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First pass at section on tannic acid
===Phenols===
[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 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.
====Tannic Acid====
Tannic acid is a sub-type of tannin. 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 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 produces the aldehyde trans-2-nonenal which is responsible for the papery flavor in stale beer. Tannic acid also increases colloidal stability (clarity) 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 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>.
One study showed that adding gallic acid (tannic acid from Chinese or Aleppo gallnuts) during the mash and sparge water extended the shelf life of beer by reducing off-flavors such as trans-2-nonanol, the aldehyde responsible for the papery taste in stale beer. It also bound to proteins containing thiols that when oxidized during mashing can cause filtering and amylolytic issues, and settled them out during the mashing/boiling which made filtration easier. The study determined that the tannic acid did not have an impact on the saccharification rest or the soluble protein content. It did have a very slightly negative impact on yeast attenuation, however the addition of zinc to worts that had gallic acid added resolved the attenuation issue (it is thought that the gallic acid dropped the zinc out of solution since it binds with metals, and thus negatively impacted the yeast health) <ref name="Aerts_2004">[http://www.mbaa.com/publications/tq/tqPastIssues/2004/Abstracts/0831-01.htm Evaluation of the Addition of Gallotannins to the Brewing Liquor for the Improvement of the Flavor Stability of Beer. Guido Aerts, Luc De Cooman, Gert De Rouck, Zoltan Pénzes, Annemie De Buck, Roger Mussche, and Joseph van Waesberghe. 2004.]</ref>.
The sensory analysis 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), and at 10 days the non-gallic beers were clearly more oxidized. These results were confirmed by storing the beers for one year at 4°C in the dark; the beers with gallic acid were always more 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 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 (papery, 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 off-flavor development <ref name="Aerts_2004" />.
Products are now being sold such as ''Brewtan B'' that contain tannic acid without the impurities that can often be found in other commercial tannic acid products. This manufacturer claims that the product enhances clarity and colloidal stability, stabilizes hop bitterness, reduces aldehyde formation, and increases flavor and color stability over time. The dosage for this product is quite low at 1.5–4.0 g/hL <ref name="Formanek _2017" />. Author Denny Conn of Experimental Brewing has noted that the use of Brewtan B increases the mouthfeel and flavor stability of beer. Recommended usage of Brewtan B is 1/4 teaspoon per 5 gallons of water <ref>[https://www.experimentalbrew.com/podcast/episode-25-brewtan-your-questions-away "Episode 25 - BrewTan Your Questions Away". Experimental Brewing podcast (~30 minutes in). Retrieved 04/17/2017.]</ref>.
===Hop Compounds===