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>.

* Pierre Tilquin explains that horizontal storage for corked bottles results in no [[Pellicle]] formation in the bottle <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1210209235673955/?comment_id=1210248462336699&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Pierre Tilquin on MTF regarding horizontal bottle storage. 01/08/2016.]</ref>. Also reported in a side by side comparison by [https://www.facebook.com/groups/MilkTheFunk/permalink/1932957630065775/ James Howat].* The internal pressure caused by CO2 inside the bottle keeps the cork moist without needing to lay the bottle on its side. Uncarbonated lambic-style beers might benefit from horizontal storage by keeping the beer in contact with the cork and keeping it moist (this doesn't matter for synthetic corks because they don't absorb moisture) <ref>[https://en.wikipedia.org/wiki/Storage_of_wine#Orientation_of_the_bottle Wikipedia. Storage of Wine. Retrieve 04/14/2016.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1284779411550270/?comment_id=1284784551549756&comment_tracking=%7B%22tn%22%3A%22R1%22%7D Conversation with Raf Soef on horizontal bottle storage for natural corks. 0414/2016.]</ref><ref>[https://web.archive.org/web/20120523183416/http://www.caterersearch.com/Articles/19/09/1996/21487/The-lay-down-on-storage.htm Caterer Search. "The Lay Down on Storage". 2001. Archive.org saved on 03/01/2012. Retrieved 02/02/2018.]</ref>. However, other sources claim that the humidity in the headpace (ullage) of uncarbonated wine is close to 100% even when stored upright <ref>[https://www.thedrinksbusiness.com/2018/06/compounds-called-corklins-found-in-cork-stoppered-wines/ Dr. Miguel Cabral. "COMPOUNDS CALLED CORKLINS FOUND IN CORK-STOPPERED WINES". The Drinks Business website article. 06/07/2018. Retrieved 06/08/2018.]</ref><ref>[https://www.thedrinksbusiness.com/2018/06/storing-wine-on-its-side-is-bullsht-says-scientist/ Dr. Miquel Cabral. "STORING WINE ON ITS SIDE IS NONSENSE, SAYS SCIENTIST". The Drinks Business website article by Patrick Schmitt. 06/11/2018. retrieved 06/11/2018.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2125390790822457/ Milk The Funk discussion on the humidity of the headspace within wine bottles. 06/11/2018.]</ref>.* Jeff Porn observes accelerated flavor development (good or bad) when comparing bottles that are stored horizontally versus ones stored vertically . Interestingly, bottles stored on their side developed [[Tetrahydropyridine]] off-flavor while the vertically stored bottles did not (Jeff bottle conditions with a house culture) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1284779411550270/?comment_id=1284814231546788&reply_comment_id=1284815964879948&comment_tracking=%7B%22tn%22%3A%22R3%22%7D Conversation with Jeff Porn on horizontal corked bottle storage. 04/14/2016.]</ref>. This is Shawn Savuto also reported THP development only in bottles aged on their side <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2149235825104620/?comment_id=2149252968436239&reply_comment_id=2149261881768681&comment_tracking=%7B%22tn%22%3A%22R0%22%7D Shawn Savuto. Milk The Funk Facebook group post on storing bottles sideways and THP development. 06/25/2018.]</ref>. Increased flavor development was also the reported by [http://allaboutbeer.com/belgian-warm-rooms/ experience of Brasserie Dupont].

As beer ages, different flavor and aroma compounds both deteriorate and form over timeand at different rates. This is a complex collection of changes with many variables involved in affecting different compounds in different ways. As molecules deteriorate or are formed, their associated flavors not only dissapate dissipate and appear, but the influence that they have on each other may emphasize or overwhelm other flavors. The compounds, which number in the hundreds or perhaps thousands, are not in equilibrium when the beer is packaged. Once bottled or kegged, this closed environment forces molecules to reach a status of minimal energy and maximal entropy, and thus many of the molecules change over time to reach equilibrium. The two largest factors that affect beer aging are temperature and dissolved oxygen in the package <ref name="Vanderhaegen_2006" />. UnfortunatelyRegular beers with a lower pH tend to age faster, however, most beer studies have focused on lager beers and some on ales and strong beers. Research on beers containing living ''Brettanomyces'' or beers that are at a lower pH of 3-3.8 is limited.

* Sulfur compounds: dimethyl trisulfide(production enhanced by low pH), 3-Methyl-3-mercaptobutylformate.

