Changes

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 (known as "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 diacetyl 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" />.  Many other compounds increase or diminish over time in packaged beer. Cyclic acetals (2,4,5-trimethyl-1,3-dioxolane, 2-isopro-pyl-4,5-dimethyl-1,3-dioxolane, 2-isobutyl-4,5-dimethyl-1,3-dioxolane and 2-sec-butyl-4,5-dimethyl-1,3-dioxo-lane) in packaged beer that has been in contact with oxygen. Heteroclyclic compounds (furan, furfural, and furanone based compounds) increase regardless of oxygen exposure but generally remain far below flavor threshold, however, their presence correlates with stale flavors in sensory tests and can thus serve as indicators for stale beer. Pyrazines such as pyrazine, 2-ethyl-6-methylpyrazine, 2-ethyl-5-methylpyrazine, 2-acetylpyrazine, 2,3-dimethylpyrazine, 2,5-dimethylpyrazine, 2-ethyl-3,6-dimethylpyrazine, 2-ethyl-3,5-dimethylpyrazine dimish over time in packaged beer <ref name="Vanderhaegen_2006" />.