Changes

Phenols such as 4-vinylphenol (4VP; barnyard, medicinal) and 4-vinylguaiacol (4-VG; clove) can be produced in beer through the decarboxylation of hydroxycinnamic acids (HCAs) by yeast, and also in small amounts by long boils with a portion of the wort coming from wheat (3+ hours resulted in 0.3 ppm of 4VG). HCAs, such as ferulic acid and p-coumaric acid, are found present in malt the non-starch polysaccharide arabinoxylan in malted barley and wheat. They are released into wort during mashing at levels that are far below their flavor thresholds (greater than 500ppm flavor threshold) <ref name="kalb_2021">[https://pubs.acs.org/doi/full/10.1021/acs.jafc.1c03018 Investigations into the Ability to Reduce Cinnamic Acid as Undesired Precursor of Toxicologically Relevant Styrene in Wort by Different Barley to Wheat Ratios (Grain Bill) during Mashing. Valerian Kalb, Torsten Seewald, Thomas Hofmann, and Michael Granvogl. Journal of Agricultural and Food Chemistry 2021 69 (32), 9443-9450. DOI: 10.1021/acs.jafc.1c03018.]</ref><ref name="lentz_2018">[http://www.mdpi.com/2311-5637/4/1/20/html#B13-fermentation-04-00020 The Impact of Simple Phenolic Compounds on Beer Aroma and Flavor. Michael Lentz. 2018. doi: 10.3390/fermentation4010020.]</ref>. Some strains of ''Oenococcus oeni'' and ''Lactobacillus'', as well as some strains of yeast such as ''Pichia'' spp, have been found to produce HCA's via cinnamoyl esterase activity in wine, although when these strains have been used in wine to increase the HCA levels, the final phenol levels produced by ''Brettanomyces'' were the same as wine that did not have an increase in HCA levels (the precursors in wine that lead to HCA's are different than they are in beer) <ref>[http://www.ajevonline.org/content/early/2018/05/02/ajev.2018.17092 Influence of Oenococcus oeni and Brettanomyces bruxellensis on Wine Microbial Taxonomic and Functional Potential Profile. Marie Lisandra Zepeda-Mendoza, Nathalia Kruse Edwards, Mikkel Gulmann Madsen, Martin Abel-Kistrup, Lara Puetz, Thomas Sicheritz-Ponten, Jan H. Swiegers, Am J Enol Vitic. May 2018. DOI: 10.5344/ajev.2018.17092.]</ref>. The esters in grape must that contain HCA's (ethyl ferulate and ethyl coumarate) can also be formed by acidic hydrolysis which occurs at the low pH of wine, and HCA's can then be released from these esters. This formation of esters is a slow process in wine, with one study reporting ~0.03 ppm of ethyl ferulate and ~0.4 ppm of ethyl coumarate at the end of primary fermentation and ~0.09 ppm of ethyl ferulate and ~1.4 ppm of ethyl coumarate after 10 months of barrel aging <ref>[https://pubs.acs.org/doi/full/10.1021/jf204908s Hydroxycinnamic Acid Ethyl Esters as Precursors to Ethylphenols in Wine. Josh L. Hixson, Nicola R. Sleep, Dimitra L. Capone, Gordon M. Elsey, Christopher D. Curtin, Mark A. Sefton, and Dennis K. Taylor. 2012. DOI: 10.1021/jf204908s.]</ref>. We are not aware of any studies that have reported an increase in HCA's from acidic hydrolysis over time in beer; however, this is a standard laboratory technique for forcing the release of HCA's from barley (although this lab technique uses a lower pH then that of sour beer). In addition, it has been demonstrated that spent yeast (''S. cerevisiae'' collected after beer fermentation) contains a small fraction of phenols and polyphenols absorbed from wort during fermentation <ref name="Cortese_2020">[https://www.sciencedirect.com/science/article/pii/S0021967319310295 Quantification of phenolic compounds in different types of crafts beers, worts, starting and spent ingredients by liquid chromatography-tandem mass spectrometry. Manuela Cortese, Maria Rosa Gigliobianco, Dolores Vargas Peregrina, Gianni Sagratini, Roberta Censi, Piera Di Martino. Journal of Chromatography A; Volume 1612, 8 February 2020, 460622. DOI: https://doi.org/10.1016/j.chroma.2019.460622.]</ref>. It is therefore conceivable that HCA levels could increase in sour beer over time.

fermentations with respect to time. Presentation at 2008 NHC. pg 12.</ref> <ref name="Yakobson_Michigan">[http://www.mbaa.com/districts/michigan/events/Documents/2011_01_14BrettanomycesBrewing.pdf Yakobson, Chad]. Brettanomyces in Brewing the horse the goat and the barnyard. 1/14/2011</ref> (Musty, Medicinal, Band-aid, Plastic) || Vinyl phenol || p-Coumaric Acid || 0.2 ppm (flavor; in beer) <ref>[http://www.scielo.br/scielo.php?pid=S1516-89132013000600018&script=sci_arttext Determination of 4-vinylgaiacol and 4-vinylphenol in top-fermented wheat beers by isocratic high performance liquid chromatography with ultraviolet detector. Mingguang Zhu; Yunqian Cui. Dec 2013.]</ref> || C₈H₈O <ref name="goodscents_4VP">[http://www.thegoodscentscompany.com/data/rw1005801.html The Good Scents Company. 4-Vinylphenol. Retrieved 08/18/2015.]</ref> || Production level is different across species/strains of ''Brettanomyces'' <ref name="Oelofse">[http://www.sciencedirect.com/science/article/pii/S0740002008002050 Molecular identification of Brettanomyces bruxellensis strains isolated from red wines and volatile phenol production. A. Oelofse, A. Lonvaud-Funel, M. du Toit. 2009.]</ref>. Coumaric acid levels vary greatly between barley varieties; for example, between 320 µg/kg to 950 µg/kg in different varities of barley husks and 73 µg/kg to 657 µg/kg in different varities of barley malt <ref name="Cortese_2020" />. Coumaric levels are generally higher in barley malt than they are in wheat malt. Coumaric acid is stable through the wort boiling process <ref name="kalb_2021" />.

