Can Whiskey 'Go Bad'?

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First, whiskey in the bottle is very sturdy stuff. It will remain unchanged indefinitely. It has only a few enemies.

'Go bad' usually means 'spoiling,' as in unpleasant bacterial activity changing some component of the product into something else. Wine becomes vinegar. Milk becomes sour. Meat becomes rancid. Fruit becomes mush. That doesn't happen with high proof spirits like whiskey because nothing can live in that much alcohol.

So no, whiskey can't 'go bad' in that sense. What whiskey can do is absorb too much oxygen, which makes it taste like somebody added way too much vanilla. This happens most often when someone leaves a small amount in the bottle for a long period of time and can be aggravated if the cork or cap isn't well seated.

The best solution is to finish the bottle. Don't leave that last quaff for a special occasion. Just drink it.

If you must save it, transfer it to a bottle appropriately sized.

Under some rare conditions you can get unbalanced evaporation, where some or all of the alcohol goes away leaving a very unpleasant-tasting brown water. An inadequate seal is always the culprit here, aggravated by high temperature. This is why you don't want long exposure to direct sunlight. Alcohol is volatile. We think of that as meaning prone to catching fire, but it actually means prone to becoming a vapor and just going away.

Some people think the solution is to store bottles on their side to keep the cork moist, like you do with wine. This is a TERRIBLE idea with whiskey. High proof alcohol is hard on corks and dissolved cork is hard on the flavor of the beverage so do not store bottles on their side, or upside down, under any circumstances.

Some people will suggest that you wrap the bottle tops with paraffin tape. Some will recommend replacing whiskey when you pour it with marbles, or clean pebbles, something to keep the fill level high. This is a bit too fussy for most people and really isn't necessary. Just drink the whiskey in due course.