Absolutely. A keg can go funky or a beer can go “off,” and there are a couple of reasons why. One is simply the length of time that has passed since a keg has been tapped. Brewers are always talking about freshness mattering when it comes to beer, and they aren’t just saying it for fun. It truly does. The longer a beer sits, whether in a tapped keg or in an unopened bottle exposed to light, the funkier it can get.
Any beer that sits around for too long is axed. It starts to degrade in flavor, which is why you’ll find many serious craft beer bars list the date a beer was tapped, or put on the draft line, next to the name of the beer on the menu or chalkboard. Similarly, many craft brewers are beginning to replace “drink by” dates with “born on” dates, so you know exactly when a beer was packaged, which is pretty awesome. With beer as with anything we consume, fresh is best. But if you aren’t at a bar that takes its beer freshness seriously, it’s very likely you could find yourself drinking from an older keg, or, worse, from draft lines that aren’t clean.
The main reason a beer on draft at a bar can taste “off” is due to dirty draft lines. When you order a beer at the bar, pulling the tap sends the beer on what is sometimes quite a lengthy journey from the keg through draft lines, and ultimately out the tap into your glass. If the draft lines, which are plastic tubes, are not cleaned regularly with a specific chemical called caustic, they can easily become dirty with material buildup and impart gnarly off-flavors in the beer — common signs of infection are buttery, or unintentional sour flavors. That’s why so many brewers are obsessive about clean draft lines — breweries are able to ensure the lines at their tasting rooms are clean, but have little control over the various bars where their beer is sold. Responsible bars hire draft technicians to clean their lines every two weeks at minimum.
If you have a bum owner who doesn’t take cleaning the lines seriously, you’ll wind up with foul-tasting beer. So choose your watering holes carefully! Read more about the importance of clean draft lines here, and how to know if you’re in a good beer bar here.