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http://www.professorbeer.com/articles/skunked_beer.html
====Damascenone====
The ketones beta-damascenone (rhubarb, red fruits, stewed apples; threshold of 25 ppb <ref name="hall_mitchell" />) can be formed from the oxidation of hop oils, as well as 3-methyl-butan-2-one and 4-methylpentan-2-one. In the case of damascenone, 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 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" />.
===Other Flavor and Non-flavor Compounds===
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, and is a result of ethanol being oxidized. The aldehyde trans-2-nonenal (threshold of 50-250 parts per trillion <ref name="hall_mitchell" />) is derived from oxidized fatty acids. It is the compound responsible for the cardboard (also described as "lipstick") flavor in aged beers and was once thought of as ''Cover lifespan 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 THP 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" />. One such aldehyde that impacts the flavor of aging beer is the malt derived compound, methional (reference THP pagethreshold of 4 ppb <ref name="hall_mitchell" />). Methional is formed by the degradation of methionine by oxidation. The flavor of methional is described as "mashed potato" or "potato chips", and is common in wheat based beers <ref name="hall_mitchell">[http://pubs.acs.org/doi/full/10.1021/jf049178l Further Insights into the Role of Methional and Phenylacetaldehyde in Lager Beer Flavor Stability. M. Soares da Costa, C. Gonçalves, A. Ferreira, C. Ibsen, diacetylP. Guedes de Pinho, proteinsand A. C. Silva Ferreira. 2004.]</ref><ref>[https://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars Robert Hall, enzymesAndy Mitchell. Beer Oxidation: Chemistry, gluten(?)Sensory Evaluation, effects of different levels of CO2and Prevention. HomebrewCon 2017 Seminar. 2017.]</ref>.''
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 diminish over time in packaged beer <ref name="Vanderhaegen_2006" />.
[[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.
====''Pediococcus'' 'sickness'====