Changes

===Bottles vs Kegsvs Cans vs PET===Chemical changes over time can be different as the beer ages. The packaging type can have a significant impact on how the beer ages. One study on lager found that PET bottles had the greatest variation in chemical changes over a 6 month period of time at ~20°C compared to glass bottles, kegs, and cans. In particular, diacetyl was higher in PET bottles as the beer aged. This was attributed to the PET bottles being more permeable to oxygen because of oxidation of acetoin and 2,3-butanediol to form diacetyl, or the oxidative decarboxylation of alpha-acetolactate, a precursor to diacetyl. Cans showed the least formation of diacetyl, while kegs and bottles displayed moderate increases in diacetyl over time compared to the PET bottles. Acetaldehyde was also highest in the PET bottles. DMS had a high spike during the first month of storage, but by the end of 6 months, the PET bottles had less DMS than the other storage types and glass bottles had the most. Other compounds had less significant differences between package type (ethyl acetate, propanol, isobutanol, isoamyl alcohol, higher alcohols, and esters) <ref>[https://www.sciencedirect.com/science/article/pii/S2214289420300089 The influence of packaging material on volatile compounds of pale lager beer. Goran Gagula, Kristina Mastanjević, Krešimir Mastanjević, Vinko Krstanović, Daniela Horvat, Damir Magdić. 2020.]</ref>.

Young finished champagne and sparkling wines produced according to the ''méthode traditionnelle'' process, which involves carbonating the champagne with sugar for 15 months and then disgorging them and corking them, begin with a CO² concentration of around 11-12 g/L (~6 volumes), while sparkling wines that are 5 years old and 10 years old have been found to have a much lower concentration of CO² at around 6-8 g/L (~3-4 volumes) <ref>[https://www.sciencedirect.com/science/article/pii/S000326700901349X?via%3Dihub CO2 volume fluxes outgassing from champagne glasses: The impact of champagne ageingaging. Gérard Liger-Belair, Sandra Villaume, Clara Cilindre, Philippe Jeandet. 2010.]</ref><ref name="Liger-Belair_2011">[https://pubs.acs.org/doi/abs/10.1021/jf104675s Losses of Dissolved CO2 Through the Cork Stopper during Champagne Aging: Toward a Multiparameter Modeling. Gérard Liger-Belair and Sandra Villaume. 2011.]</ref>. The gradual loss of carbonation in sparkling wines has been attributed to the porous nature of corks allowing for the slow diffusion of gasses through them, which is highly variable based on the density of the cork <ref>[https://www.ncbi.nlm.nih.gov/pubmed/19215133 Kinetics of CO(2) fluxes outgassing from champagne glasses in tasting conditions: the role of temperature. Liger-Belair G1, Villaume S, Cilindre C, Jeandet P. 2009.]</ref><ref>[https://www.sciencedirect.com/science/article/pii/S0003267009013981?via%3Dihub#tbl1 Foaming properties of various Champagne wines depending on several parameters: Grape variety, aging, protein and CO2 content. Clara Cilindrea, Gérard Liger-Belair, Sandra Villaume, Philippe Jeandet, Richard Marchal. 2010.]</ref>, as well as the interface between the cork and the neck of the bottle <ref name="Liger-Belair_2011" />. An interesting observation is that there wasn't a large difference in carbonation loss between 5-year-old sparkling wines and 10-year-old sparkling wines, indicating that the loss of carbonation could greatly slow down once the liquid inside reaches around 3-4 volumes of CO².

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 required since other chemicals can serve this purpose. Oxidation can occur slowly (e.g. metal rusting) or quickly (e.g. 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 or metal ions losing electrons to either oxygen molecules or free radicals. Oxygen itself in its ground state is not particularly reactive in beer, however, oxygen in beer reacts with transition metal ions found in beer such as copper, iron, and manganese(these include two types of redox reactions called the Fenton, and Haber-Weiss reactions) to form "reactive oxygen species" (ROS) which causes then react with other compounds in the beer to cause 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 reactions increase the amount of off-flavor compounds, as well as dulls the aroma of beer. Examples of off-flavors produced by these redox processes include aldehydes and ketones (e.g. acetaldehyde), trans-2-nonenol, and diacetyl. Beer also darkens in color when exposed to oxygen, probably through an oxidation process known as [https://en.wikipedia.org/wiki/Food_browning enzymatic browning]. 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 Decrease of Aged Beer Aroma by the Reducing Activity of Brewing Yeast. Daan Saison, David P. De Schutter, Nele Vanbeneden, Luk Daenen, Filip Delvaux and Freddy R. Delvaux. 2010.]</ref><ref name="hall_mitchell" /><ref name="Barnette_2018_Masters" />.

