There are many different things that skunk a beer, but the most important are:
Light can quickly ruin a good beer, especially if the beer is in a green or clear bottle. This
is the main reason why many brewers prefer cans or dark bottles.
Beer also goes “bad” if left in a hot place for an extended period of time. You won’t ruin
a beer by taking it out of your fridge and putting it back in, but leaving a case of beer in a
hot car for a couple days is never a good idea.
Time is also an enemy of beer. Hops are very volatile, and hop compounds can start to break down in as little as a couple weeks. Non-harmful bacteria, present in commercialbeers in very small levels, can also multiply over time creating off-flavors.
For these reasons, we believe it is best to drink your beer BREWERY FRESH. Will drinking a year-old IPA you found in the back of your car kill you? Certainly not – but it could taste a helluva lot better.
Beer Geek Note: If kept in the right conditions, some beers (mostly those with 10%+
ABV and “sour” beers) will actually improve with age. The only way to know for sure
how a beer will age is ask other people who have tried aging it, or experiment yourself.