The life of a beer doesn't end once it's finished fermenting and is sealed into bottles. Like wine, beer continues to age and change over time. They're at their peak of freshness and flavor the first few months after bottling, but then things slowly decline. Flavors start to fade, oxidization occurs, and the protein structure that gives the beer body starts to break down.
But saying that beer actually expires is a bit misleading. It doesn't actually spoil or become unsafe to drink. It will just start to taste flat, flavorless, and unappealing.
How long a beer stays fresh and how fast it becomes unpalatable depends on the specific beer. Most craft beers stored at a stable temperature and out of the light will be good for about a year before starting to turn. Look for a bottling date or an expiration date on the bottle's label, as well. Many breweries have started printing this information for our benefit.
Some beers, particularly bottle-conditioned ales with an alcohol content of 9% or higher, can be aged for longer than a year. They experience the same breakdown in flavor and structure, but this has been calculated to actually improve the flavor of the beer. Hop bitterness fades away while malts take on a deeper, more honey-like flavors. Other flavors emerge that may not have been perceptible before. It can be a fun experiment to put away a few bottles of a strong beer like a barley-wine or a barrel-aged stout, and then open a bottle every few months!
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