In general, the best practice is to limit dissolved oxygen (DO) levels to 30-60 ppb (or 40-150 ppb [https://tapintohach.com/2014/03/18/dissolved-oxygen -in-beer-how-it-compares-to-total-package-oxygen/ total package oxygen (TPO)]) <ref>[https://www.hach.com/cms-portals/hach_com/cms/documents/pdf/LIT2149-how-to-measure-DO-in-a-brewery.pdf "HOW TO MEASURE DISSOLVED OXYGEN IN THE BREWERY". Hach Company pamphlet. Retrieved 10/23/2018.]</ref>, much of which is picked up at packaging time, although brewers have had success packaging beers with living ''Brettanomyces'' without purging the bottles with CO². Dissolved oxygen should be measured with a dissolved oxygen sensor/meter during production and immediately at packaging; Bradley Barnette's Masters thesis showed that beers that were oxygenated at packaging to test the effect on dry hopping at 200 ppb had similar DO levels to beers oxygenated at 50 ppb after two weeks of storage, which lead to the hypothesis that oxygen is consumed via oxidation reactions over time during the storage of beer <ref name="Barnette_2018_Masters" />. 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>.  Oxygen has a large and negative impact on highly hopped beers. Dry hopping also serves as a greater risk of oxidation in beer. Portable dissolved Oxygen can enter the beer during the dry hopping process. It's also been shown that iron ions increase from hop additions, which react with oxygen sensors and cause oxidation. Higher DO can be used to detect dissolved oxygen at various points in slightly increase the brewing process rate of IBU degradation. Interestingly, during two weeks of storage it was shown by Barnette that hop compounds such as monoterpenes did not decline even though flavor analysis reported decreased hoppy, fruity, and help troubleshoot citrus character, suggesting that this is caused by the source production of contaminating oxygenother oxidized compounds in the beer rather than a decrease hop-derived compounds <ref name="Barnette_2018_Masters" />.

The temperature at which beer is stored has also has a major impact on how beer ages. The effect that temperature has on a given reaction depends on the type of reaction; not all reactions are increased at the same rate. For example, it has been reported that beer stored at 25°C tends to develop more caramel flavor, while the same beer stored at 30°C develops more cardboard flavor <ref name="Vanderhaegen_2006" />. Bamforth and Lentini proposed a simplified version of the Arrhenius model known as [https://web.calpoly.edu/~bio/EPL/pdfs/SampleLectureBIO162.pdf Q₁₀] to generalize the rate of chemical reactions in beer based on temperature. Q₁₀ is a measure of the temperature sensitivity of chemical and enzymatic reactions due to an increase in temperature by 10°C. The equation is expressed as a ratio: Q₁₀ = Reaction Time + 10°C / Reaction Time. Bamforth recommends that for beer, Q₁₀ will be 2 or 3 (most chemical reactions fall in this range). For example, assuming a Q₁₀ of 3, storing beer at 30°C for 2 weeks is equivalent to 6 weeks at 20°C, 18 weeks at 10°C, and 54 weeks at 1°C. Assuming a Q₁₀ of 2, storing beer at 30°C for 2 weeks is equivalent to 4 weeks at 20°C, 8 weeks at 10°C, and 16 weeks at 1°C <ref>[https://www.sciencedirect.com/science/article/pii/B9780126692013000038 Charles Bamforth and Aldo Lentini. Beer: A Quality Perspective. 2009. Pgs 85-109.]</ref><ref name="Barnette_2018_Masters" /><ref>[https://web.calpoly.edu/~bio/EPL/pdfs/SampleLectureBIO162.pdf Temperature Regulation PowerPoint. Cal Poly. Environmental Proteomics Laboratory.]</ref>. Barnette's Masters thesis found that warmer temperatures had a greater negative impact on hop flavor and aroma than high levels of dissolved oxygen over a two week storage period in dry hopped beers <ref name="Barnette_2018_Masters" />.

