Question about aging

[Jackie]

We are of the opinion that the alcohol in bourbon becomes more intense with aging - or that the proof becomes higher. A friend disagreed and said he thinks that the alcohol content becomes less with aging. Are either of us correct, or does the length of aging not affect the amount of alcohol at all? Thanks.

[kickert]

We are of the opinion that the alcohol in bourbon becomes more intense with aging - or that the proof becomes higher. A friend disagreed and said he thinks that the alcohol content becomes less with aging. Are either of us correct, or does the length of aging not affect the amount of alcohol at all? Thanks.

You both are right. In general, a bourbon aged in alcohol increases in proof as it ages. However, that is not always the case. I hear that scotches actually decrease in proof.

At a recent distillery tour I went on, I was told the higher barrels in a warehouse significantly increase in proof because of the higher tempature and lower humidity. Conversely, barrels stored on the bottle floor tend to lower in proof in the cooler, higher humidity areas.

[squire]

Hey Jackie, welcome aboard. Whiskey aged in warehouses under the warm Southern Sun will increase in alcohol, though not at the same rate depending on placement in the warehouse. Whiskys aged in the colder climate of Scotland will decrease, but again not at the same rate for each barrel. Doesn't really affect the final product though, that is the province of the Masters who choose the barrels to be mingled into a certain profile.

[Squash]

The proof increases in warm climates because the water molecules are smaller than the ethyl alcohol molecules, and therefore they have an easier time getting through the oak and out to the air. So more water loss than alcohol loss yields higher proof.

In the cooler climates, the water that gets through the oak does not want to jump into the air (evaporate) as much because of the lower temperature and the high partial pressure of water in the air - the air is already holding as much water as it can. This is why we don't dry clothes with cold air. The alcohol is much more volatile, and can still jump into the air at lower temperatures, and the partial pressure of alcohol in air is near zero, so there's plenty of room for more alcohol in the air. The alcohol leaves more than the water - decrease in proof.

[smokinjoe]

The proof increases in warm climates because the water molecules are smaller than the ethyl alcohol molecules, and therefore they have an easier time getting through the oak and out to the air. So more water loss than alcohol loss yields higher proof.

In the cooler climates, the water that gets through the oak does not want to jump into the air (evaporate) as much because of the lower temperature and the high partial pressure of water in the air - the air is already holding as much water as it can. This is why we don't dry clothes with cold air. The alcohol is much more volatile, and can still jump into the air at lower temperatures, and the partial pressure of alcohol in air is near zero, so there's plenty of room for more alcohol in the air. The alcohol leaves more than the water - decrease in proof.

Nice work, Squash. Now, if someone will confirm this, (it sounds good to me)I can put this page in my Bourbon Bible.

[Josh]

The proof increases in warm climates because the water molecules are smaller than the ethyl alcohol molecules, and therefore they have an easier time getting through the oak and out to the air. So more water loss than alcohol loss yields higher proof.

In the cooler climates, the water that gets through the oak does not want to jump into the air (evaporate) as much because of the lower temperature and the high partial pressure of water in the air - the air is already holding as much water as it can. This is why we don't dry clothes with cold air. The alcohol is much more volatile, and can still jump into the air at lower temperatures, and the partial pressure of alcohol in air is near zero, so there's plenty of room for more alcohol in the air. The alcohol leaves more than the water - decrease in proof.

I agree w/joe. That's the first reasonable explanation of the phenomenon I've heard. Well done squash!

[barturtle]

Here'a nice couple posts by Tim Dellinger:

The first one shoots down the whole water molecule theory

And the second is a more complete explanation.

EDIT: Actually the whole thread that the second post is on is worth a read.

EDIT 2 here's a third one

[Josh]

Here'a nice couple posts by Tim Dellinger:

The first one shoots down the whole water molecule theory

And the second is a more complete explanation.

EDIT: Actually the whole thread that the second post is on is worth a read.

EDIT 2 here's a third one

I'm sure ppl much smarter than I are saying "aha! now I get it!" but I could really use an executive summary of these posts. Having trouble following the whole thing.

[Squash]

If temperature and humidity were the only factors involved here, all spirits would loose proof as they aged in wood barrels because alcohol evaporates faster than water in all aging environments (Scottland, Kentucky, etc...).

The idea that water evaporates faster that alcohol in Kentucky, as Tdelling stated, is not true. If you don't believe this, take a glass of Stagg and a glass of water and put then in your attick on a warm, summer's day (similar to the environment where bourbon gains proof). I think we all know the result, so don't waste your Stagg.

Barreled Stagg gains proof. Open Stagg looses proof, quickly. Bottled Stagg has constant proof. Since we can all agree on the above three statements, it must have something to do with the barrel.

The oak barrel must be acting as a partially semipermeable membrane.

[barturtle]

If you have a membrane, which the barrel certainly is (and I refer to it as such in the second post-I'm the guy with the questions), and you put two compounds that can both get through it on one side of the membrane and only one of the two compounds on the other side, then only the compound that doesn't exist on the second side will flow across the membrane until both sides have equal amounts of both compounds.

So taking this to whiskey: if you have a location that is extremely damp (like Scotland) the same thing applies, the water in the barrel isn't trying to get across as there is already plenty of water on the other side, but the alcohol will continue to flow through the pores as there is little alcohol on the outside of a barrel (at least in comparison to the amount of water vapor)

Put it in a dry environment and the water catches up and surpasses the evaporation of the alcohol.

Permeability of a membrane is, I believe, and Tim states, unrelated to the size of the molecule. As a matter of fact he quotes a study here and gives these figures:

Yoahizawa et. at (J Agric Chem Soc Jpn 55: 1063-8, 1981) studied "Subastances
Evaporated Through Barrel of Whisky", and found the following losses over
a given time:

acetaldehyde 32%
ethanol 12.7 %
acetic acid 1.0%

These molecules are very close in size, but very different in barrel
permeability!