What is the Angels' share?

The Angels’ share is the romantic term for the annual rate of whisky lost during cask maturation due to evaporation. As the liquid would evaporate into the heavens, it was dubbed the angels’ share. From a scientific point of view, however, it is the volume of the liquid that turns into gas and then leaves the barrel.

The amount lost in Scotland amounts to only 1-2%, in contrast with considerably warmer climates such as India or Australia where evaporation can reach as high as 12%. Regardless even in Scotland that adds up quickly.

Ewann Gunn, Keeper of the Quaich and one of DIAGEO’s Global Whiskey Masters, estimates there are currently around 22 million barrels of whiskey mature in Scotland. Most of these are hogsheads with a capacity of ~250 liters. If you assume that around 2 percent of the content of the cask lost as Angels’ share, this corresponds to around 110,000,000 liters of whisky disappearing annually. Phrased another way 440,000 casks, enough to fill 44 olympic sized swimming pools!

Evaporation as a process

The process of evaporation begins as soon as a barrel of fresh distillate, i.e. New Make, is filled. Since barrels are seldom completely filled, there is usually a small air reservoir between the liquid surface and the top of the barrel commonly called the head space. In addition within 48 hours after filling a fresh oak barrel the staves absorb about 2% of the total volume of spirits with the result is that the headspace increases accordingly. Chemical substances (e.g. water, alcohols, esters, volatile sulfur compounds, etc.) within the liquid rise in to the headspace or come into contact with the staves and can either escape through the pores of the oak staves or through transformation in the headspace.

Fill level and evaporation

As oak barrels are filled with fresh distillate with an ethanol content of 63.5% ABV as a rule. This means that the remainder, around a third consists primarily of water. It is therefore important to consider vapour of water and ethanol separately, because both escape from the barrel at different speeds. Ethanol is generally considered easier to get out of the barrel through evaporation then, since it is more volatile. Indeed while this is true it has been demonstrated that the temperature is an important factor for the evaporation process. Likewise differences in the ambient conditions of the warehouse have a measurable impact in the relative evaporation rate from the barrel. The reason for this is a fundamental of physics: Similar systems tend toto adjust. Accordingly, the amount of water in the oak constantly is balanced with the water concentration in the environment outside the barrel. The higher the humidity in a warehouse, the lower the amount of water that will escape from the cask due to evaporation bcause the atmosphere is already “saturated” with water. Conversely drier air in the warehouse will trigger a comparatively stronger evaporation.

Temperature

An increase in the temperature in the warehouse increases the evaporation losses of ethanol and water in the barrel alike, while the humidity increases the relative ratewith which both are lost. Both physical parameters, i.e. temperature and humidity, influence by the type of maturation. In a squat, traditional dunnage warehouse with an open clay floor, in which the barrels are stacked up to a maximum of three rows, the humidity is usually higher than in a racked warehouse with up to 12 floors. This shelf storage with a concrete foundation is also much larger and taller, has thinner walls and a metal roof. As a result, external temperature fluctuations are more easily transferred to the interior than is the case with the Dunnage warehouse, which has thick stone walls and a slate roof. While in more temperate Scotland the top barrels in a racked warehouse reach an average ambient temperature of around 20°C in summer and only slightly exceed them - compared to 10-15°C in the lower rows - the conditions in the southern USA are more drastic. Unless natural or forced air circulation is provided, temperatures of 50°C and more can be reached in the upper floors of the warehouses in summer, compared to 18-21°C in the lower part. The higher you get in a rack or pallet warehouse, the lower the humidity.

Humidity

When the air humidity is high, relatively more ethanol is lost than water in the barrel and the ethanol content decreases as the whisky matures. This is the typical situation in Scotland where the relative humidity is often 80-90 percent, especially in winter. On the other hand, there is a loss of ethanol in relatively hot and dry climates. This results in an increase in the ethanol concentration during barrel maturation, which means that the ethanol content increases during maturation. A changing amount of ethanol in the barrel influences the maturation speed and the maturation progress of the whisky. For example, a decrease in ethanol strength can impair the solubility of wood and distillate components, and consequently the amount of ethanol-soluble long-chain esters, Fats and lignin components decrease and the water-soluble sugars and tannins increase. Tannins make a significant contribution to the structure and balance of the whisky, influence its color and also contribute to a bitter, woody and astringent taste. After all, the increase in the content of extracts and wood aromas with prolonged maturation could, in part, clearly obscure the distillery character of the maturing whisky.

