Whiskey and Wine, All Gone Bad
In this edition of booze talk I'm going to get into something that we all fear - booze gone bad. Yes, it's the one thing that I often hear when people complain about a spirit, wine or beer that is not tasting the way *they* wish it to taste and just assume that it has gone bad.
In the case of Whiskey, the moment you open that bottle is the moment the flavors start to change. It's oxidization. It's not a bad thing, and for the most part your Whiskey will stay good for a long time. You should just mind the bottle you're keeping it in. If it's only a little bit left in that bottle, perhaps you should just kill it - or just change it into a different, more smaller container.
As for wine. The term you often hear is it being corked. That's when opened, is bad. Oh so bad. But what exactly causes it? Well, corked wine is wine that has been spoiled by TCA. It has nothing to do with the flavor of the cork - uncontaminated cork has no TCA taint. Typically the taint is from the cork in the bottle, though to some extent TCA can be transferred through the cork from barrels, in transport, or at some other time.
TCA is a metabolite of TCP, a common fungicide. There are a wide variety of bacteria - including Botrytis Cinerea, typically quite desired in certain sweet wines, and then the fungi which can metabolize chlorinated phenolic compounds into anisole derivatives like TCA. But wine which is overly oxidized before its time would be described as prematurely oxidized, or premoxed. This is a big problem with White Burgundy right now, but people aren't exactly sure of the cause.
Prematurely oxidized wines have an almost sherry like character. Often premoxed wine will be so due to air leakage as Wormil suggested, but it's often more complex than that, and is a long term result of some complex of factors between racking, aging, stirring, time on lees, barrel treatment,bottling style, sulfur levels, &etc.
Skunked beer is beer that has been hit by the power of the sun. Leave a beer in the sun for a while and the chemicals inside will break down and make the beer smell like... well, a skunk. You can combat this by buying beer that comes in a dark bottle. Clear bottles are far more easier to get tainted by the sun. So be warned.
That's a good primer for now.