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[https://en.wikipedia.org/wiki/Phenols Phenols] are a large class of organic compounds. One way that phenols can be classified is by how many carbon atoms they include (see the [https://en.wikipedia.org/wiki/Phenols#Classification phenols] Wikipedia article). Examples of classes of phenols include the [https://en.wikipedia.org/wiki/Phenol phenol] (the simplest form of phenols with 6 carbon atoms), [https://en.wikipedia.org/wiki/Hydroxycinnamic_acid hydroxycinnamic acids] (ferulic acid, caffeic acid, etc.), and complex polyphenols (multiple phenol structure units) <ref>[https://en.wikipedia.org/wiki/Phenols Phenols. Wikipedia website. Retrieved 04/17/2017.]</ref><ref>[https://en.wikipedia.org/wiki/Polyphenol Polyphenol. Wikipedia website. Retrieved 04/17/2017.]</ref>. Many phenols have an impact on beer aging or are impacted by beer aging.  ====Phenolic Monomers====Phenolic monomers include phenolic acids ([https://en.wikipedia.org/wiki/Hydroxycinnamic_acid also known as "hydroxycinnamic acids"]), [https://en.wikipedia.org/wiki/Flavonols flavonols], and [[Brettanomyces#Phenol_Production|volatile phenols]] (4-vinylphenol, 4-vinylguaiacol, 4-vinylcatechol, and their ethyl derivative) <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>.  Hydroxybenzoic acids (e.g. vanillic acid, gallic acid, syringic acid) and hydroxycinnamic acids (e.g. p-coumaric, acid, ferulic acid, sinapic acid, caffeic acid) are extracted from polysaccharides within the cell walls of malted grains during the mashing process, and are generally considered to have some antioxidant qualities, however at least one study found that they did not positively impact the oxidative reactions in aging beer (some phenolic acids are antioxidants, but others are oxidizers, and the net result is possibly a non-effect of oxidation and preventing oxidation). They generally do not impact flavor because of their high flavor threshold in beer (52 ppm for p-coumaric acid, 66 ppm for ferulic acid), however yeast metabolism can lead to flavorful volatile phenols such as 4-vinylphenol (plastic) and 4-vinylguaiacol (clove flavor; 0.3 ppm flavor threshold in beer) <ref name="Iyuke_2008" />. The aging of volatile phenols in bottled beer hasn't received much attention from science. One study looked at the evolution of various volatile phenols in several Belgian beers, including one Trappist beer that was conditioned with ''Brettanomyces'' (probably Orval). All beers were aged at 20°C/68°F in a dark room. The study found that Belgian beers that did not contain ''Brettanomyces'' and that had a high levels of 4-vinylguaiacol (4VG) saw a drop of about 50% between months 3 and 6, and then a very slow increase from months 6 to 14. This also corresponded with a fairly sharp increase of vanillan beginning after 6 months. This decrease in 4VG and increase in vanillan was suggested to be caused by both the oxidation of 4VG and acid hydrolysis of [[Glycosides|glycosides]]. Both guaiacol (roasted coffee flavor) and 4-methylphenol (burnt flavor) saw a sharp rise after 6 months to 14 months of aging, particularly in the dark Belgian beers (from 6 to 15 ug/L and 2 to 5 ug/L respectfully) <ref name="Scholtes _2014">[https://www.ncbi.nlm.nih.gov/pubmed/25174984 Guaiacol and 4-methylphenol as specific markers of torrefied malts. Fate of volatile phenols in special beers through aging. Scholtes C, Nizet S, Collin S. 2014.]</ref>.  For the Belgian beer that was conditioned with ''Brettanomyces'', 4-ethylphenol (4EP), which is responsible for the plastic off-flavor in beers and wine with ''Brettanomyces'', steadily increased from 1500 ug/L to 2000 ug/L from when bottled to 3 months. From month 3 to 6, the level of 4EP was stable. From month 6 to 14, 4EP showed a steady decline from 2000 ug/L to 500 ug/L, indicating that 4EP can age out of bottled beers. The phenol 4-ethylguaiacol (4EG) was steady at 1600 ug/L with only a very slight increase from bottling date until 6 months. From month 6 to month 14, 3EG dropped from 1600 ug/L to 1200 ug/L. This indicates that 4EG is relatively stable compared to 4EP, but is not immune to breaking down <ref name="Scholtes _2014" />.