How Single Malt Whisky is Made – Part 4: Maturation and Bottling

In the fourth and final step of the whisky production process, the distilled spirit will be aged for some length of time, and then bottled. This is done in order to further develop the flavoring of the whisky and obviously to package it so it can be sold.

So far in the previous three parts of this series, we’ve grinded and mashed together the grain ingredients that make the whisky and mixed with hot water. Then, we fermented this mixture and finally distilled it. Now that the distillation process is complete, we’re now ready to age the whisky.

After distillation and pre-maturation dilution, the spirit is put in oak casks to mature. The maturation period can take a variable length of time. All Scotch whisky must be aged for at least three years (although it is usually aged for a lot longer). Anywhere between 8–30 years is common, and the longer the spirit has aged, the smoother the taste, but also the more costly the end product will be for the consumer to buy.

Other distilleries such as Jack Daniels have no predefined length of time for aging their whiskey, and instead they determine when the maturation process should end according to the taste of the spirit.

Over time, and as the spirit interracts with the oak of the cask it is encased in, the spirit will change taste, smell, color and body. Additionally, a small amount of the alcohol in the cask will evaporate over time, which is often referred to as the “angel’s share”.


During the maturation period, whisky is matured in oak casks. Depending on the type of whisky, the spirit may be matured in a new or reused casks.

New vs. used casks

American whiskey is required by law to be matured in new oak casks. Outside of the US, and especially with Scotch and Irish whiskey, the spirit is matured in reused casks—that is, a cask that was previously used to mature a different spirit is then used as a “second hand” cask to mature the whisky. This is common practice outside of the US, as other distilleries find these used casks to impart a less oakey flavoring so the wood notes aren’t overpowering to the taste of the whisky. The most common reused casks are those that previously aged Sherry or Bourbon, however Rum or Port Wine casks are also used.

A reused Sherry or Bourbon cask is referred to as “Ex-Sherry” or “Ex-Bourbon” casks respectfully.

Effects of Cask Type on Flavor

New Bourbon casks are charred from the inside prior to first use. This is done to transfer vanilla and oakey notes to the Bourbon, and also to filter out some of the impurities and oils in the Bourbon. Thus, Ex-Bourbon casks will often give a non-American whisky a sweeter vanilla flavoring. Sherry casks, on the other hand, will make the whisky a bit heavier and also give it a subtle sweet Sherry taste to it.

The size of the cask can also make a difference in flavor. The larger the cask, the greater the surface area of oak for the whisky to interact with and mature. The smaller the cask, the less surface area, but the whisky can also mature at a faster rate.


The age refers to how long a whisky has matured in its cask. Unlike other spirits such as wine, Whisky does not mature in a bottle—it only matures when interracting with oak. Scotch is required to age for at least three years, but you’ll often find Scotches aged anywhere between 8–30 years. There are even some rare reserves that are aged even longer. For example, Glenfiddich has reserves that have been matured for 40 and 50 years. Generally speaking, the greater the age, the more expensive the bottle of whisky.

Additionally, some distilleries such as Jack Daniels have no predefined length of time they age their whisky. Instead, they determine when the maturation process should end according to taste of the spirit—the Master Distiller will occasionally taste the whisky from the cask to determine if the spirit is ready for bottling.

Before bottling, it’s common for the whisky to be “vatted”, or combined with other casks (in the same distillery) in order to have a more consistent taste. Thus, the age statement on a whisky bottle reflects the youngest age of the whiskies that were vatted together. The exception to this is “Single-Cask” bottles, which haven’t been vatted.

It is also common practice for the whisky to be diluted with water before bottling, in order to bring the ABV down to about 40% – 50%. The exception to this is “Cask Strength” bottles, where the whisky has not been diluted.

So there you have it. This concludes our summary of how whisky is created. Once the whisky has been diluted prior to aging, it is then bottled and ready for sale.

Have any questions or notes about how whisky is made? Let us know in the comments below.

1 comment… add one
  • Pieter Dorhout says:

    Good day,

    I have a 250L sherry cask at Bruichladdich, filled in 2002. I tasted it last summer, and found it quite outspoken and/or unbalanced (I’m not good with the terminology). If I leave it for – say – another four years, will it ‘mellow’, or is this outspokenness a lasting feature of this single cask, which I’m unused to as I never drink single-cask whisky?

    Best regards, Pieter Dorhout

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