Distillers and whisky enthusiasts often use the term “angels’ share” to refer to the small quantity of whisky that is lost to evaporation. The name comes from the fanciful idea that heavenly beings were partial to a drop of whisky and would secretly take their portion when the distiller wasn’t looking. There is also a more scientific explanation for this phenomenon; as whisky ages in the cask some of the liquid escapes through the porous oak as gas, thus reducing the overall quantity in the cask.
The amount of liquid lost each year can vary depending on the climate and humidity levels. For example, in Scotland annual loss might be 1.5-2.5% per year, whereas in hotter climates like India it could be as high as 12% per year.
The climate can also affect how much alcohol is lost. In Scotland the percentage of alcohol loss slowly decreases due to the humidity level, mild climate and consistent year-round temperatures. In more humid climates like Kentucky more water tends to leave the cask which has the effect of making the whisky stronger over time.
One big advantage of the angels’ share is that the evaporation process helps to make the remaining whisky smoother and give it a clean and pure flavour profile. However, there are also downsides to this annual loss since for older whiskies the overall volume can fall significantly. For example, a 12-year old Scotch might have lost as much as 24% of its initial volume.
The angels’ share can also be a concern when it comes to the alcohol percentage. To be labelled as whisky the cask must have a minimum ABV of 40%. If the alcohol percentage falls below this level the liquid can no longer be legally called whisky. This can be rectified by topping up the cask with another whisky, but going down this route normally reduces the value of the cask.
The amount of whisky lost each year due to evaporation has prompted distillers and whisky brands to research methods to manage and reduce this loss. Some have experimented with alternative ageing processes including oak inserts and infusion systems, while Johnnie Walker has even tried wrapping barrels in cling film.
For producers of premium whiskies, though, the angels’ share is considered part of the process of making an exceptional dram as it helps the whisky mellow and mature. Levels are monitored regularly through a process known as “regauging” when the alcohol percentage is calculated to make sure it meets minimum legal requirements. This is typically done every 1-3 years depending on the age of the cask to make sure the angels don’t take more than their fair share!