endless nature of what the whisky category has to offer.
Cask sizes and their impact on whisky maturation
Irrespective of the type of oak used, or if it had a previous occupant – the cask size will substantially impact the maturation process.
This is because the size of a cask influences both the chemical and sensory profiles of the whisky. Namely, the surface area ratio wood-to-spirit affects the speed at which the whisky interacts with the cask.
Here are the common sizes utilised for whisky maturation:
- Quarter Casks (50-70 litres). These allow for a faster ageing process as they offer a lot more interaction with the wood than larger casks. They sometimes yield a less refined product – which is why they are predominantly used for tests and early indicators.
- American Standard Barrel (200L). This size is one of the most widely used, offering an optimal balance between volume and surface area, lending predictable maturation progess.
- Hogshead ( 230-250 litr). Created by reassembling American Standard Barrels with extra staves, these barrels allow for slower maturation and great flavour development. The added size optimises space in the warehouse.
- Wine Barrique (about 300 litres). Usually made of French oak and used to store wine or cognac. These are extremely popular with New World Whisky makers.
- Butt. (500 Litre). Butts are casks that have become popular with whisky makers (namely as most are used to mature sherry). They are often the largest casks in a dunnage warehouse.
- Port Pipe (350 litres upwards). Named for the Portuguese word ‘pipa’ – meaning cask – historically these were used to mature port. Pipes are now commonly used to mature whisky. It’s not unusual for them to be the same size as a butt and bigger than 500L.
It’s easy to see how scale plays a role in terms of the way the wood and whisky interact. The smaller the faster is true – but it’s also worth noting the other consequences. A faster maturation in a smaller cask also means that a higher percentage of the spirit is lost over the years to evaporation.
You might fast forward some aspects of maturation, but it comes at a price!
Oak types and maturation dynamics
The type of oak chosen greatly influences the additive and subtractive interactions during maturation, thus making huge differences in the whisky’s flavour, colour, and mouthfeel.
- American Oak (Quercus Alba). Generally imparts vanilla, caramel, and coconut notes to the whisky, coupled with a rich colour.
- European Oak (Quercus Robur). Bestows rich, spicy, and dried fruit characteristics, offering a deeper hue and more tannins than American oak.
Beyond the commonly used Quercus Robur and Quercus Alba there are other interesting types on the market.
- Japanese Oak (Mizunara). Infuses complex spicy notes and a distinct coconut aroma, creating a unique profile sought after in the industry.
- Quercus Petraea (Sessile Oak). This oak type is common in Europe, with a notable presence in the forests of Hungary and other Eastern European countries. It’s similar to Quercus Robur (French oak) in terms of its tight grain and rich flavours. Spirits matured in casks made from Quercus Petraea often exhibit rich, spicy, and chocolaty notes.
- Quercus Mongolica (Mongolian Oak). Predominantly found in regions of Northern Asia, including parts of Russia, China, and Korea. Whiskies aged in these casks may exhibit diverse flavours profiles that include sweet, vanilla notes and a lighter, more delicate coloration compared to spirits aged in other oak types.
- Quercus Pyrenaica (Pyrenean Oak). Native to the Iberian Peninsula, notably in regions of Spain and Portugal. New World Whiskies and wines aged in Pyrenean oak casks often boast dark, rich colours and flavours characterised by dark fruits, spices, and occasionally, hints of balsamic.
Each oak type, with its distinctive geographical, structural, and biochemical characteristics, contributes uniquely to the flavour, colour, and texture of the spirit. The type of oak offers distillers a palette of options to explore and create varied expressions of their spirits.
Additive and subtractive reactions during maturation
While the focus of a lot of flavour notes and product descriptions are on what the type of oak has brought to a whisky through its interaction, maturation isn’t all about creating flavour.
It’s more accurate to look at maturation as altering flavour. It does this in two ways; additive and subtractive.
Additive interactions involve the extraction of flavour compounds from the wood into the whisky. They contribute to the whisky’s flavour and colour.
The main additive extractions are those derived from of wood compounds
Lignin. This complex polymer breaks down to infuse the whisky with sweet and spicy notes, including vanilla and clove, augmenting the depth of its flavour profile.
Hemicellulose. This component releases simple sugars during the ageing process, contributing a sweetness to the whisky and enhancing its mouthfeel.
