To clarify my explanation regarding the vapor pressures: Under the same conditions ethanol (higher vapor pressure) will always evaporate faster than water (lower vapor pressure).Originally Posted by tallison
Of course, humidity has an effect on the evaporation of water. Less water will transit from the liquid to the gaseous phase as the humidity rises (the air gets saturated). This also means that there can be no "crossing of the lines", unless the ethanol saturation of the air is unrealistically high.
There is no doubt that humidity is responsible for this phenomenon. I find it rather astounding that none of the articles and references deliver a profound scientific explanation. Maybe the relevant literature is just not publicly available.We have some decent evidence that the loss rates of ethanol and water can be inverted in the barrel, now it's just a hunt for a reasonable physical model as to how that happens. I've read several wine references now that only mention humidity, but that isn't definitive, I'll grant you.
Maybe we are talking past each other. While the humidity is low the wood cells will try to absorb (not adsorb) water by attracting it from the whiskey up to the most outer layer of cells. The water will, of course, not pass through the outside layer of cells ("last stage" is reached) - thus "permeability" might be a mistakable phrasing here. If this absorption process is faster than the evaporation of ethanol (which seems likely), alcohol content in the whiskey rises.The only data I could find on polarity and wood permeability, though, actually argues against your hypothesis determining that the more polar substance is slower due to adsorbtion by cell walls (Transport of Polar and Nonpolar Liquids in Softwood Imaged by Neutron Radiography | SpringerLink). Do you have evidence you can point to to support the case you make?
My most recent other suspect for this -- if humidity isn't enough to explain it (which I admit is not all that satisfying in pure conjecture form) -- is that perhaps capillary action could be the differentiator. With respect to the physics of the capillary action favoring water transport over ethanol transport through the barrel walls, see: Washburn's Equation. The relevant physical parameters are surface tension (gamma) in the nominator and viscosity (eta) in the denominator, the values of which both very much favor water over ethanol in terms of capillary flow or "wicking" rate (for surface tension: water >> EtOH; for viscosity: water < EtOH - who knew?).
Back to drinking whiskey: I'd like to buy my first bourbon (under $30), but don't know which one to choose:
- Buffalo Trace (saw this one being recommended in a different thread)
- Four Roses Single Barrel
- Maker's Mark
- Knob Creek
- or a completely different one
Which one should I go with?
