Every beer at every bar, restaurant or backyard endures an arduous journey from brewery to customer. On this journey, bottled, canned or kegged beer meets a series of touchpoints that, if mishandled, can alter its flavor from what the brewery intended. These are called beer faults.
In most commercially brewed beers, chances are, the fault is not the brewer’s, says Michelle Tham, Certified Cicerone and head of education at Labatt Breweries of Canada. Beer faults and off-flavors almost always happen after beer has been packaged and left the brewery, where it faces a series of threats that Tham refers to as “beer’s enemies: light, oxygen, heat, time and bacteria.”
The good news is that most beer faults are preventable. Learn how to spot the most common beer faults below.
Five Common Beer Faults
Fault: Light struck
Symptoms: Skunky, catty, cannabis
When beer is exposed to ultraviolet (UV) light, the light reacts with alpha acids in hops, or the compounds that give beer its bitterness. This reaction forms a compound that creates a “skunky” aroma.
“We say that it’s skunky because it’s the same compound that makes a skunk smell like a skunk,” says Jen Blair, Advanced Cicerone, Cicerone Certification Program exam manager, and National BJCP Beer Judge.
Of course, skunky can be a relative and geo-specific term.
“If you’re from a part of the world where there are no skunks, some people associate that aroma [with] sewage…or cannabis upon combustion,” says Tham. “Basically, it smells like weed.”
Any beer can become light struck if the liquid is exposed to sunlight, but those packaged in glass bottles face a higher risk than beer in cans. Tham compares bottle glass color to sunscreen: Budweiser’s brown bottles offer the most protection, “like wearing SPF 50,” she says. Meanwhile, Stella Artois and Heineken, in green bottles, are closer to SPF 20. Miller High Life and Corona, in clear bottles, “is [like] going out there in baby oil. You’re gonna get burned.”
Prevention is simple. Protect beer from light by keeping it in the refrigerator, shade or cardboard cases until refrigeration is available. When drinking outdoors, choose a cooler or ice bucket that has a cover on it. “And there’s nothing wrong with a red Solo cup,” says Tham.
Fault: Diacetyl
Symptoms: Buttery, butterscotch, movie theater popcorn butter
Ever order a pint of Pilsner and find that it smells like a pint of popcorn?
“One of the first things to do, if you’ve got your detective cap on, is find out when the last time the draft lines were cleaned,” says Blair. “If the answer is, ‘We never clean our draft lines,’ then you can be pretty confident that the diacetyl is coming from dirty draft lines.”
On the other hand, if the bar you’re at has clean lines and a beer program you trust, “then you can be fairly confident that the diacetyl came from a faulty fermentation process,” says Blair.
Should you suspect an infection, “there’s nothing wrong with asking for a different beer and saying, ‘I don’t like this,’ or ‘I think something might be wrong with this,’ ” she adds.
Fault: Oxidation
Symptoms: Stale, papery, wet cardboard
Oxygen is beer’s enemy in nearly every stage of its life. Heat and time catalyze this process.
“Oxidation is a very large umbrella term,” says Blair. “There’s a ton of different ways that beer can taste aged.” For instance, with exposure to oxygen, hop aroma and bitterness can decrease, and malt might develop sweet honey- or toffee-like flavors in lieu of notes of fresh grain or bread. Over time, it can develop papery notes.
Like bread, “beer is subject to staling or getting old,” says Tham. “Not as quickly, but in about a month or two.”
That’s why fresh is best.
“Even if you like day-old pizza, nothing’s ever going to taste as good as pizza right out of the oven,” says Tham. “Beer is at its prime and at its peak when it’s super fresh.”
Fault: Acetaldehyde
Symptom: Fresh-cut green apple
Per the Oxford Companion to Beer, acetaldehyde “is an organic compound found in almost all plant materials,” including fruit, coffee and beer. If present in excess amounts in beer, it gives a “fresh-cut green apple” aroma that, Blair says, is almost always due to a fermentation issue. Brewers can avoid this issue by ensuring yeast is healthy and fully attenuates during a vigorous fermentation. Also, allowing beer to “sit” on yeast for a few days after primary fermentation can help reabsorb acetaldehyde into the yeast.
Acetaldehyde can also result from oxidation. If acetaldehyde is detectable alongside other oxidative flavors, Blair says, this can “indicate you’re tasting an aged beer.”
Fault: Acetic acid
Symptoms: Vinegary; vinegar-like sourness
“If your beer is sour, and it’s not a sour beer, that’s a good sign that there is some sort of bacterial infection,” warns Tham. An infection of acetobacter bacteria produces acetic acid, which has a sour, vinegar-like aroma and flavor.
The most likely culprit? Dirty draft lines.
Diacetyl thrives in this environment, too. “If the issue is dirty draft lines, you’re going to find diacetyl and acetic acid together most times,” says Blair.
That means draft beer will have a buttery and sour aroma and taste. “If you were to mix apple cider vinegar with movie theater popcorn butter, it’s basically what draft line infection smells and tastes like,” says Tham.
Tham compares improper draft maintenance to poor dental hygiene: If not cleaned properly, a plaque-like residue called “beer stone” can build up over several days, eventually causing an infection.
“If you don’t brush your teeth every day, then you build up plaque,” she says. “It’s basically a plaque inside the beer line, and that plaque is where bacteria likes to live.” To prevent this, bars should clean draft lines at least every two weeks.
It’s worth noting, however, that this and other beer faults are not lethal by any means. “It’s not unsafe, it’s just unsanitary,” says Blair. Should you encounter this at a bar or restaurant, opt for a canned or bottled beer instead. “And as a beer person, maybe it’s a place [you] don’t want to come back to.”
Last Updated: July 12, 2023
