Evaporative loss, storage conditions, and fluctuating alcohol by volume (ABV) can present significant challenges for distilleries during whisky maturation.
Between them, they create many situations that need to be managed to produce exceptional flavours. Let alone to do it consistently.
Understanding what distillers are dealing with helps to understand why their expressions taste the way they do. It also explains why there is such fascination around what is going on in warehouses across the world.
Managing maturation is clearly a dark art! Here’s everything you need to know…
As whisky matures in oak barrels, a certain percentage of the liquid naturally evaporates. It’s a phenomenon affectionately known as the “Angel’s Share”.
This process can result in significant volume loss over the years.
While the evaporative loss may involve a literal loss in quantity, it’s also a transformative process that influences the whisky’s quality and flavour profile. It’s not necessarily a bad thing. It can even be the opposite!
For example, the concentration of the spirit enhances as water and alcohol evaporate, allowing for an ever evolving interaction with the wood. This ultimately creates a more complex flavour profile.
To a certain extent, distillers can influence the rate of evaporative loss by managing the ageing environment’s humidity and temperature.
High humidity minimises water loss, reducing the ABV in a cask. By contrast, a dry environment encourages water evaporation, increasing the ABV.
You can’t fight against your geography however. The likes of Bourbon get stronger over the years due to the hot and dry Kentucky climate, Scotch loses ABV due to the rainy, cold Scottish weather. Trying to optimise the rate at which it happens however, is the reason you’ll see warehouse managers in deep concentration…
On average, the annual evaporative loss can range between 1% and 4%.
In cooler climates, such as those found in Scotland and Ireland, the evaporation rate tends to be on the lower end of the scale, averaging around 1-2%. Conversely, in hotter climates, such as Kentucky in the USA, the rates can be higher, averaging between 2% and 4%, or sometimes even higher.
New world whisky makers in tropical environments also tend to have thirsty angels too.
Another way to look at it is that over a ten year maturation period – over a fifth of the cask will have evaporated. Look at that with a wider lens and the scale of what is happening is mind boggling. Over 20 million bottles worth of whisky are lost to the angel’s share each year in Scotland alone.
Commercially, the amount of money evaporating into thin air is astonishing. However, seeing this perspective helps explain why while older doesn’t always mean better – it usually means more expensive…
It’s not just a regionally variable factor either, differences occur within a single warehouse.
The specific location of a cask within a warehouse can also substantially affect the rate of evaporation. Casks stored at higher elevations where temperatures are higher might experience a faster rate of evaporation compared to those stored closer to the ground.
It’s not unheard of for some warehouse managers spray water down on a dunnage warehouse floor to increase the humidity levels. Meanwhile others actively ventilate to reduce humidity build ups.
The Angel’s Share is not a fixed figure but a variable one. It’s deeply influenced by the geographical, climatic, and infrastructural nuances of the whisky’s maturation environment.
It’s unavoidable and the rate at which it happens is hard to control.
Storage conditions during whisky maturation
Beyond the Angel’s Share, climate will significantly affect the final product in other ways. Temperature variance, humidity, and even the air quality will impact flavour.
For example, the environment surrounding the distillery introduces unique compounds into the spirit, with the air carrying microbes, salts, and other trace elements that can influence the whisky’s flavour.
It’s entirely possible that the location of the coastal distilleries might add a saline character to the whisky simply due to the where the casks are being stored. It sounds far-fetched and over romanticised at first. But think it through and it’s hard to rule out entirely…
Meanwhile, as temperature governs the rate of chemical reactions occurring between the liquid and the cask’s interior, it can influence how the whisky absorbs compounds from the wood and the rate at which this happens.
Equally, temperature fluctuations make a big difference too. Hot days and cold nights allow for the liquid to expand and contract. The more dramatic the fluctuation, the greater the interaction between cask and spirit.
Ten years in the tropics or in a Kentucky rickhouse is very different to a decade in the English countryside!
Alcohol by Volume (ABV):
The ABV refers to the alcohol content present in the whisky. During maturation, the ABV will change due to various factors, including evaporative loss.
ABV is not just a measure of alcoholic strength though; it influences the whisky’s texture, flavour profile, and mouthfeel. Where to start and at what ABV to fill a cask is a big decision.
Industry standard is 63.5% for Scotch, but many go a few increments either side.
The chosen ABV can significantly impact the extraction of flavours from the wood, with higher ABVs potentially extracting flavours faster. The trade off is the type of compound being extracted. At a higher ABV, you will get more lipophilic compounds (and therefore a higher concentration of tannins, lignin, and other wood constituents). More booze, yes, but more bitter too…
Warehouse managers and blenders keep close tabs to ensure they have a good barometer of where their inventory is at any given point. Liquid availability and the condition of it for the blending team aside – it’s also a vital part of compliance for the duty excise due.
Striking a balance between ABV and time in the cask is a delicate dance. What you fill at, and how much you let it fluctuate to vs. what’s gained is subjective. It’s a pursuit of harmony where both factors are intertwined in shaping the whisky’s character.
How much does ABV change year on year?
On average, it is not uncommon to witness a change in ABV to the tune of 0.5% to 1% per annum during the maturation process. This figure can vary significantly based on a host of factors though, so each distillery will be marginally different. Initial ABV, cask size, maturation environment, and the age of the whisky all play a role.
It’s worth remembering that it can go both ways. In hot, dry climates, more water evaporates and the ABV increases over the years. In damp, colder climate the ABV decreases.
As to what can be done if it’s headed in the wrong direction?
Not much!
It is possible to change the location of the cask. Because temperature and humidity impact the rate of evaporation, they influence ABV change too. A different location may be less “active” as a storage space.
Distillers sometimes employ corrective measures such as re-racking the spirit in different casks. However, this has less to do with steering the ABV evolution in a different direction (again, you can’t change your geography) and more to do with what is likely to be an issue with the cask leaking / seeping.
Typically, re-racking will be about addressing a clear volume of liquid loss rather than about ABV.
The dark art of whisky maturation
Understanding the nuances of evaporative loss, storage conditions, and ABV during the whisky maturation process greatly help contextualise why something tastes the way it does.
Many describe whisky maturation as being an alchemy of time and place. It’s why such reverence is paid to those who can master the fine balance year after year.
It really is a complex series of seemingly minor details that amount to a huge difference. Yet, in this intricate dark art of maturation, every detail counts. Every choice is significant, and every year imparts a new chapter to the liquid story that each cask contains.