===General Effects of OxygenOxidation===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 required since other chemicals can serve this purpose. Oxidation can occur slowly (This 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 probably 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 then react with other compounds in the most important thing beer to talk about firstcause 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 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 <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 beergeneral, oxidation the best practice is the process of carbon based molecules losing electrons to limit dissolved oxygen atoms (DO) levels to 30-60 ppb (or free radicals 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://enwww.wikipediahach.orgcom/wikicms-portals/Redox Wikipediahach_com/cms/documents/pdf/LIT2149-how-to-measure-DO-in-a-brewery. pdf "RedoxHOW TO MEASURE DISSOLVED OXYGEN IN THE BREWERY". Hach Company pamphlet. Retrieved 0910/0323/20172018.]</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. Oxidation increases 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 amount effect on dry hopping at 200 ppb had similar DO levels to beers oxygenated at 50 ppb after two weeks of off-flavor compoundsstorage, as well as dulls which lead to the hypothesis that oxygen is consumed via oxidation reactions over time during the aroma storage of beer <ref name="Barnette_2018_Masters" />. Other compounds can serve as anti-oxidants in beer. Brewers 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 ''Brettanomyces'' hops (20-30%), are great thought to be free radical scavengers and anti-oxidants, however other polyphenols have been identified as pro-oxidants and the effectiveness of oxygenantioxidant 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 adding fresh yeast and sugar at packaging can help reduce dissolved oxygen 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 packageproanthocyanidins and flavonols derived from Maillard reactions, have been found to be a source for oxidation and even reverse some affects beer staling. Alpha acids and iso-alpha acids have been shown to react with transition metal ions (iron), thus reducing the impact of the oxidationof iron ions <ref name="Barnette_2018_Masters" />. Adding fresh yeast Lactic acid and sugar can reduce aldehydes and ketones such lactic acid fermentation are thought to also help serve as acetaldehyde, transanti-2-nonenoloxidants, although this has not be studied in sour beer <ref>[https://www.ncbi.nlm.nih.gov/pubmed/10904049 Free radical scavenging and diacetylantioxidant effects of lactate ion: an in vitro study. Groussard C, Morel I, Chevanne M, Monnier M, back into ethanol after packaging Cillard J, Delamarche A. 1985.]</ref><ref>[httphttps://pubswww.acssciencedirect.orgcom/doiscience/absarticle/10.1021pii/jf9037387 Daan SaisonS0740002011000530 Effect of lactic acid fermentation on antioxidant, David Ptexture, color and sensory properties of red and green smoothies. De Schutter Raffaella Di Cagno, Giovanna Minervini, Nele VanbenedenCarlo G. Rizzello, Luk DaenenMaria De Angelis, Filip Delvaux and Freddy R. DelvauxMarco Gobbetti. 20102011.]</ref>.  Oxygen has a large and negative impact on highly hopped beers. Dry hopping also serves as a greater risk of oxidation in beer. 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 and cause oxidation. Higher DO can slightly increase the 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 citrus character, suggesting that this is caused by the production of other oxidized compounds in the beer rather than a decrease hop-derived compounds <ref name="hall_mitchellBarnette_2018_Masters" />.  See also:* [https://www.morebeer.com/articles/oxidation_in_beer "Controlling Beer Oxidation" by George Fix.]* [https://jp.hach.com/asset-get.download.jsa?id=50544340479 Industry standards of dissolved oxygen levels in beer throughout the brewing process, by Hach.]* [https://www.facebook.com/groups/MilkTheFunk/permalink/3835629766465209/ MTF thread on anecdotal accounts of ''Brettanomyces'' affecting oxidation character in beer.]* [https://suigenerisbrewing.com/index.php/2022/03/12/metabisulfite-7-year-experiment/ Dr. Bryan Heit explanations chemical oxidation pathways, and the use of metabisulfite to limit oxidation in packaging.]

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 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]]. While most ester formation and hydrolysis (breakdown) in beer during aging are mostly acid-catalysed, some of this activity is due to esterase enzymes. It has been shown that some esterases are released by yeast autolysis during beer aging. This release of esterase enzymes is strain dependent, with ale yeast having more potential for this activity than lager yeast. The enzyme is most active at 15-20°C and is destroyed by pasteurization <ref name="Vanderhaegen_2006" />.

Volatile phenols include 4-vinylphenol, 4-vinylguaiacol, 4-vinylcatechol, and their ethyl derivativeswhich are produced by ''Brettanomyces''. Although the production of volatile phenols during fermentation has been heavily researched, 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 level of 4-vinylguaiacol (4VG) saw a drop in 4VG 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 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). The phenols guaiacol and 4-methylphenol have been associated with darker roasted malts <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>. The degradation of 4VG has also been reported to be 25% after 20 days at room temperature, or 50% after 20 days at 40°C/104°F <ref name="Callemien_2010" />.

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" />.

- http://www.scieloprofessorbeer.brcom/scieloarticles/skunked_beer.php?pid=S0100-40422000000100019&script=sci_arttext&tlng=eshtml

http - https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/j.20501521-3765%2820011105%297%3A21%3C4553%3A%3AAID-0416CHEM4553%3E3.20020.tb00568.x/abstractCO%3B2-0

http - 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.professorbeerhomebrewersassociation.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/articlesdictionary/eIXf22Zwnt/ "Lightstruck", Craft beer and Brewing Magazine website]. * [https://www.youtube.com/skunked_beerwatch?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.html]

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" />.

===External Resources=== * [https://tapintohach.com/ Tap Into Hach; blog about understanding the technical aspects of and dealing with dissolved oxygen in beer.]