CiceroneWhat The Skunk? A Beer Off Flavor Primer


Skunk. Soured. Oily. Just plain “off.”

We’ve all had beer – whether craft or macro – that tastes wrong. In the brewing world, these imperfections are called “off flavors,” and they can manifest in the brewing process or in packaging, storing, or serving beer.

According to the Cicerone Certification Program, there are six basic off flavors, each with a unique flavor, aroma, and cause. These six off flavors are: diacetyl, dimethyl sulfide (DMS), acetaldehyde, lightstruck, oxidized (trans-2-nonenal), and infection. Identifying off flavors is a requirement for Cicerone Certification.

Yes, to be a Certified Cicerone, you unfortunately have to drink bad beer.

This may seem like an advanced chemistry lesson, but the root causes of many off flavors are easy to understand. Below we get into the specifics, which will help you identify these nasty flavors, the culprits behind each, and what a brewer, retailer, or consumer can do to prevent them.

Diacetyl (Movie Theater Popcorn Of Beer)

You may have seen this chemical compound listed on another item in your kitchen – buttered popcorn. While it’s generally accepted for consumption in small amounts, the presence of diacetyl in beer can be a sign of bacterial infection.

In brewing, diacetyl occurs naturally during fermentation. It’s one of the many organic compounds derived from brewer’s yeast. As that yeast ferments, it consumes the diacetyl, reducing its levels to below human perception.

Improper sanitation of brewing equipment is the leading cause of diacetyl in beer. If it tastes of butter or butterscotch, it’s likely unwanted bacteria has infected the beer. Diacetyl can also present as an oily, slick mouthfeel.

Getting Geeky: Diacetyl, as a chemical, is quite controversial. Increased exposure to inhalation of diacetyl can lead to what’s known as “popcorn workers lung.” The medical term is bronchiolitis obliterans, and it’s an irreversible lung disease. Many popcorn manufacturers have removed diacetyl from their products. Don’t worry about your beer though; you’re safe to continue inhaling all the wonderful aromatics of your favorite brew.

Diacetyl is commonly found in lagers, which are fermented at a lower temperature than ales. To eliminate this off flavor, brewers use a process called “diacetyl rest.” As the craft beer is finishing fermenting, brewers temporarily crank up the temperature a few degrees to help stimulate yeast into consuming more diacetyl.

Acetaldehyde (Taffy Apple Of Beer)

Like diacetyl, acetaldehyde is naturally occurring in the brewing process. It’s an organic compound found in ripe fruit and coffee, and has a tart flavor reminiscent of green apples.

Brewers yeast produces acetaldehyde as glucose (sugar) converts to alcohol. In a healthy fermentation, the yeast will fully convert acetaldehyde into alcohol. Proper fermentation and packaging protocols – described more here – are critical in reducing this off flavor. Brewers have to give yeast ample opportunity to remove this tart off flavor.

Acetaldehyde is also produced if you expose beer to oxygen. This is why breweries take careful measures to cram cans and bottles with carbon dioxide (CO2). CO2 is heavier than oxygen and thus forces it out of the container.

Small amounts of acetaldehyde is acceptable in some beer styles, typically light American lagers. It’s also possible that a sour ale intentionally includes tart, green apple flavor, but that doesn’t mean it comes from acetaldehyde.

Dimethyl Sulfide (Corniest Of Beer)

Once again, this off flavor presents naturally in the brewing process but it’s up to the brewer to ensure it doesn’t get to the nose or palate of a consumer, unless intentionally. A proper boiling and cooling procedure will eradicate it.

Getting Geeky: All malt contains a variant of the amino acid methionine, called S-methyl methionine (SMM). SMM presents in the wort during mashing, and as the wort heats, SMM degrades and converts into DMS. A vigorous boil followed by immediate cooling will eliminate most DMS. Allowing the brew to remain hot – but not boiling – for too long will allow DMS to take hold, producing a creamed-corn beer. If the brewer forgets to lift the lid during boiling, they may also see high levels of DMS. Condensation from the brew is teeming with that corny off flavor, and will drip back into the tank.

Like most off-flavors, DMS can affect beer due to poor sanitization of brewing equipment or packaging. Infection-related DMS will taste like cabbage, shrimp, or celery.

Tiny amounts of DMS in some beer styles is acceptable. You may have had a light lager with a subtle corn flavor. The key word is “tiny.” Larger amounts will taste like you’re kicking back a can of creamed corn.

