Thu Nov 12, 2015 10:17 am
Because it's fortified... it has defenses.
Seriously though, I would guess it's the introduction of the spirits. Most spirits oxidize very, very slowly compared to beer/wine. It has a lot to do with the alcohol content... or rather the ratio of ethanol compared to volatile compounds. It's similar to why herbs in herbal liqueurs don't break down.
Basically, it has a lot to do with the ABV & what other volatiles are in solution. Beer has a low ABV with lots of easily oxidized compounds. Fortified wine has a much higher ABV & a lower concentration of volatiles. Whiskey has a high ABV & nearly no volatiles. Beer can go bad in hours, whiskey can last for months if not years with nearly no effect. Fortified wine... I don't really know since I don't really drink it or have it around all that often.
It will go stale, but if you keep it sealed tightly & in a cool place, you'll inevitably get some increased longevity out of it.
Lee
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BN Army // 13th Mountain Division