| 4-Vinylguaiacol <ref name="Doss"></ref><ref name="Yakobson_Michigan"></ref> (Clove) || Vinyl phenol || Ferulic Acid || 0.3 ppm (flavor; in beer) <ref>[http://www.aroxa.com/beer/beer-flavour-standard/4-vinyl-guaiacol/ Aroxa Website. 4-Vinylguaiacol. Retrieved 08/19/2015.]</ref> || C₉H₁₀O₂. Also known as 2-methoxy-4-vinyl phenol <ref name="goodscents_4VG">[http://www.thegoodscentscompany.com/data/rw1005101.html The Good Scents Company. 2-methoxy-4-vinyl phenol. Retrieved 08/18/2015.]</ref>. || Produced by some strains of ''S. cerevisiae'' (see [[Saccharomyces#Phenolic_Off_Flavor_Strains|''Saccharomyces'']]) <ref name="Coghe_2014">[http://pubs.acs.org/doi/abs/10.1021/jf0346556 Ferulic Acid Release and 4-Vinylguaiacol Formation during Brewing and Fermentation:  Indications for Feruloyl Esterase Activity in Saccharomyces cerevisiae. Stefan Coghe, Koen Benoot, Filip Delvaux, Bart Vanderhaegen, and Freddy R. Delvaux. 2004.]</ref>. Some ''Brettanomyces'' species/strains may also be able to produce this compound at varying levels <ref name="Joseph"></ref><ref>[http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6968.1995.tb07374.x/abstract The biotransformation of simple phenolic compounds by Brettanomyces anomalus. Duncan A.N. Edlin1, Arjan Narbad, J. Richard Dickinson1 andDavid Lloyd. 2006.]</ref><ref name="Oelofse"></ref>. Organic malts have been linked to higher levels of 4VG, vanillan, and their malt precursor ferulic acid <ref name="Iyuke_2008">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2008.tb00773.x/full The Effect of Hydroxycinnamic Acids and Volatile Phenols on Beer Quality. S. E. Iyuke, E. M. Madigoe, and R. Maponya. 2008.]</ref>. Ferulic acid is released during mashing, with an optimal mash temperature of 40-45°C (104-113°F) and a mash pH of 5.7-5.8 (enyzmatic release of ferulic acid is optimal at a pH of 7.5, but this high of a pH is difficult to achieve during mashing and would cause other enzymatic problems during the later steps of the mash) <ref name="Coghe_2014" /><ref>[https://onlinelibrary.wiley.com/doi/full/10.1002/j.2050-0416.2000.tb00036.x Extraction and Assay of Ferulic Acid Esterase From Malted Barley. F. J. Humberstone D. E. Briggs. 2012.]</ref>. Ferulic Some studies have found that ferulic acid is generally more efficiently extracted from a combination of 70% barley malt and 30% wheat malt (not raw wheat), despite studies showing that barley malt often contains more ferulic acid than wheat malt (see [https://www.facebook.com/groups/MilkTheFunk/permalink/2053354874692716/ this MTF thread] that explains why this is) <ref>[https://onlinelibrary.wiley.com/doi/abs/10.1002/jib.189 Enhancing the levels of 4‐vinylguaiacol and 4‐vinylphenol in pilot‐scale top‐fermented wheat beers by response surface methodology. Yunqian Cui, Aiping Wang, Zhuo Zhang, R. Alex. Speers. 2005. DOI: https://doi.org/10.1002/jib.189.]</ref><ref name="Coghe_2014" /><ref name="lentz_2018" /><ref>[https://www.sciencedirect.com/science/article/pii/S0308814608003348 Release of phenolic flavour precursors during wort production: Influence of process parameters and grist composition on ferulic acid release during brewing. Nele Vanbeneden, Tom Van Roey, Filip Willems, Filip Delvaux, Freddy R.Delvaux. 2008. https://doi.org/10.1016/j.foodchem.2008.03.029]</ref><ref>[https://pdfs.semanticscholar.org/74cd/c0ad3811d95b92c1ecb55ddea392de95ba59.pdf Ferulic Acid in Cereals – a Review. Hüseyin BOZ. 2015. doi: 10.17221/401/2014-CJFS.]</ref>. A more recent studies disagreed and found a linear increase soluble ferulic acid correlated with higher percentages of wheat malt <ref name="kalb_2021" />. Malting parameters also affect the levels of ferulic acid in malt; for example, wheat malt with higher germination temperatures (24-26°C versus 12-18°C) were shown to form more ferulic acid in one study that looked at the impact of germination temperature and aeration during germination of barley and wheat malt <ref name="kalb_2021" />. Ferulic acid is also There is also a correlation between how dark a malt is (or how highly kilned it is and how much melanoidin content it has) and how much ferulic acid the malt has; the darker the malt, the more ferulic acid (however, roasted malts were not tested in the referenced study) <ref>[https://www.mdpi.com/2076-3921/10/7/1124 Shopska V, Denkova-Kostova R, Dzhivoderova-Zarcheva M, Teneva D, Denev P, Kostov G. Comparative Study on Phenolic Content and Antioxidant Activity of Different Malt Types. Antioxidants. 2021; 10(7):1124. https://doi.org/10.3390/antiox10071124.]</ref>. Ferulic acid is stable through the wort boiling process <ref name="kalb_2021" />.