What is the explanation of barreled spirits (at 62.5% ethanol) that undergo changes in concentration of ethanol. Whereas the volume after aging is typically reduced, the proof varies between low 100 to low 140 (50-70%). Does the presence or formation of azeoptopes play a role? I contend that the barrel interface is important and not so much the differences in evaporation, that the industry states: Angel's Share and Devil's Cut.

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    $\begingroup$ The barrel is sealed so simple evaporation can't explain the loss of volume. The liquid obviously permeates the wood and evaporates from the outside of the barrel. Water permeates wood better than ethanol, so the liquid inside the barrel becomes enriched in ethanol. // If the mixture were left in an open glass to evaporate, then the liquid would lose ethanol faster, and hence the proof would be reduced. $\endgroup$ – MaxW Nov 4 '19 at 20:42
  • $\begingroup$ Thanks for the input. I think this is a big factor, but doesn't explain loss of proof. Most barrel proof bourbons are 55-59%; just slightly less than entry proof of 62.5. $\endgroup$ – Bill Welch Nov 5 '19 at 20:53
  • $\begingroup$ Both the barrel/cask and the storage environment matter. For single malts matured in casks in Scotland, proof typically declines year after year, so really old whisky (30 YO and up) is rare. For straight bourbon matured in barrels in Kentucky, proof can go up, as in the famous G.T. Stagg whiskey. Too many variables ... $\endgroup$ – Ed V Mar 7 at 21:14

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