Changes

As beer ages, different flavor and aroma compounds both deteriorate and form over time. This is a complex collection of changes with many variables affecting different compounds in different ways. As molecules deteriorate or are formed, their associated flavors not only disappate dissapate 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 equilibiumequilibrium. The two largest factors that affect beer aging are temperature and dissolved oxygen in the package <ref name="Vanderhaegen_2006" />. Unfortunately, most beer studies have focused on lager beers and some on ales and strong beers. Research on beers containing living ''BrettanoycesBrettanomyces'' or beers that are at a lower pH of 3-3.8 is limited.  Overview of compounds formed in beer during storage <ref name="Vanderhaegen_2006" />:* Linear aldehydes: acetaldehyde, trans-2-octenal, trans-2-nonenal, derivatives of nonadienal and decadienal.* Strecker aldehydes: 2 and 3-methyl-butanal, 2-phenylacetaldehyde, benzaldehyde, 3-(methylthio)propionaldehyde.* Ketones: trans-beta-damascenone, 3-methyl-2-butanone, 4-methyl-2-butanone, 4-methyl-2-pentanone, diacetyl, 2,3-pentanedione.* Cyclic acetals: 2,4,5-trimethyl-1,3-dioxolane, 2-isopropyl-4,5-dimethyl-1,3-dioxolane, 2-isobutyl-4,5-dimethyl-1,3-dioxolane, 2-sec butyl-4,5-dimethyl-1,3-dioxolane.* Heterocyclic compounds: furfural, 5-hydroxymethyl-furfural, 5-methyl-furfural, 2-acetyl-furan, 2-acetyl-5-methyl-furan, 2-propionylfuran, furan, furfuryl alcohol, furfuryl ethyl ether, 2-ethoxymethyl-5-furfural, 2-ethoxy-2,5-dihydrofuran, maltol, dihydro-5,5-dimethyl-2(3H)-furanon, 5,5-dimethyl-2(5H)-furanon, 2-Acetylpyrazine, 2-methoxypyrazine/2,6-dimethylpyrazine, trimethylpyrazine, tetramethylpyrazine.* Ethyl esters: ethyl 3-methyl-butyrate, ethyl 2-methyl-butyrate, ethyl 2-methyl-propionate, ethyl nicotinate, diethyl succinate, ethyl lactate, ethyl phenylacetate, ethyl formate, ethyl cinnamate.* Lactones: nonalactone, hexalactone.* Sulfur compounds: dimethyl trisulfide, 3-Methyl-3-mercaptobutylformate.

Heavier polyphenols such as procyanidin and prodelphinidin also contribute to beer haze (see [[Aging_and_Storage#Haze|Haze]] above) and darkening in color after packaging. While beer darkening is also caused by oxygen and storage at higher temperatures, and possibly also by slow Malliard reactions, yellow-brown pigments can also come from oxidized polyphenols. The catechin polyphenol from hops can oxidize into yellow pigments such as dehydrodicatechin . This oxidative reaction occurs more so at a pH of 6 rather than at a pH of 3, and is greatly inhibited by storing beer cold. These reactions have been studied more so in wine and port; anthocyanidins bind with flavanols or acetoacetic acid, producing yellow-brown pigments. Pyruvic acid, vinylphenol, cinnamic acid, glyoxylic acid, ad furfural derivatives have also been identified as precursors to yellow-brown pigmentation in wine <ref name="Callemien_2010" />.

''Cover lifespan and effects of: THP (reference THP page), diacetyl, proteins, enzymes, gluten(?), effects of different levels of CO2.'' Several volatile [https://en.wikipedia.org/wiki/Carbonyl_group carbonyl compounds] are formed during beer aging. Acetaldehyde is one such major flavor compound formed during aging and one of the first documented in science. The aldehyde trans-2-nonenal is the compound responsible for the cardboard flavor in aged beers and was once thought of as ''the'' molecule responsible for beer staling. This interpretation was updated by studies by Van Eerde et al. and Narziss et al. where it was shown that trans-2-nonenal formation was dependent on temperature (a lot is formed at 40°C, but none is formed at 20°C), and other authors observed that trans-2-nonenal develops independently of dissolved oxygen. Carbonyl scavengers such as [https://en.wikipedia.org/wiki/Hydroxylamine hydroxylamine] might help to diminish the effects of carbonyl staling compounds such as trans-2-nonenal. Other aldehydes ("strecker" aldehydes) are formed during beer storage and increase depending on oxygen. While many of them are not impactful on flavor, their presence is indicative of an oxidation issue <ref name="Vanderhaegen_2006" />. Ketones are also formed during the storage of bottled beer. The compound beta-damascenone (rhubarb, red fruits, strawberry) can be formed, as well as 3-methyl-butan-2-one and 4-methylpentan-2-one. The buttery tasting compounds diacteyl and 2,3-pentanedione are also formed during beer aging. These are more pronounced in beers that have higher dissolved oxygen during packaging. Diacetyl, in particular, can form levels that are above flavor threshold <ref name="Vanderhaegen_2006" />.