Changes

Iso-α-acids, which are the main component of bitterness in beer, degrade over time in the bottle. Iso-α-acids are comprised of ''trans'' isomers and ''cis'' isomers of isohumulone, isocohumulone, and isoadhumulone. The ''trans'' forms of these iso-α-acids degrades much faster than the ''cis'' versions. This degradation is attributed to oxidative reactions. According to one study this included oxidative reactions in the mash when the mash was exposed to oxygen versus flushed with CO2. Although the difference was slight, the mash that was exposed to oxygen had slightly more iso-α-acids degradation over time in the packaged beer than a mash that was flushed with CO2 (this degradation was emphasized more with time rather than hotter temperature once packaged). Beers stored at 20°C had their ''trans'' iso-α-acids degrade gradually (10% at day 20, 25% at day 50, 35% at day 100, and 45% at day 200), while ''cis'' iso-α-acids stayed stable. This rate of decay was nearly tripled when the beer was stored at 30°C. The ratio of remaining ''trans'' iso-α-acids to the more stable ''cis'' iso-α-acids also corresponded to the stale flavor of beer, and thus this ratio was offered as a quantitative measurement of the oxidation/staling of beer <ref>[http://www.asbcnet.org/publications/journal/vol/abstracts/1207-01a.htm A New Parameter for Determination of the Extent of Staling in Beer. Shigeki Araki, Masachika Takashio, and Ken Shinotsuka]. 2002. DOI: 10.1094/ASBCJ-60-0026.</ref>. The ''cis'' isomers are generally considered more bitter than the ''trans'' isomers, so aged beer will still maintain some amount of bitter character <ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2011.tb00471.x/abstract 125th Anniversary Review: The Role of Hops in Brewing. Schönberger and Kostelecky. 2012. DOI: 10.1002/j.2050-0416.2011.tb00471.x.]</ref>.