Some volatile esters that give the beer a fruity flavor, such as isoamyl acetate that gives the beer a banana flavor, can decrease to levels below their flavor thresholds over time in packaged beer. Other volatile esters such as ethyl 3-methyl-butyrate (derived from oxidized alpha and beta acids in hops and contributes a wine-like character<ref name="williams_wagner_1979">[https://www.asbcnet.org/publications/journal/vol/Abstracts/37-03.htm Contribution of Hop Bitter Substances to Beer Staling Mechanisms. Williams and Wagner 1979.]</ref>), ethyl 2-methyl-butyrate (derived from oxidized alpha and beta acids in hops and contributes a wine-like character<ref name="williams_wagner_1979" />), ethyl 2-methyl-propionate, ethyl nicotinate, diethyl succinate, ethyl lactate, ethyl phenylacetate, ethyl formate, ethyl furoate and ethyl cinnamate (fruity, sweet character) are formed during beer aging. Lactones, which are cyclic esters, such as the peach-like hexalactone and nonalactone tend to increase during beer storage and have a significant impact on the flavor of aged beer <ref name="Vanderhaegen_2006" />. See also [[Hops#Esters|Esters From Hops]].

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 by scavenging free radicals and binding with oxidative metals (iron, for example). 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, and therefore a portion may not react with polyphenols. 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](this reaction increases in the presence of acids <ref>[https://en.wikipedia.org/wiki/Quinone#Reduction "Quinone". Wikipedia. Retrieved 12/23/2023.]</ref>). 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 [[Aging_and_Storage#Tannic_Acid|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" />.

- https://beersensoryscience.wordpress.com/2011/03/17/lightstruck/

- http://www.scielo.br/scielo.php?pid=S0100-40422000000100019&script=sci_arttext&tlng=es

- http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2002.tb00568.x/abstract

- http://www.professorbeer.com/articles/skunked_beer.html

- https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/1521-3765%2820011105%297%3A21%3C4553%3A%3AAID-CHEM4553%3E3.0.CO%3B2-0  - https://pubs.rsc.org/en/content/articlelanding/2004/pp/b316210a/unauth#!divAbstract Iso-alpha acids will skunk if exposed to UV light. Oxidized alpha acids (humulinones) will also skunk if exposed to UV light <ref>[https://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars Dr. Patricia Aron. "Bitterness and the IBU: What’s It All About?" HomebrewCon 2017 Presentation. ~32 mins in. Retrieved 09/05/2017.]</ref>.This compound is known as 3-methyl-2-butene-1-thiol (3MBT). Brown bottles filter most UV light, while green bottles only filter a portion of UV light. See [https://beerandbrewing.com/dictionary/eIXf22Zwnt/ "Lightstruck", Craft beer and Brewing Magazine website]. * [https://www.youtube.com/watch?app=desktop&v=W4vJ9DhoLp4&t=774s Olivier Dedeycker explains why Saison Dupont is packaged in brown bottles for the US market and green bottles for the European market.]

The ketones ketone beta-damascenone (rhubarb, red fruits, stewed apples; threshold of 25 ppb <ref name="hall_mitchell" />) can is thought to be formed from the oxidation of hop oils, as well as 3-methyl-butan-2-one and 4-methylpentan-2-onealthough it has also been found in unhopped aged beers. In Potential precursors are allene triols and acetylene diols formed from the case degradation of damascenone, it the carotenoid neoxanthin. It was found to increase greatly in aged beer that was at a pH of 3 or 4.2 versus a higher pH. This was attributed to the acidic hydrolysis of glycosides. The release of flavor compounds from glycosides could be present in acidic beers that are aged on fruit or herbs <ref>[https://pubs.acs.org/doi/full/10.1021/jf020085i Investigation of the β-Damascenone Level in Fresh and Aged Commercial Beers. Fabienne Chevance, Christine Guyot-Declerck, Jérôme Dupont, and Sonia Collin. 2002. DOI: 10.1021/jf020085i.]</ref><ref name="Vanderhaegen_2006" />. See [[Glycosides#Acidic_Hydrolysis|Glycosides]] for more information on acidic hydrolysis of glycosides. Damascenone is also found in grapes and is a major flavor component of bourbon <ref name="hall_mitchell" />.