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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><refname="wikipedia_polyphenols">[https://en.wikipedia.org/wiki/Polyphenol Polyphenol. Wikipedia website. Retrieved 04/17/2017.]</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- above flavor threshold these acids are generally bitter and astringent in flavor), 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" /><ref name="Callemien_2010" />. 4-vinylguaiacol can be partially oxidized or reduced into smaller compounds such as vanillan, 4-ethylguaiacol (reduced by ''Brettanomyces''), and guaiacol as beer ages <ref name="Callemien_2010" />.

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 level 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 in 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>. The degradation of 4VG has also been reported to be 25% after 20 days at room temperature, or 50% after 20 days at 40°C/104°F <ref name="Callemien_2010" />.

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 _2014Scholtes_2014" />.  The volatile phenol 4-vinylsyringol (smokey, burnt) has been identified in lagers that have aged and is thought to stem from the slow acidic hydrolysis of a glycoside during aging <ref name="Callemien_2010" />.  ====Polyphenols====[https://en.wikipedia.org/wiki/Polyphenol Polyphenols] are a large group of organic chemicals characterized by many phenol structures combined. Subclasses of polyphenols include tannic acid, tannins, and flavonoids <ref name="wikipedia_polyphenols" /><ref>[https://en.wikipedia.org/wiki/Flavonoid Flavonoid. Wikipedia website. Retrieved 05/02/2017.]</ref>.  Polyphenols have an ambiguous role in the aging of beer. Flavonoids (for example catechin, which comes from hops and is a major source of polyphenols in beer), are antioxidants and protect more sensitive compounds such as isohumulones from oxidation. However, they themselves can also be oxidized over time to possibly create off-flavors. In addition to their own oxidation, [https://en.wikipedia.org/wiki/Hydroxyl_radical hydroxyl radicals] that cause oxidation also react highly with ethanol. After a lag period of 5 weeks in the bottle, it was found that levels of tannins actually increase. This is thought to be caused by smaller flavonoids reacting with acetaldehyde. Polyphenols were also oxidized into [https://en.wikipedia.org/wiki/Quinone quinones], which are a stepping stone in the reaction that causes [https://en.wikipedia.org/wiki/Food_browning oxidative food browning]. The use of polyphenols during mashing and boiling has been shown to decrease trans-2-nonenal (cardboard flavor) and trans-2-nonenal that is protected from fermentation by being bound to proteins (see Tannic Acid below). In two studies, there appeared to be no significant effect on free radical formation by polyphenols, probably due to the fact that they readily react with ethanol <ref name="Callemien_2010" />. Higher temperatures increase the rate of oxidized polyphenols. In one study on aged lagers, 6.5% of the polyphenols were oxidized after 5 days at 40°C/104°F, but only 0.6% of the poly[phenols were oxidized after 9 months at 20°C/68°F <ref name="Callemien_2010" />.  Polyphenols generally contribute to an astringent taste in beer, and this can be intensified at a lower pH (4-4.2). Oxidation of polyphenols might make them more astringent depending on the degree of "polymerization degree" (see [https://winemakermag.com/1045-tannin-chemistry-techniques this article]), although residual sugars reduce their astringency. Sensory analysis of lagers has shown that aged lagers became less bitter and more astringent over time (especially at a higher temperature or a higher pH), probably due to a decrease in IBU's and bitter polyphenols like catechin, and an increase in oxidized polyphenols <ref name="Callemien_2010" />.  Heavier polyphenols such as procyanidin and prodelphinidin also contribute to beer haze after packaging. See Haze above.