questionshow much has my 11 year old alcohol fermented?

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I don't think those airline bottles seal very well. You've probably lost alcohol, so the proof would be lower. Aren't liquors distilled, not fermented? That would also rule out any increase.

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They should be the same, but after all is said and done just enjoy it responsibly and strike the pose. Be safe!

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If the bottles are still just as full as when they were new, then they should be just as strong.

If they've lost volume, most likely the loss is alcohol so the proof would be lower.

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there would have to be live yeast in the bottle to ferment the remaining sugars into additional alcohol. Had they been opened, there's a chance some wild airborne yeast could have gotten in before they were re-sealed, and then the proof may have gone up a little. I doubt there has been any change to the rum in the bottles you have, though.

Had these been bottles of wine or beer, there may have been some additional fermentation going on.

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A few lessons on alcohol:
1 The highest alcohol content produced by a commercial yeast strain is 21%. (Most strains are much lower.)
2 The process of distillation kills off all the yeast and also concentrates the alcohol to a point that will kill any microorganisms.
3 Alcoholic beverages no longer mature once they've been bottled.
4 Exposure to light and heat can adversely affect the taste and color of alcohol.

@kamikazeken Only way that beer will continue to ferment in the bottle is if it is what is called bottle conditioned. This is the natural method of producing carbonation in beer by adding a small amount of fermentable sugars to the beer when you bottle it, and leaving live yeast in it as well. Most commercial beers kill off and filter out the yeast prior to bottling and use forced carbonation.

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my guess : since it's distilled and not fermented, it likely has not changed to a noticable degree...

if it was a wine, I'd comment that you can NOT get a wine above around 11-13% because the alcohol gets to a point where it kills the bacteria doing the fermenting. Or another way to say it is that the bacteria drown in their own excrement.

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@theant: Different strains of yeast can tolerate different concentrations of alcohol. Most yeasts for beer can only survive to 5 or 6%, while champaigne yeast can survive to 16 to 18%. Then there's a variety called "The Water of Life". It'll survive to around 21%.

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Let's please not forget that fermentation gives off gas. Said gas would have popped the seal or the bottles themselves if there were any fermentation going on. Trust me, there isn't. These aren't home-brewed beer, they're distilled liquors. @woadwarrior has it right.

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Also a note: distilled spirits do not improve in the bottle. They remain as they were when they were bottled. That 8 year old bottle of Maccallan you bought 10 years ago is not going to drink like an 18 year old.