How to Tell if Your Beer Is Spoiled

It’s happened to all of us: you walk into a bar and order a beer expecting something nice. But when you bring it to your lips, maybe something doesn't smell right. You forge on and take a sip... and now you're sure there's something wrong. Even if it doesn't taste downright terrible, it's pretty clear that somewhere along the line, something's bringing that beer down.

Just like wine or any other food product made with natural ingredients, beer is susceptible to a long list of potential problems that can affect the way you enjoy it. They could stem from as early as the first stages of the brewing process to the basement of the very bar you're drinking in. We spend plenty of time looking for what it is in a beer that makes it great... but what about what makes it bad? Because you only deserve the best, we've come up with a list of some of the most common off-flavors in beer you'll find in the wild and how to spot them.

Author's Note: If you’re looking to get a feel for these flavors, you can actually buy an off-flavor spike kit online to try at home through the Cicerone® Certification Program. They’re not cheap by any means, but if you’re serious about learning how to detect any of these, it’s the way to go. If you don’t feel like spending the money on a kit, Draft magazine has a completely free glossary of off-flavors that can be super helpful for getting started.

That's acetaldehyde
It may sound bizarre, but some level of acetaldehyde is actually present in all beers (and, as a truly crushing footnote, is the compound that gives you that hangover, and why you can sometimes smell green apple on a drinker’s breath). Scientifically speaking, this is because yeast cells are just like Kanye and they just need time to perfect their work: higher levels of acetaldehyde are found in beer when brewers remove it from fermentation too early and those little cells haven’t had enough time to finish their job. Unfortunately, unlike Life of Pablo, brewers can’t re-release batches of green beer quite as easily. If you get this aroma or flavor in your glass at a brewpub or brewery, switch over to another option.

That's diacetyl
Diacetyl has a tricky relationship with the off-flavor world. Some noticeable amount is actually OK in styles like light lagers (especially Czech Pilsners) and even stouts. This is because diacetyl naturally makes its way into beer as a precursor formed by yeast during fermentation. But in some cases, bacterial infection can send the levels way out of whack, and it’s especially noticeable when styles are being brewed where no diacetyl flavors are acceptable. If your pint smells like a Werther's candy or a bag of Pop Secret, chances are you're dealing with an infected batch.

That's DMS (dimethyl sulfide)
Like diacetyl, DMS is at an odd position in the off-flavor scale. The fancy way of describing this off-flavor is “a bacterial infection during fermentation that comes from malt-derived precursors during wort production.” Translation: a brewer didn’t use their ingredients the right way when making a specific type of beer. Where this gets tricky is when you start considering different styles, since some low-level DMS presence is perfectly at home in pale ales and lighter lagers. This has to do with the malts in lighter beers, as darker malts are so roasted that the precursors to DMS are cooked away during kilning. You’re more likely to catch this in a brewpub or brewery’s taproom in an off-batch, since quality control can catch this defect before canning or bottling (although it certainly still happens).
 

That's line infection
It’s happened to everyone at some point. You walk into a bar and order something on draft, and what you get immediately hits your nostrils like a revolting slap in the face. This flavor is the stuff nightmares are made of, and is the result of bacteria living in the draft lines that just housed the beer in your glass. It’s not fair to judge a book by its cover, since line infection can happen as easily in a dive bar as it could in a fancy restaurant. Quite simply, this is the result when businesses don’t perform proper maintenance and regularly clean their lines with solution. The rule of thumb here should be to trust your gut and order a small taste of a beer you know when drinking draft at a place you’re not familiar with: the smell alone should make it immediately apparent if you should order a bottled option instead.

It's lightstruck
The word that gets thrown around most for bad beer is probably “skunked,” which makes sense since there’s an incredibly high chance you’ve had one recently. How do you know you’re drinking a lightstruck beer? Pick up any beer sold in clear, green, or blue glass bottles, open it, and take a sip. It’s that easy!

This is because beer has three mortal enemies: oxygen, heat, and light. In the case of going lightstruck, the deterioration process begins almost immediately, and can be affected by any form of light, from the sun to fluorescent bulbs in a cooler case. Brown glass blocks over 99% of the wavelengths of light that will deteriorate a beer (and cans will block 100%), whereas others only get as good as around 30%. But if you stop to think about how popular certain well-known beers sold in clear or green glass bottles are, you’ll quickly realize that a lot of people actually like this defect.

It's old (and slightly oxidized)
Old beer! It gets sold and consumed in more places around the world every day than anyone would care to admit. That papery flavor in your nine-month-old bottle of pale ale is thanks to a compound called trans-2-nonenal that becomes apparent over time as beer is stored. Contact with oxygen (which you’ll remember as one of beer’s mortal enemies) slowly brings it forward. This is why anyone who cellars beer stores it upright, so as to minimize surface contact with any airspace in the bottle. The easy way around this? Check the packaging dates on bottles (avoid anything older than four months unless you’re going for big styles like imperial stouts or barleywines) and use a trusted vendor who removes coded beer from its fridges.

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Zach Mack is Thrillist's contributing beer writer, the owner of Alphabet City Beer Co. in NYC, a newly minted Certified Cicerone, and nothing else. Follow him: @zmack.