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updated info for Phenol production
Phenols such as 4-vinylphenol (4VP, barnyard, medicinal) and 4-vinylguaiacol (4-VG, clove) can be produced in beer by the decarboxylation of hydroxycinnamic acids (HCAs), which are found in malt. While both ''Saccharomyces'' (only by "phenolic off flavor positive/POF+" strains) and ''Brettanomyces'' strains have varying capabilities based on strain of converting hydroxycinnamic acids to their vinyl derivatives <ref name="Lentz">[http://www.mdpi.com/2304-8158/4/4/581/htm Analysis of Growth Inhibition and Metabolism of Hydroxycinnamic Acids by Brewing and Spoilage Strains of Brettanomyces Yeast. Michael Lentz and Chad Harris. 2015.]</ref>, ''Brettanomyces'' is also able to reduce these vinyl phenol derivatives to ethyl phenol derivatives. Phenolic acid decarboxylase (PAD) is the enzyme that converts the HCAs into vinyl phenols. Vinyl reductase (VA) is the enzyme that reduces vinyl phenols to ethyl phenols <ref>[https://www.facebook.com/groups/MilkTheFunk/1632316743463200/ Analysis of phenolic acid decarboxylase enzyme from the wine spoilage yeast Brettanomyces bruxellensis (poster). Mike Lentz, Jamie Lynch, Pricilla Walters, Rachel Licea, Henok Daniel, Kimberly Pereira. 2017.]</ref>.
These vinyl derivatives have similar tastes to the ethyl derivatives but have lower flavor thresholds. Levels of all compounds produced vary depending on species and strain of ''Brettanomyces''. Although the production of ethyl phenols has been identified to occur higher in substrates with more available sugars, and this has also correlated with higher growth <ref name="Barata_2008">[http://onlinelibrary.wiley.com/doi/10.1111/j.1567-1364.2008.00415.x/full The effect of sugar concentration and temperature on growth and volatile phenol production by Dekkera bruxellensis in wine. André Barata, Daniela Pagliara, Tiziana Piccininno, Francesco Tarantino, Wilma Ciardulli, Manuel Malfeito-Ferreira, Virgílio Loureiro. 2008. DOI: 10.1111/j.1567-1364.2008.00415.x]</ref>, some data supports that pitching rate does not have an effect on how much phenol content is produced by ''Brettanomyces''<ref name="MTF_Brett_Secondary">[http://www.milkthefunk.com/wiki/Brettanomyces_secondary_fermentation_experiment Brettanomyces secondary fermentation experiment. Milk The Funk Wiki. Lance Shaner and Richard Preiss. Retrieved 04/21/2016.]</ref>. Additionally, Curtin et al. (2013) showed that while both cell growth and attenuation was inhibited in anaerobic conditions in wine, phenol production was not (in fact, the phenol production was inhibited by aerobic conditions). They also showed that each of the three strains of ''B. bruxellensis'' tested all produced the same amount of phenols, while other flavor compounds such as esters were produced at different levels by each of the strains <ref>[https://www.ncbi.nlm.nih.gov/pubmed/24010603 Impact of Australian Dekkera bruxellensis strains grown under oxygen-limited conditions on model wine composition and aroma. Curtin CD, Langhans G, Henschke PA, Grbin PR. 2013]</ref>. Perhaps growth itself is not as much of a factor in producing phenols, but having sugars available for metabolism is. This contradicts the somewhat popular belief that under-pitching ''Brettanomyces'' produces more "funky" flavors. Additionally, perhaps some strains are perceived as "funkier" than others due to less ester production and more fatty acid production (isobutyric acid, for example), rather than more phenol production.
Another study by Tyrawa et al. found that fermentation temperature also did not have a significant effect on phenol production in 9 strains of ''B. bruxellensis''. Given the same wort composition, strains of ''B. bruxellensis'' produced similar levels of phenols at both 15°C and 22.5°C. The ester production was affected by this temperature difference in some strains but not others (see [[Brettanomyces#Ester_Production|Esters]] above). Assuming that phenols contribute the "funky" flavor characteristics, this suggests that perhaps a lower balance of esters to phenols produces a more "funky" tasting beer more so than a beer with more phenol content. If so, a lower fermentation temperature may be one way to emphasize phenols over fruity esters <ref name="Tyrawa_2017" />.