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update to Bottles vs Kegs vs Cans vs PET
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" />: