Identifying off-flavours in beer: Phenols

This article is the fourth installment of a mini-series on the most important off-flavours and compounds that a homebrewer should know. Last month we talked about acetaldehyde. This month, we discuss phenols.


Phenols are part of a large group of compounds that are responsible for a number of qualities in beer. Phenols can play both positive and negative roles in flavour, mouthfeel, colour, and clarity. In regards to flavour they can be perceived as spicy, peppery, clove, smoky, barnyard, plastic, Band-Aid, and even medicinal. A beer judge might use the term “phenolic” as an umbrella term for beers that have a combination of these more undesirable descriptors.

The most important thing to know about phenols is that once phenols are in your beer—at any stage of the process—they cannot be removed. There are a number of sources and precursors where phenols can develop during the brewing process, as well as a wide variety of ways to mitigate and control their qualities.

Chlorophenols and brewing water

Chlorophenols specifically make plastic, Band-Aid, and medicinal flavours. Chlorophenols are formed by the presence of chlorine and chloramine in your brewing water that bond to natural phenolic compounds that yeast produce during fermentation. The yeast binds these phenols to the chlorine compounds as a way to protect themselves, which is smart, but leaves us with a pretty undesirable taste.

Most municipal water suppliers these days use chloramine as it is more stable compound. Back in the day when homebrewers were only faced with chlorine, all you had to do was simply allow the water sit out overnight. Chloramine will not dissipate or evaporate at all, which means you must use Campden tablets, a.k.a. potassium metabisulfite.

Use half a tablet for every 20 litres (5 gallons) of water. Simply crush the half-tablet and mix it into your water and allow to sit for up to 30 minutes. This will force the chloramine out of suspension. Campden tablets work just as well on regular chlorine too, if you know for a fact that your water supply still uses it.

Reverse Osmosis water is another option to remove chlorine or chloramine, as is bottled spring water. Distilled water is also an option, as long as you calculate and add in minerals and yeast nutrient.

Also note, there are often trace phenols and other compounds in some water supplies that can lead to other flavours in your water that you might not want. If you do not like the taste of your tap water, then don’t use it. Bad water makes bad beer.

Sanitizing and chemical cleaners

While bleach is an effective household cleanser, it is not recommended to use on your brewing equipment. Bleach is a concentrated chlorine source and can linger in porous materials such as plastic brewing buckets, hoses, tubing, and plastic siphons. Just as with brewing water, any lingering bleach in your brewing equipment can lead to chlorophenol production and unwanted off-flavours.

The best solution is to discontinue using bleach and replace any equipment that might have come into contact with it. Instead, opt for cleaners such as OxiClean or PBW and using a no-rinse sanitizer like Star San.

Smoked malts and volatile phenols

Smoky phenols (guaiacol and syringol) are most likely to come from smoked malts or rauchmalt. This includes, but is not limited to, beechwood, cherrywood, mesquite, and peated smoked malts.

If you sense smoky phenolic flavours but you didn’t use any smoked malt, check that there is no other source of char or anything burnt introduced to your beer. This includes charred oak, roasted or toasted adjuncts, or high percentage of roasted barley in the grain bill.

Some strains of Brettanomyces and wild yeast can also produce smoky phenols.

Phenolic characteristics and yeast

The desirable clove, spicy, peppery, and herbal flavours in some Belgian beers and Hefeweizens owe these complex notes to the presence of 4-Vinyl guaiacol (4VG). Yeast produces this phenol naturally when they interact with ferulic acid.

For brewers wanting to enhance these distinct Belgian and Hefeweizen flavours, you can employ a ferulic acid mash rest of 45° C (113° F) just as it has been done in Europe for centuries. Wheat is also naturally high in ferulic acid, which is no wonder many of these beers feature a healthy dose of wheat malt.

In addition to its souring capabilities, Brettanomyces is famous for flavours and aroma of barnyard and “wet horse blanket” which are the result of the phenol 4-ethylphenol (4-EP). Some strains can also produce smoky or meaty flavoured phenols in the wrong conditions, so be sure to read about the strains you are using and how to utilize them appropriately.

If you don’t want any the flavours mentioned, simply select appropriate yeast strains and take measures to reduce the amount of ferulic acid possible in your process. If you are getting any of these qualities in a beer that isn’t supposed to have them, you might have a contamination of wild yeast. Simply ensure your equipment is properly cleaned and sanitized.

Tannins and astringency

While this might come as a surprise, tannins are actually a phenolic compound belonging to a class of polyphenols. In small amounts these can provide anti-oxidants that preserve beer and stabilize colour and clarity. In excess, though, some of these compounds can also contribute to chill haze or premature staling in the bottle.

The easiest way to know if you are getting an excessive level of tannins and polyphenols is if you experience unpleasant grainy or husk-like flavours, an over-dry finish, or excessive bitterness – not to be confused with hop burn or hop bitterness.

The easiest way to mitigate tannins is to control your mash and sparge regimen. Most specifically:

  • do not sparge with water that is over 77° C (170° F)
  • do not over-sparge (use the calculated amount of sparge water)
  • decrease the amount of time you are sparging (if possible)
  • treat your sparge water to ensure it is not too alkaline
  • Do not press on your grains to get the “extra juices” out

Also, try to avoid getting grain pieces into your brew kettle pre-boil. Strain these out when possible, as boiling them can leach tannins into the wort.

Final thoughts

The biggest takeaways you should come away with are to do your best to remove chlorine and chloramine from your brew water, do not use bleach to clean your equipment, be aware of your ingredients and yeast strains, and to be mindful of your sparge regimen. If you can stick to these, you can be sure to get control of phenols and their various qualities in your final product.

David Boggs has been homebrewing for over 10 years and currently works as a professional brewer in Edmonton. He holds a certificate in Applied Craft Brewing and is a Recognized BJCP Judge. He is a fan of Belgian ales and anything barrel-aged.

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