Lesson 11: Why does the alcohol content increase when whiskey ages?

When whiskey is distilled, the proof can be as high as 160 (80% ABV). The law forbids, however, a higher proof than 125 (62.5% ABV) from entering the barrels – so the manufacturers have to add some water, to lower the proof. Most manufacturers go for the maximum allowed 125 proof – but some go as low as 100 proof (50% ABV). It may even be lower, but 100 proof is the lowest I have come across. The alcohol level when the whiskey is added to the barrels for ageing is referred to as “barrel entry proof” and I will make a post about this topic in the future. This post will also explain why they use different levels.

So … as the whiskey is sitting there in the barrels, the proof/ABV will typically slightly increase, as the years go by. As I have written about in previous posts, this is a little counterintuitive, as you would expect it to go down, given that alcohol is more volatile than water. I will explain in a moment what is going on.

I also wrote “typically” go up – because it is not always the case. For most types of American whiskey, it does go up – but in other parts of the world, such as Scotland, things are different. You are probably confused at this point, so I will explain both scenarios:

The alcohol content increases

By law, most American whiskey needs to be stored in containers (i.e. barrels) made from oak. Almost everybody uses American white oak, as it has some amazing properties, when it comes to the ability to hold/contain liquid. The molecule structure in white oak is so dense, in fact, that the alcohol (ethanol) molecules have significant issues penetrating the wood. Other oak types like red oak, for instance, do not have the same properties. 

Water molecules (H2O) are much smaller than ethanol (C2H6O) and will therefore have an easier time penetrating the wood. And as you can probably guess: When water evaporates faster than ethanol, the proof/ABV increases.

But why would the water try to penetrate the wood? Other than normal soaking as you would expect, the water would also try to penetrate the wood if the pressure in the barrels increases. This happens when the whiskey is heated up during summer; the temperature increases, which makes the liquid expand, which increases the pressure inside the barrels (as there is not enough room). In Kentucky and Tennessee, for instance, the temperature can get sometimes break 100F/38C, and you can just imagine the pressure inside those barrels, if they were filled to the top at e.g. 70F/21C.

In other parts of the US this situation can be even worse, when you for instance age whiskey in Texas and Arizona where temperatures can get insanely high. That is why you will see whiskey from those areas with a high proof and a deep brown color, even though they have only been aged for e.g. two years.

The location in the warehouse is also a contributing factor. As you may know, heat rises towards to top of a building, so the top-most floors in the warehouse is where the will be the highest evaporation and therefore the highest proof.

Here is an example of a Bourbon where the alcohol content has increased during the ageing process:

A Bourbon with a staggering (no pun intended) high alcohol content.


The alcohol content decreases

When the barrel ages in a climate that is milder than the aforementioned US states, evaporation is slower. This is what you see in e.g. Scotland and Ireland. A contributing factor is also the fact that many whiskeys from those countries are aged in used barrels (typically Bourbon, Sherry or Port) – and those barrels will have less integrity compared to new barrels and the ethanol will have an easier time escaping.

It is also important how the warehouses are built; if they are made from brick, the temperature is lower than the ones made from wood. A famous example of that is MGP in Indiana (one of the largest distilleries in the world) whose warehouse generally make the proof go down a little with time, for this reason.

And as a final reason, the proof can also go down when there is a lot of moisture coming from the floor – especially if the floor is made from dirt. The moisture clings to the barrels and can even find its way into the wood if the conditions are right.

If you know of more reasons, please comment below.

Here is an example of a barrel proof whiskey (i.e. not diluted with water) that has a lower proof than when it started out:

A Scotch whisky that has been aged for 25 years. The proof in the barrels have decreased to 98.8 (49.4% ABV)

And that concludes this lesson.

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Corrections, suggestions for other posts, or any other feedback? Then please leave a comment below or write me on henrik@thebourbonnerd.com

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