For The Love Of Port

Hey all,
I was discussing some decanting methods with some friends and we bumped into a question noone could answer.

When decanting a port in an open decanter (no cap), is there a substential amount of alcohol that evaporates ?

I've seem some people post some TN's after 72 hours of decanting and I'm wondering wether or not the alcohol % could drop way below 20%. Is that why some port loose that hot, alcohol taste after spending a while in a decanter?

And a corollary question : Is there any variation of the alcohol % during port maturation, wether in cask or in bottle-aged vintages ?

Here is my chemical engineering answer.

Ethanol is not overly volatile with a boiling point of around 78 celcius, so you are only seeing the change due to diffusion into the air from the liquid state (ie physical chemistry in general tells us that molecules want to be where they currently aren't so some C2H5OH molecules want to get into the air, and some O2,N2, and CO2 want to get in the wine).

The amount of time allowed to sit will certainly increase the amount of alcohol diffused into the air, but this diffusion rate would be expected to relatively small, the ethanol would probably equally or more diffusive than the water content of the wine, but not considerable....so if say 1% of the volume of the wine were to be evaporated through diffusion I would estimate that maybe you would see limited change, maybe a small decrease of .1 to .5% at most (indicating alchohol evaporation via diffusion being slightly quicker than water diffusion)....but after 72 hrs I would not expect that much change. But us humans are very sensitive beasts it does not take much ethanol vapour for us to smell it probably just a few hundred parts per million is likely detectable by the human nose...I could probably look up diffusion constants and work all this out for you, but do not have the time. Cask aged over long period of time does see a reduction in alcohol content. Bottle aged would not.

Chemistry around wine oxygenation and subsequent reactions probably have more to do with the changes in taste than alcohol content. Also I would like to see if any one in science has ever considered the affect that CO2 has as CO2 can also be a reactive component...but have never read anything on that...