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* [https://encyclopedia.pub/item/revision/d2232da8945e383a5a0cdf189b38a2d8 "Bottle Conditioning," Topic Review by Kateřina Štulíková and Pavel Dostálek, Scholarly Community Encyclopedia.]
* [[Packaging#Oxygen_exposure|Packaging and Oxygen Exposure.]]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/2282287018466166/ Justin Amaral's can conditioning project with DO tracking for canned conditioned beers on MTF.]
Cellaring, or extended age in the bottle once the beer is ready to drink, is common for many mixed fermentation beers. Cellaring is typically carried out at cooler temperatures.
===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>.
===Corks vs Caps===
====CO<sup>2</sup> Loss Over Time====
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 CO<sup>2</sup> 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 CO<sup>2</sup> 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 CO<sup>2</sup>.
The construction of the cork itself is a variable that makes it difficult to predict the exact rate of CO<sup>2</sup> loss. Corks are composed of two distinct parts: the mushroom of the cork is made up of agglomerated cork small granules, while the foot of the cork is made up of two large cork slices. This lower part is made up of several [https://en.wikipedia.org/wiki/Lenticel lenticels], which are parts of the plant that allow gasses to flow in and out of the plant. These lenticels vary from cork to cork. Nevertheless, a model has been proposed by Liger-Belair et al. that estimates the amount of CO<sup>2</sup> loss over time. In this model, two other variables have been identified as playing a large role in how much CO<sup>2</sup> is lost: storage temperature and bottle size. The warmer the storage temperature, the faster the rate is of losing of CO<sup>2</sup>, and the larger the bottle volume the slower the rate is of losing CO<sup>2</sup>. Below are some estimated CO<sup>2</sup> levels based on the Liger-Belair model in g/L and then converted to volumes in parenthesis at various points in time. The first table shows the estimated amount of CO<sup>2</sup> loss when stored at three different temperatures (4 °C, 12 °C, and 20 °C). The second table shows the estimated amount of CO<sup>2</sup> loss in different sized bottles (1.5 L, 750 mL, and 350 mL) when stored at 12 °C <ref name="Liger-Belair_2011" />:
[http://www.sciencedirect.com/science/article/pii/S0740002014002548 Spitaels et al., 2015 microbes in bottles of gueuze]
[https://www.facebook.com/groups/MilkTheFunk/posts/7284659538228864/ Anecdotal evidence that ''Brettanomyces'' can reverse discoloration from enzymatic browning.]
See also [[Commercial Sour Beer Dregs Inoculation]].
* [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.]
===General Effects of Temperature===
[https://en.wikipedia.org/wiki/Polyphenol Polyphenols] are a large group of organic chemicals characterized by many phenol structures combined. Subclasses of polyphenols include tannic acid, tannins, and flavonoids <ref name="wikipedia_polyphenols" /><ref>[https://en.wikipedia.org/wiki/Flavonoid Flavonoid. Wikipedia website. Retrieved 05/02/2017.]</ref>.
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" />.
Higher temperatures increase the rate of oxidized polyphenols. In one study on aged lagers, 6.5% of the polyphenols were oxidized after 5 days at 40°C/104°F, but only 0.6% of the polyphenols were oxidized after 9 months at 20°C/68°F <ref name="Callemien_2010" />.
===Hop Compounds===
- https://www.tandfonline.com/doi/full/10.1080/03610470.2019.1705037?needAccess=true
- https://www.tandfonline.com/doi/abs/10.1080/03610470.2020.1843898
====IBU Degradation====
====Lightstruck====
- 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.scieloprofessorbeer.brcom/scieloarticles/skunked_beer.php?pid=S0100-40422000000100019&script=sci_arttext&tlng=eshtml
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.]
====Damascenone====
Products of Maillard reactions, which include a diverse range of reactions, have also been found in beer, although research in this area is limited. Some Maillard compounds found in aging beer remain under taste threshold, for example, furfural and 5-hydroxymethyl furfural. It is hypothesized that a wide range of unknown Maillard reactions and their intermediates might play a role in the aging of beer. In particular, the bready, sweet, caramel and wine-like character of stale beer might be due to Maillard reactions <ref name="Vanderhaegen_2006" />.
In general, lower storage temperatures preserve hop compounds. Cans also help preserve some hop compounds versus bottles because bottle caps can strip certain hop compounds such as myrcene and caryophyllene when stored at room temperature (less so when stored cold). For example, one study found a moderate amount of degradation of humulinones, iso-α-acids, and residual α-acids when dry hopped beers were stored at 20°C versus 3°C. There was also an overall decrease in hop aroma compounds during warm storage, with some esters, hop monoterpenes, and sesquiterpenes showing poor storage stability compared to other ester compounds, monoterpene alcohols, and ketones which increased during warm storage. After 10 months of storage, the dry hopped beers stored at 20°C had a significant drop in floral, citrus and tropical fruit notes when compared to the same beers stored at 3°C <ref>[https://onlinelibrary.wiley.com/doi/full/10.1002/jib.667 Kemp, O., Hofmann, S., Braumann, I., Jensen, S., Fenton, A., and Oladokun, O. (2021) Changes in key hop-derived compounds and their impact on perceived dry-hop flavour in beers after storage at cold and ambient temperature. J. Inst. Brew., https://doi.org/10.1002/jib.667.]</ref>.
[[Tetrahydropyridine]] (THP) is a compound that tastes like Cheerios® or corn tortilla chips that often develops soon after packaging beers that contain ''Brettanomyces'' or heterofermentative ''Lactobacillus''. It is usually detected after swallowing the beer. This compound is stimulated by oxygen, and often ages out after a few months. See the [[Tetrahydropyridine]] page for more information.