an overview of matuation and evaporation

In summary, the evaporation rate is influenced by the following factors:

• Type of maturing storage
• Ambient temperature
• Air humidity
• Filling strength
• Position of the barrel in the storage

Not all of the ethanol that escapes from the oak barrels is reserved for the angels alone however. An ancient organism competes for their share: Baudoinia compniacensis. This black, soot-like fungus also feeds on the evaporating ethanol, but needs a high level of humidity for at least a few seasons in order to support its colony formation. It grows on the walls of warehouses and even spreads to surrounding trees, shrubs, and buildings. Mature colonies of this fungus can reach a thickness of 1-2 cm. If you do not see a blackened warehouse, this is an indication that the whiskies are not maturing locally but elsewhere.

Evaporation & whisky pricing

The angels share also goes a long way to explaining the incredible prices attached to much older whisky, beyond merely accounting for the storage costs they also factor in rarity. While a 10 year old cask may have lost only 18-20% of its volume, in contrast a 25 year old cask typically loses around 40% abv.

The complicated, complimentary process of evaporation

While on paper this sounds like a devestating financial loss that should be halted at any cost, which led to some amusing confusions regarding Diageo and clingfilm, the Angels’ share is not a one-way street.

The gap left by evaporating whisky is filled by air from the environment. This penetrates through the pores of the oak staves into the barrel and reacts with the chemical compounds of the whisky. This process is how distilleries eliminate unwanted flavours, allow stronger sulphurous notes to disipate and of course allow the ABV to drop gradually over time. The Angels’ share is a prerequisite for this.

External influences on the angels’ share

How much whisky actually has to be shared with the angels depends heavily on external influences. While no distillery has complete control over the Angels Share there are a number of factors which can reduce, or increase the rate of evaporation:

• Temperature
• Humidity
• Barrel Storage
• Air pressure
• Cask Size
• Cask fill level

Temperature

No distillery has any influence on the climatic conditions of the country in which it produces but it can control the climatic conditions within its own warehouses. Heating systems are not uncommon, ensuring that the temperature is not too subject to the fluctuations of the seasons. The Angels’ share and thus the oxidative maturation ceases to be a roller coaster ride as tempereatures rise and fall, but a constant process.

In addition, the following applies: the warmer the temperature inside the warehouse, the faster the whiskey matures. The chemical basis for this is provided by the Van’t Hoff rule, according to which the speed of chemical reactions is at very least doubled as soon as the temperature rises by only 10°C.

Humidity

Under predominantly humid climatic conditions, such as those found in Scotland at an average humidity of 88% the air that surrounds the barrel is already saturated with water. The pressure of the Angels’ share, with which it pushes gases out of the barrel, is too weak to counteract the ambient air. The result is that more alcohol evaporates than water, so that over time the alcohol content of the spirit decreases.

In predominantly dry climatic conditions, such as in Kentucky, however, more water evaporates than alcohol in the oak barrels. The result is that alcohol to water ratio actually increases during maturation and the whiskey leaves the barrel stronger than it was poured into it.

Barrel Storage

The iconic dunnage warehouses with barrels slowly maturing in dark stone buildings are the best for consistency. Racked or palletised warehouses are vast metal structures in which the barrels are stacked dozens of meters into the sky and huge ranges in temperature are found between the casks. Those in the warmest areas, typically closer to the roof mature more rapidly.

Air pressure, Cask Size and Fill Level

The list of factors that influence the angel’s share could go on and on, from air pressure, to the size of the cask and the volume of whisky in it. Every little change to the cask can change the angels’ share with some significant effects on the quality of your whiskey. Amazingly even moving a whisky cask can increase the Angels’ share by 1-2%.

The Film Angels’ Share

The Angels’ Share is also a the name of a comedy drama directed by Ken Loach in 2012. The film which derives its name from this process, is happily centred around a whisky distillery and fictional cask of Islay’s Malt Mill. Young Robbie is given community service after a crime. During an excursion with his supervisor, they visit a distillery, where Robbie’s special tasting talent comes to the fore. When he learns that one of the most expensive whiskey barrels is about to be auctioned off comedy ensues.

Fungas and Angels

It’s not only the angels that enjoy the evaporating whisky however. The black stuff in the picture above is Baudoinia Compniacensis, better known as the whisky fungus, a fungus that consumes airbourne alcohol and are heavily found in the vicinity of distilleries. Baudoinia Compniacensis is also millions of years older than humans.

References

Ageing gracefully Do Dunnage Warehouses Create Better Whisky?