How to Detect Off-Flavors in Beer

As a beginner homebrewer, I’m interested in learning more about off-flavors in beer. I can admit it: I’m that annoying perfectionist and only want to make awesome beer, which means understanding how these off-flavors happen and how to identify them.

Conveniently, our local Barley’s Angels chapter and Boulder-based Upslope Brewing Co. teamed up to host a beer-tasting sensory class, led by Mara Miller, Upslope’s laboratory and quality control manager. Mara would spike a few beers with common off-flavors and explain why they occur. We’d then blindly sample the spiked beers and guess the off-flavors. Perfect. I signed up as soon as I heard about the class, and about 25 other women and I jumped into the world of terrible beer.

Upslope Brewing

Before tasting the spiked samples, Mara went over the basics of tasting beer critically. While flavor is certainly a key element to tasting beer, your other senses go a long way in determining how you evaluate a beer. Here are some of Mara’s tips for judging the appearance, aroma and flavor.

  • Appearance: Hold the glass to the light and examine the color and clarity. Take note of the carbonation, which varies in color and body. Judging a beer’s appearance is best done in natural light.
  • Aroma: Gently swirl the glass. Bring the glass to your nose and take short, sharp sniffs. Note the different malt and hop aromas.
  • Flavor: Take two small sips and let the beer sit on your tongue to warm. Then, take larger sips, which gives you the beer’s mouthfeel, such as smooth, sharp, crisp or puckering.

Mara then reviewed five common off-flavors that infect beer. Brewing is a tricky process that can produce a number of off-flavors, so this is just a sampling of potential beer infections. If you’re interested in a comprehensive list of off-flavors, Mara recommends consulting the Beer Judge Certification Program.

  • Acetaldehyde: This gives off-flavors of green apple, cut grass or pumpkin. Typically, this is caused by poor wort aeration, slowed fermentation or prematurely removing the beer from yeast.
  • Dimethyl sulfide: Also known as DMS, it tastes similar to cooked corn. DMS is created when heating the wort, so it’s important to vigorously boil long enough to cook off this compound. Once the boil is finished, cool the wort quickly because DMS can also form during this step.
  • Diacetyl: Think movie theater popcorn. Diacetyl leaves a slick feel in your mouth and often stems from microbial contamination, which is avoidable with proper sanitation. Fermentation also causes diacetyl. Give the yeast enough time to fully ferment and rid the beer of any remaining diacetyl.
  • Ferrous sulfate: This produces a metallic, tin-like flavor. Using water with high levels of iron is a common cause.
  • Oxidation: Oxidized beer has a papery, stale, cardboard-like taste. Aerating hot wort and introducing oxygen once fermentation has started can cause oxidation.

After explaining the common infections, Mara gave us three samples spiked with mystery off-flavors and the Upslope Craft Lager as a palate cleanser. Honestly, American lagers usually do nothing for me, but tasting that clean lager next to infected beers gave me a new appreciation for the style.

With each sip, we kept our comments to ourselves, so no one would be influenced by public opinion. Once Mara disclosed the off-flavor, we were free to share our thoughts.

Sample A tasted like lawn clippings, so I immediately guessed acetaldehyde, which it was. Oddly enough, most of the women thought Sample A was acetaldehyde because of what they perceived as a green apple flavor. I didn’t get green apple whatsoever. To me, it was straight-up lawn clippings. For the record, I’ve never actually eaten lawn clippings, but that’s a perfect example of why aroma is such an important part of the tasting process. Just from the smell, I can imagine exactly what lawn clippings would taste like.

Now on to sample B. For me, there was no mistaking the flavor of movie theater popcorn in this one, which was diacetyl. Most of the women agreed that this off-flavor was easier to spot.

Sample C is where it got tricky for me, and it was by far the most offensive taster of the batch. Unfortunately, I had such a problem figuring out this one that I ended up drinking the whole sample. I initially thought it was ferrous sulfate, and then I had moments where I thought it could have been DMS or oxidation. How I thought it could be any of the three when they’re all so different in flavor is beyond me, but it was oxidation.

While I went into this class with homebrewing on the mind, it’s an awesome experience for any craft beer lover. With all of the craft breweries popping up throughout the country, we need an educated customer base to ensure that the quality doesn’t fall behind.


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