Tannins. In moderate amounts they contribute structure and complexity, fostering a rich mouthfeel. Excessive tannins can impart a bitter, astringent note however.
Oak lactones. These are responsible for the coconut and woody notes often associated with whiskies aged in American oak barrels.
Compounds being leeched into the whisky aren’t the only additive element going on ether. There are chemical reactions that add to the development of complexity.
A significant one is esterification. This is the reaction between organic acids and alcohols to form esters. As you might have already read in our articles on Fermentation and Distillation, esters contribute significantly to the spirit’s fragrance, imparting floral and fruity notes.
Subtractive interactions
Subtractive interactions refer to the removal or breakdown of undesired elements.
The role of oxidisation is an example of a subtractive interaction. Oxidation is a chemical reaction that involves the interaction between oxygen and the various components present in the whisky. It occurs gradually over time as the spirit matures in the casks.
The reason oxidisation is possible is because casks are not entirely airtight. Over time, the process has a transformative effect on the whisky, engendering a series of chemical reactions that influence the flavour and aromatic profile of the spirit.
Here’s how:
Altering volatile compounds. Volatile compounds that might have been formed during the distillation process (generally undesirable), can be altered or even removed through oxidation. This can include the reduction of sharp alcoholic notes, paving the way for a smoother spirit.
Esterification. Oxidation also encourages the formation of esters — compounds resulting from the reaction between acids and alcohols present in the whisky.
Tannin transformation. Oxidation aids in the mellowing down of harsh tannins, transforming them into more pleasant and softer tasting compounds. This not only makes the whisky smoother but it can also replace a stringent or bitter profile with a more rounded, palatable character.
Charring casks
To achieve these additive and subtractive interactions, each cask undergoes a mandatory toasting process before its initial use. This is known as charring.
Charring refers to the controlled burning of the interior of oak casks, a practice mastered and performed by coopers (cask makers). Through the application of direct flame, the inner surface of the cask is subjected to high temperatures, causing the wood to burn and form a layer of charcoal.
This process leaves behind a distinctively rugged, cracked appearance. So much so, it is sometimes likened to an alligator’s skin due to its textured and blackened state.
Charring is helpful in many ways.
- Charring alters the chemical compounds in the oak (maillard reaction), enabling the creation of complex flavour compounds. It also makes the whisky to wood interaction possible as the resulting cracks and crevices increase the inner surface area of the cask. This allows the liquid to penetrate the oak further too.
- The charcoal layer created through charring acts as a natural filter, adsorbing unwanted elements such as sulphur compounds and harsh precursors, which might otherwise impart off-flavours to the whisky.
- Charring can break down harsh tannins, balancing the influence of wood on the final product and preventing the whisky from becoming overly tannic. As mentioned earlier – by managing the tannin levels, charring also plays a role in maintaining the subtle sweetness and preventing it from being overshadowed by bitter notes.
The role of previous occupants in shaping flavour profiles
Last but not least – the choice of a cask’s previous occupants plays a critical role in the final whisky product, influencing its flavour and aroma profile.
Ex-Bourbon. Barrels previously holding bourbon can add notes of vanilla, caramel, and sometimes, a sweet corn-like undertone.
Ex-Sherry:.These barrels tend to confer a rich, fruity, and nutty profile to the whisky, adding complexity and depth.
Ex-Port. Utilising port barrels can offer whisky a sweet and rich profile, with notes of dried fruits and chocolate.
Ex-Wine. Depending on the type of wine, casks can add a wide array of nuances, ranging from tart notes to rich fruity undertones.
All of this amounts to a few key takeaways – three areas to remember!
No two casks are the same, starting from the type of oak to how it has been charred. Believe it or not, we only scratched the surface above and even based on that, it’s easy to see why.
Size influences rate of interaction, but it’s not just about what’s being added (e.g. leaching compounds from the oak). It’s also about what’s been subtracted too (e.g. through oxidisation).
Add the third layer – the previous occupant. What has been in a cask before it is filled with whisky is incredibly varied. Each will sway the flavour of the whisky being matured because when you refill it, the cask will still be wet (at least 5 litres for most are still soaked into the wood).
This all speaks to why you’ve got an incredibly complex equation being calculated for every bottle of whisky that gets made!
