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first pass at haze
===General Effects of Oxygen===
(This is probably the most important thing to talk about first.)
===Haze===
Also referred to as ''colloidal instability'', haze in beer is often attributed to the interactions between polyphenols and proteins. Haze generally limits the shelf life of beer. Beer generally contains much more haze-active proteins than haze-active polyphenols. Haze-active proteins are acidic hydrophilic polypeptides that originate from barley and are rich in proline, glutamic acid, and glycosylated. Smaller phenols such as phenolic acids and flavonols do not contribute to haze, but heavier polyphenols such as procyanidin and prodelphinidin are strong haze inducers. These proteins and polyphenols combine and cause haze in beer. The polyphenol found in hops, catechin, does not form haze immediately but can cause haze after a period of storage. The pH has a huge impact on this reaction; much more haze is formed at a ph of 4.0 than it is at a pH of 3.0 or above 4.2. Higher ABV beers also encourage more haze formation from proteins and polyphenol reactions. This haze generally forms after a period of storage (called the "lag phase"); the longer this lag phase, the better the beer's colloidal stability. Haze can also be induced by oxidation, the presence of aldehydes, shaking, higher temperature, polyphenol-rich raw materials, light, and heavy metals <ref name="Callemien_2010" />.
Chill haze (or reversible haze) is the combination of polyphenols and proteins via non-covalent bonds at colder temperatures. This haze generally goes away after the beer is warmed up again, but chill haze can become permanent as well <ref name="Callemien_2010" />.
Brewers generally remove haze by additives such as tannic acid, papain, or silica gel. Some of these additives can also remove foam forming proteins. Because of this, a chemical known as PVPP is often used because it does not remove foam forming proteins. Several products are available that contain combinations of PVPP and other compounds <ref name="Callemien_2010" />.
===Acids and Esters===
Cover microbiologically driven changes: over-attenuation, Brett expression under pressure, autoylsisautolysis
* [http://horscategoriebrewing.blogspot.com/2016/02/thoughts-on-spitaels-and-van.html "Thoughts on Spitaels and Van Kerrebroeck et al, 2015", by Dave Janssen on Hors Catégorie Blog.]
====Tannic Acid====
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 paper/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 _2017Formanek_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 (including head retention proteins). It did have a very slightly negative impact on yeast attenuation, however the addition of zinc 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's 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>.