Troubleshooting - Phenols

Production of phenolic aroma compounds is an example of biotransformation by yeast since it involves conversion of flavourless precursors into flavour-active molecules. However, this transformation is not desirable in every beer style.

How does yeast make phenols?

Yeast makes volatile phenols such as 4-vinylguaiacol (clove) from phenolic acid precursors such as ferulic acid in your wort, using enzymes called PAD1 and FDC1. Different base malts and crop years can result in different amounts of phenolic precursors. This means that malt selection can impact the phenol aroma you get from POF+ (phenolic) yeasts. This is important to consider since phenolic flavours are a large driver of overall beer flavour.

Production of volatile phenolics is another example of biotransformation by yeast. However, it is not always desirable in the final beer.

Controlling Yeast Phenols

How do I control yeast phenols?

This is a tricky one, because ferulic acid (and other phenolic acids) is impacted by plant stress. This means that the phenol potential of malt can actually change from year to year! The increased plant stress seems to increase the total phenolic acid concentration of the grain typically, so it is possible that more stressed barley results in more ferulic acid. As a result, the amount of ferulic acid may change from year to year and location to location.

How can I predict phenol potential of my malt?

We suggest doing a paired forced fermentation test using a French Saison (POF+) and a California Ale (e.g. Cali Ale strain by pitching the yeast at 4x the usual inoculation rate into two separate flasks of wort prepared from the malt you are testing, on a stir plate for 72 hours. A sniff test of the POF+ sample should reveal the phenol potential of the malt you are using. Currently, phenolic acid content is not a standard feature of malt specification sheets.

How does malt production and kilning impact phenol precursors?

During the kilning process of malts, the free phenolic acid in the husk will start to polymerize with other compounds in the husk of the grain. This will in turn make the phenolic acids insoluble in water which reduces the amount is then extracted into the wort in the brewing process. This will ring true whether it is American or European malt. As a result, phenol-variable yeasts like Wit yeasts will be highly responsive to the free PA content in the malt. To reduce phenols with phenol-variable yeasts, we suggest an English or North American pale ale malt as your base. To enhance phenols with Wit and Weizen yeasts, we suggest using a North American or European Pilsner malt as a base malt.

How do I reduce phenolics when using a POF+ (phenolic) yeast strain?

  1. Use Pale Ale malt rather than Pilsner malt as your base.
  2. Avoid releasing ferulic acid into the wort by avoiding a ferulic acid rest step during mashing.
  3. Avoid trub carryover in your wort.
  4. Select a temperature and pitch rate that optimizes the production of fruity esters.

Troubleshooting Phenolics

  1. Is the yeast you used capable of producing phenolics?
  2. What malt variety was used?
  3. What was your fermentation temperature?
  4. Is it possible the beer has a wild yeast contamination leading to phenolics?
  5. Were any fruit or spices added to the beer? Some fruits such as strawberries can introduce unwanted phenolics into beer.

Further Reading

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