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Hops
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The flavor and aroma compounds found in leaf/pellet hops are different than the hop-derived flavor and aroma compounds found in finished beer (other than in the case of dry hopping). The brewing process (particularly boiling), and fermentation greatly affect the composition of flavor and aroma compounds that are found in beer. For example, boiling wort and hops isomerizes non-bitter alpha acids into bitter iso-alpha acids. During the boiling of the wort, many compounds found in hops are evaporated, such as many of the various sulfur compounds found in hops. The terpene hydrocarbons which make up most of the hop oil content in hops (myrecene, humulene, and caryophyllene) are completely removed by fermentation. It is believed that these terpene hydrocarbons stick to the yeast cells and fall out of solution during fermentation <ref name="Praet_2012">[http://www.sciencedirect.com/science/article/pii/S1373716311001636 Biotransformations of hop-derived aroma compounds by Saccharomyces cerevisiae upon fermentation. Tatiana Praet, Filip Van Opstaele, Barbara Jaskula-Goiris, Guido Aerts, Luc De Cooman. 2012.]</ref>.
A "biotransformation" is any change in a chemical's structure that is initiated by a living organism <ref>[https://en.m.wikipedia.org/wiki/Biotransformation "Biotransformation". Wikipedia. Retrieved 05/10/2019.]</ref>. It has been hypothesized that biotransformations of some kind are taking place in beer during fermentation and explain changes to hop compounds during fermentation and beer storage. Some carbonyl compounds found in hops (citral, geranial, nerol, [https://en.wikipedia.org/wiki/Citronellal citronellal], and methyl ketones) can be used as a food source by yeast during fermentation. ''Cyclic ethers'' such as linalool oxides, karahana ether, hop ether, and rose oxide (aroma of roses <ref>[http://www.thegoodscentscompany.com/data/rw1035651.html "(Z)-rose oxide ". Good Scents Company. Retrieved 12/29/2016.]</ref>), increase after fermentation and have been identified as secondary metabolites produced by yeast during metabolism from hop derived precursors. ''Esters'' found in hops can be converted into ethyl esters by yeast during fermentation; for example, geranyl esters found in Cascade hops can be hydrolyzed into geraniol (flowery). The terpenoid [https://en.wikipedia.org/wiki/Citronellol citronellol] (citrus and floral <ref>[https://eic.rsc.org/magnificent-molecules/citronellol/2000020.article "There are no flies on Emma Stoye". Emma Stoye. Education in Chemistry website. 06/01/2016. Retrieved 01/10/2017.]</ref>) can be esterified by yeast fermentation into citronellyl acetate (fresh, rosy, fruity odor reminiscent of geranium oil <ref>[https://shop.perfumersapprentice.com/p-6034-citronellyl-acetate.aspx "Citronellyl acetate". Perfumers Apprentice website. Retrieved 01/10/2017.]</ref>). Yeast strains differ in their ability to convert these compounds. For example, one study found that lager yeast was able to form acetate esters of geraniol and citronellol, but ale yeast was not <ref name="Praet_2012" />.
Terpenes and terpenoids (monoterpene alcohols) can also be transformed by fermentation. Studies have found that geraniol and nerol can transform into linalool by a strain of ''S. cerevisiae'', as well as nerol and linalool into alpha-terpineol, which can then be further transformed to terpin. Geraniol can also be converted into citronellol, and the content of geraniol and citronellol can be increased in finished beer by increasing the initial content of geraniol, which is found in higher quantities in some varieties of hops (Citra, for example). Linalool, nerol, and alpha-terpineol gradually decrease during fermentation and aging (perhaps being transformed into [https://en.wikipedia.org/wiki/Ether ethers], which is a class of organic compound that contains an oxygen atom connected to two alkyl or aryl groups), while nerol and citronellol gradually increase. Geraniol also decreases during fermentation, but not as drastically as linalool. It has been hypothesized that the bioconversion of geraniol into citronellol could be by means of glycosidic activity (although another study found that glycosidic activity in ''S. cerevisiae'' is not very strong). Post-fermentation dry hopping preserves linalool and alpha-terpineol, and limits citronellol to trace levels <ref name="Praet_2012" />.