When you have an extra keg of beer left over from a party, it can seem attractive to keep it around for the next one. However, even untapped, the beer in your keg can still go bad. Your keg of beer should have a stamp signifying the expiration date.
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The shelf life of beer will vary depending on the brand and whether or not is has been pasteurized. An unpasteurized beer has a life expectancy of 45 to 60 days -- however, once you account for shipping time, this can drop to about a month from the date of purchase. If beer has been pasteurized, it can last three to four months. Most American beer is not pasteurized.
While the flavor will be a sure indicator of whether or not your beer has gone bad, you can look for some other signs before taste-testing. If the beer has gone cloudy or is overly foamy, it may have already gone bad.
For unpasteurized beer it is crucial to keep the keg cool, below 38 degrees; anything above this will cause your beer to foam excessively when dispensed. Because unpasteurized beer contains dormant bacteria, any temperature above 50 degrees is likely to cause the bacteria to grow and spoil the beer.