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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. They are introduced from malt, hops, and yeast fermentation. Some phenols directly impact the flavor, astringency, haze, body, and fullness of beer. Some phenols also have health properties. Degradation of some phenols leads to the changing of fresh beer taste. Other phenols act as antioxidants and can protect the beer to some degree from oxidative degradation as beer ages <ref>[http://www.tandfonline.com/doi/abs/10.1080/87559120903157954 Structure, Organoleptic Properties, Quantification Methods, and Stability of Phenolic Compounds in Beer—A Review. D. Callemien and S. Collin. 2010. DOI: http://dx.doi.org/10.1080/87559120903157954.]</ref>.

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 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 of 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>.

Tannic acid is a subclass of tannin (tannins are a subclass of polyphenols). Tannic acid is generally extracted from four sources: Chinese gallnuts, Aleppo gallnuts (these both produce ''gallic acid''), Sumac leaves, and Tara pods (produces ''quinic acid''). Forms of tannic acid can also be extracted from oak galls (seeds from oak trees) and oak bark <ref>[https://en.wikipedia.org/wiki/Tannic_acid Tannic acid. Wikipedia website. Retrieved 04/17/2017.]</ref>. Tannic acid extracts are of interest to the brewing industry because they have been found to improve flavor stability through its antioxidant Properties. Specifically , these properties include free radical scavenging activity, bonding of the metals involved in beer staling (specifically iron ions and perhaps copper ions), and preventing oxidative degradation of lipids that produce the aldehydes such as trans-2-nonenal (trans-2-nonenal is responsible for the paperypaper/cardboard off-flavor in stale beer). Tannic acid also increases colloidal stability (clarity over time) by binding with the proline-rich proteins that cause chill haze. The higher molecular weight tannic acids (Chinese gallnuts or Sumac leaves) will reduce chill haze by removing these proteins, while medium molecular weight tannic acids (Allepo gallnuts) can result in a stable haze for beer styles where some haze is desirable such as Belgian Wit or German Hefeweizen <ref name="Formanek _2017">[http://dx.doi.org/10.1094/TQ-54-1-0112-01 Use of Tannic Acid in the Brewing Industry for Colloidal and Organoleptic Stability. Joseph A. Formanek and Pieter Bonte. MBAA Technical Quarterly. 2017.]</ref>.

The sensory analysis in this study showed that all of the fresh beers with or without gallic acid scored positively by tasters, although they preferred the beers with gallic acid because of their "fullness of taste and mouthfeel". After 5 days of storage at 40°C, the beers with gallic acid continued to rate high while the non-gallic acid beers began to show signs of staling (darker in color, cardboard flavor, slight sweetness, and a sharp unpleasant bitterness). After storing the beers for 10 days at 40°C the non-gallic beers were clearly more oxidized. These results were confirmed by also storing the beers for one year at 4°C in the dark; the beers with gallic acid were always preferred by tasters. The beers with gallic acid added during the mashing and sparging had better flavor stability, and better stability of hop compounds such as iso-alpha acids. They also had a higher polyphenol content from the malts (only hop extract was used, so the polyphenol content had to come from the malts). The [https://en.wikipedia.org/wiki/Lipoxygenase lipoxygenase activity ('''LOX''')], which is the mechanism by which lipids are oxidized to create off-flavors, was also decreased in the beers with gallic acid added. They also found that the "Strecker" and "Maillard" derived aldehydes phenylacetaldehyde, furfural, benzaldehyde were reduced by the use of gallic acid (fatty-acid derived aldehydes were not reduced, but there is evidence that these are already present in malt and are not formed during the mashing process, unlike the previously mentioned aldehydes). Trans-2-nonenal (paperypaper, cardboard staling flavors) was also reduced in beers brewed with gallic acid. The study found that mashing with gallic acid at 62°C and 5.3 pH had the most positive impacts on both flavor stability and the prevention on of off-flavor development <ref name="Aerts_2004" />.