[tomsawyer]

I've got a bet going with a friend on this subject.  He contends that barrel aging increases ABV, but I can't think of any good reason why it would.  I think the observations he's referred to are probably due to whiskey in the barrel staves causing increases by leaching back into beer (similar to swish), I've heard of that incresing ABV by a couple of points.  I'm talking here about using a new barrel or wine barrel where the wine is not much different in ABV than the beer.  Is there a selective mechanism that allows water to escape while retaining ethanol?  I know it doesn't happen that way when you distill.

[tumarkin]

actually, he may be right.... in that when aged in a low humidity environment the loss (the angel's share) is mostly water which increases abv. however, in a high humidity environment the lost is mostly alcohol.

[tomsawyer]

Do you have a reference for that?  I find it hard to accept.  Even in low humidity there is still as great a gradient between outside/inside for ethanol as there is for water.  Ethanol is the more volatile of the two, and its azeotrope of 95% ABV is lower yet.  When you distill you move towards the azeoptrope, but you start at something near the original ABV and the ethanol only goes up, not down.

[tumarkin]

maybe not the most authoritative reference, but per wikipedia -

"Angels' share
Sherry aging produces fungus on the walls"Angels' share" is a term for the portion (share) of a wine or distilled spirit's volume that is lost to evaporation during aging in oak barrels. The barrels are typically French or American oak. In low humidity conditions, the loss to evaporation may be primarily water. However, in higher humidities, more alcohol than water will evaporate, therefore reducing the alcoholic strength of the product. In humid climates, this loss of ethanol is associated with the growth of a darkly colored fungus, Baudoinia compniacensis, on the exterior surfaces of buildings, trees and other vegetation, and anything else that happens to be nearby."

I've read in other places, but can't recall where, that this loss of alcohol is typical with single malt in Scotland (manyof the distilleries have the black fungus around their stillage warehouses) but the reverse is the case with bourbon storage.

[denny]

I'm not saying it's impossible, but I've never seen it happen in the maybe 4-5 barrel aged beers I've done.

[beersk]

I wouldn't think there'd be a significant amount of alcohol that leaches from the barrel to impact the abv.  What an interesting debate.

[tschmidlin]

I really don't know, but if it's true it might be because H2O is a significantly smaller molecule than CH3CH2OH.  Perhaps it can more easily penetrate the wood to the outer surface where it can evaporate, so you're losing both water and ethanol, but water faster so you get a net increase in ethanol per volume.  In a humid environment the water is slower to evaporate so the net is a loss in alcohol per volume.  Just a guess, so take it for what it's worth

[tomsawyer]

Yes the moecular size was mentioned, and ethanol is 3.5 times the size of a water molecule.  Hard to imagine that oak pores are just in that range where water can escape while ethanol stays behind, but I suppose the range of pore sizes would include those that favor one over the other.  Anybody got a definitive reference for any of this?  I haven't seen anything yet that I'd call scientific.

I will say one thing, these small barrels (6gal & 11gal) do give a bigger angels share than the big barrels, due to so much surface area per unit volume.

[tschmidlin]

I'm not suggesting the ethanol is exluded, just slower to move to the surface.  But it's a guess, I don't know of any definitive sources for this.

[morticaixavier]

could it have anything to do with the fact that in a high humidity environment the air is already largly saturated with water so water evap rates require higher temps while ethanol still evaporates at lower temps? not a scientist so just guessing.

[tomsawyer]

It seems to be the case for spirits, and to some extent wine.  So this molecular size must be at the root of things.

I did see French oak is less dense/more porous than American oak, so possibly there are qualitative differences in this effect.

Quite an interesting topic.  Whenever I see something that flies in the face of chemistry and common sense, I think its interesting.

Morticai, thats true but I think ethanol always has the edge with respect to the gradient inside vs outside so I would discount this as a reason why water would move faster.  It does explain how you can manipulate things to get the ethanol moving faster than water.

[tschmidlin]

I'm thinking the rate of ethanol evaporation probably doesn't change with humidity, just the water does.  That doesn't explain why the water would evaporate faster than the ethanol though.

What happens to vodka left in an open container?  I haven't tried that.  How long is it supposed to take to evaporate?  I can leave some open and then measure it, it will give us some information.

[tumarkin]

with single malts, the evaporation rate roughly 2% per year. whisky remains in the barrel for long periods, beer (which was the original topic) generally for short periods. so I'm guessing there is little impact on the beer abv in the weeks/months that are the usual time frame.

big +1 to the differences between small casks & large. wood has a lot more impact in a lot less time with the smaller casks.

[denny]

I will say one thing, these small barrels (6gal & 11gal) do give a bigger angels share than the big barrels, due to so much surface area per unit volume.

That might be why I haven't seen it, given that my experiences are all with 55-60 gal. barrels.

[kramerog]

What happens to vodka left in an open container?  I haven't tried that.  How long is it supposed to take to evaporate?  I can leave some open and then measure it, it will give us some information.

Years ago, I had some home made slivovic in a bottle.  When I first got it it was really rough.  I poured it into a bottle cap and lit it.  The stuff stayed lit until there was no more.  Years later I lit it in a bottle cap and it would stay lit for only a few seconds.  The slivovic was much smoother.  I conclude that alcohol evaporated faster and the ABV went down (my daughter wasn't born yet so I'm pretty sure nobody was secretly doing shots and backfilling the bottle with water).  So this idea that water moves more easily through the wood has merit.

In Europe, where wine is aged underground under humid conditions, alcohol content goes down with aging.  In the US, where wine is aged in dry, refrigerated conditions, alcohol goes up with aging.