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===General Effects of Oxidation===
Oxidation, also known as a redox reaction, is the chemical process of atoms losing electrons to other atoms. The atom that loses an electron is called the "oxidizer", and the atom that gains the electron is called the "reducer". Despite the name of this process being called "oxidation" and oxygen often being the reducer, oxygen is not always the reducing agent since other chemicals can serve this purpose . Oxidation can occur slowly (metal rusting) or quickly (fire), and applies to a large range of simple and complex processes <ref>[https://en.wikipedia.org/wiki/Redox Wikipedia. "Redox". Retrieved 09/03/2017.]</ref>. In beer, oxidation takes the form of carbon-based molecules losing electrons to either oxygen molecules or free radicals. Oxygen reacts with transition metal ions found in beer such as copper, iron, and manganese, which causes beer staling <ref name="Barnette_2018_Masters">[http://scholar.google.com/scholar_url?url=https://ir.library.oregonstate.edu/downloads/dv140033b&hl=en&sa=X&d=799257176923188618&scisig=AAGBfm23Uy0QqVLXJEUSylw-LILNTHHd7Q&nossl=1&oi=scholaralrt&hist=CYJIrnMAAAAJ:10241589793194662084:AAGBfm17pAuQUDgk8QVeubsITC7flr3nZQ Evaluating the Impact of Dissolved Oxygen and Aging on Dry-Hopped Aroma Stability in Beer. Bradley M. Barnette. Masters Thesis in Food Science and technology, Oregon State University. 2018.]</ref>. Oxidation increases the amount of off-flavor compounds, as well as dulls the aroma of beer. Brewers yeast and ''Brettanomyces'' are great scavengers of oxygen, and adding fresh yeast and sugar at packaging can help reduce dissolved oxygen in the package, and even reverse some effects of oxidation. Adding fresh yeast and sugar can reduce aldehydes and ketones such as acetaldehyde, trans-2-nonenol, and diacetyl, back into ethanol after packaging <ref>[http://pubs.acs.org/doi/abs/10.1021/jf9037387 Daan Saison, David P. De Schutter, Nele Vanbeneden, Luk Daenen, Filip Delvaux and Freddy R. Delvaux. 2010.]</ref><ref name="hall_mitchell" />.
In general, the best practice is to limit oxygen at packaging time, although brewers have had success packaging beers with living ''Brettanomyces'' without purging the bottles with CO<sup>2</sup>. Other compounds can serve as anti-oxidants in beer. For example, sulfates are converted into sulfites by yeast, and sulfites postpone the formation of free radicals. Lower-weight polyphenols, which originate from malt (70-80%) and hops (20-30%), are thought to be free radical scavengers and anti-oxidants, however other polyphenols have been identified as pro-oxidants and the effectiveness of antioxidant activity in general for polyphenols is debatable in the scientific literature (although their impact in the mash and boil has been established as positive). Maillard reactions from malting/roasting and wort boiling also create anti-oxidants, and in general the darker the roasting the more anti-oxidant the malts will be <ref name="Vanderhaegen_2006" />, although compounds in kilned malts, hypothesized to be the proanthocyanidins and flavonols derived from Maillard reactions, have been found to be a source for oxidation and beer staling. Alpha acids and iso-alpha acids have been shown to react with transition metal ions (iron), thus reducing the impact of the oxidation of iron ions <ref name="Barnette_2018_Masters" />. Lactic acid and lactic acid fermentation are thought to also help serve as anti-oxidants, although this has not be studied in sour beer <ref>[https://www.ncbi.nlm.nih.gov/pubmed/10904049 Free radical scavenging and antioxidant effects of lactate ion: an in vitro study. Groussard C, Morel I, Chevanne M, Monnier M, Cillard J, Delamarche A. 1985.]</ref><ref>[https://www.sciencedirect.com/science/article/pii/S0740002011000530 Effect of lactic acid fermentation on antioxidant, texture, color and sensory properties of red and green smoothies. Raffaella Di Cagno, Giovanna Minervini, Carlo G. Rizzello, Maria De Angelis, Marco Gobbetti. 2011.]</ref>. Portable dissolved oxygen sensors can be used to detect dissolved oxygen at various points in the brewing process and help troubleshoot the source of contaminating oxygen.