Lightstruck (Pepé Le Pew Of Beers)

Skunked beer may be the most commonly experienced flavor for the average beer drinker. Simply put: clear and green glass bottles allow light in, and your favorite craft beer skunks.

Getting Geeky: Some hop compounds remain photosensitive after brewing. Isohumulones, a primary bittering compound in finished beer, will turn into MBT (methylbutane thiol) if exposed to light. MBT, simply put, reeks of skunk spray, and humans can detect it at very low levels. (One can guess this is evolutionary, making sure we really stay clear of these stinky mammals.)

Skunk is common in several macro-brands, simply look for a clear or green glass bottle. Some have argued that adding limes to Mexican lagers started as a way to mask a skunky taste or smell.

This off flavor and scent is easy to avoid. If you’re homebrewing, use dark bottles and keep your beer out of sunlight. If you’re looking to enjoy a crisp beer, avoid clear bottles, especially if they hold imported beers. Those have had to survive a long trip across the sea and may have caught a sun tan on their journey.

If you’re drinking a craft beer outside on a sunny day, avoid pouring it into a clear glass. A porcelain mug or stein, or even a classic red plastic cup, will block sunlight. (And of course, we always recommend drinking craft brews from a glass, not the can.)

Oxidized (The Old Papery Book Of Beer)

If you’ve ever had a beer – especially one that has been aged – that tasted of paper or cardboard, then you’ve experienced the unpleasant oxidation off flavor.

The chemical compound that causes oxidation is trans-2-nonenal, which presents when you store beer above 40 degrees Fahrenheit. While brewers work diligently to remove all oxygen from packaged beer, it’s inevitable that some will remain. Storing beer at room, or even cellaring, temperature will increase the rate of oxidation.

Oxidation sometimes also presents as a musty smell or flavor – reminiscent of digging through old boxes in grandma’s attic, a really old paper book, or of wet and rotting wood.

Guess what else can cause oxidation or musty flavors? That’s right – improper sanitization! Mold or mildew remaining in reused bottles (if you’re homebrewing), or condensation and water buildup in fridges and freezers can all contribute to this off flavor in stored beer.

The simplest way to reduce oxidation is to ensure your craft beer cooler is set to around 37 degrees Fahrenheit. You should also crack open a craft beer as fresh as possible. Check for dates on your cans. These tell you when the beer was packaged. They are typically stamped on the bottom of the can or around the collar. Any trace amounts of oxygen in the can will worsen over time. If the brewery accidentally “low-filled” the beer, you will be able to squeeze the can inward. This can often mean that the can has more oxygen in it than it should and will likely be stale faster than normal.

The short of it: keep cold and drink fresh.

Infection (The “I’ll Never Order Beer From Here Again” of Beer)

If you’ve learned anything from above, it’s that clean equipment is critical to great tasting craft beer. Generally speaking, the “infection” off flavor – which is a combination of DMS and acetic acid – comes from unwanted bacteria or wild yeast. Infected beer will taste spoiled and sour, or like soy sauce and vinegar.

Infections are, in practice, easy to stave off. It boils down to proper cleaning procedures, from the mash tank to the lines that fill the kegs, cans, bottles, and your pint glass. Brewers must also ensure the wort is covered while it’s cooling. Cooling wort is a breeding ground for unwanted bacteria. 

Even if a brewer does everything right, it’s still possible the craft beer becomes infected once it lands in a bar or tavern that doesn’t care for their beer lines. It’s critical that your favorite watering holes are cleaning their lines regularly to prevent infection. The Brewers Association recommends cleaning draught lines every two weeks. This goes for your kegerators at home, as well.

If a spoiled beer makes it all the way into your cup, there isn’t much you can do, except dump it down the drain. It is worth telling the bottle shop or retailer. You could be saving someone else from buying a bad beer!

Of course, wild yeast is often used for certain sour style brews. In these cases, brewers closely monitor the yeast strains they want, and avoid those they don’t. Some sour pucker is OK, granted you’re actually drinking a sour beer.

Even More Nasty Beers!

We’ve only scratch (and sniffed?) the surface of off flavors. Poorly produced or packaged beer can also taste like plastic, paint thinner, Band-Aids, lawn clippings, and cough medicine. Yum….not! 

If you’re interested in partaking in an off flavor tasting, check out our Tastings and Trainings page. We hold events like this at the bar regularly. (And for even more geeky stuff, check out this official Cicerone guide to off flavors.)

The next time you’re drinking a craft beer, be thankful it tastes like, well, beer. And if it doesn’t, don’t be afraid to dump it down the drain. Life is too short to drink bad beer.


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