Changes

During fermentation, it is believed that esters are produced by yeast metabolism from hop compounds such as alpha acids, beta acids, polyphenols, and hydrocarbons because they are not found in unhopped beer or in hops themselves. These esters include ethyl 2-methylpropanoate (citrus, pineapple, sweetness), ethyl 2-methylbutanoate (citrus, apple-like), ethyl 3-methylbutanoate (citrus, sweetness, apple-like), 2-phenylethyl 3-methylbutanoate (floral, minty), and 4-(4-hydroxyphenyl)-2-butanone (citrus, raspberry) <ref>[http://pubs.acs.org/doi/abs/10.1021/jf061342c Comparison of the Odor-Active Compounds in Unhopped Beer and Beers Hopped with Different Hop Varieties. Toru Kishimoto, Akira Wanikawa, Katsunori Kono, and Kazunori Shibata. 2006.]</ref>. [https://researchmap.jp/?action=cv_download_main&upload_id=110987 Kishimoto et al.] found that some beer esters were increased when using unidentified pellet hops (described in the study only as "a bitter variety of 11.5% alpha acid") that were aged for 30 days at 40°C versus using fresh pellet hops that were stored cold (4°C). Specifically, in the beers that used the aged hops, they found a significant increase in citrus esters (ethyl 2-methylbutanoate, ethyl 3-methylbutanoate, and 4-(4-hydroxyphenyl)-2-butanone), and a decrease in "green, hop-pellet-like, and resinous" compounds such as myrcene and (Z)-3-hexen-1-ol in the beers made from aged hops. The beers brewed with aged hops were described as more citrusy, while the beers brewed with fresh hops were described as more "hop pellet-like", resinous, floral, and "green". The authors speculated that since these esters were not present in beers brewed without hops that they were derived from the humulone and lupulone oils in the hops during yeast fermentation <ref name="kishimoto_2007">[https://researchmap.jp/?action=cv_download_main&upload_id=110987 Odorants comprising hop aroma of beer: hop-derived odorants increased in the beer hopped with aged hops. Toru KISHIMOTO , Katsunori Kono , Kenkichi Aoki. 2007.]</ref>.  ====Thiols==== [https://researchmap.jp/?action=cv_download_main&upload_id=110987 Kishimoto et al.] found an increase in the thiol 3-methyl-2-butene-1-thiol (MBT) in beers that were brewed with unidentified pellet hops (described in the study only as "a bitter variety of 11.5% alpha acid") that were aged for 30 days at 40°C versus using fresh pellet hops that were stored cold (4°C). Interestingly, this thiol was higher in beers where the aged hops were added to the boil rather than when they were added after the wort was cooled. The authors were not able to determine whether or not the MBT was derived from yeast fermentation, or from boiling the hops, but aging the hops increased the precursors for MBT <ref name="kishimoto_2007" />. MBT has been described as the thiol that produces the "skunky" aroma in light struck beer <ref>[http://www.aroxa.com/beer/beer-flavour-standard/3-methyl-2-butene-1-thiol/ 3-methyl-2-butene-1-thiol. Aroxa website. Retrieved 11/22/2016.]</ref>.