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Hops
,update to bioconversion by other yeast
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 hydrolized 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 to alpha-terpineol, which can then by further transformed to terpin. Geraniol can also be converted into citronellol. Linalool, nerol, and alpha-terpineol gradually decrease during fermentation and aging (perhaps being transformed into ethers), while nerol and citronellol gradually increase. Geraniol also decreases during fermentation, but not as drastically as linalool. Citronellol might be created from geraniol by glycosidic activity (although another study found that glycosidic activity in ''S. cerevisiae'' is not very strong). Likewise, dry hopping preserves linalool and alpha-terpineol, and limits citronellol <ref name="Praet_2012" />. Other yeast species can also convert monoterpenes. For example, a strain of ''Kluyveromyces lactis'' was found to reduce geraniol to citronellol. This strain and a strain of ''Torulaspora delbrueckii'' produced linalool from both geraniol and nerol, and could also form geraniol from nerol <ref>[https://www.ncbi.nlm.nih.gov/pubmed/10790686 Biotransformation of monoterpene alcohols by Saccharomyces cerevisiae, Torulaspora delbrueckii and Kluyveromyces lactis. King A1, Richard Dickinson J. 2000.]</ref>. Many species of ''Debaryomyces'', ''Kluyveromyces'', and ''Pichia'' were found to transform geraniol into linalool, and nerol into linalool and alpha-terpineol <ref>[https://www.ncbi.nlm.nih.gov/pubmed/18357555 Biotransformation of acyclic monoterpenoids by Debaryomyces sp., Kluyveromyces sp., and Pichia sp. strains of environmental origin. Ponzoni C, Gasparetti C, Goretti M, Turchetti B, Pagnoni UM, Cramarossa MR, Forti L, Buzzini P. 2008.]</ref>.
Sulfur based compounds known as ''thiols'' have also been shown to be produced by yeast fermentation from hop derived precursors (suspected to be S-glutathione). So far, science has found that these include the volatile thiols 3-sulfanyl-4-methylpentan-1-ol (3S4MP; grapefruit) and 3-sulfanyl-4-methylpentyl acetate (3S4MPA; passionfruit, grapefruit). These thiols were found in beers dry hopped separately with Amarillo, Hallertau Blanc, and Mosaic hop varieties. The amounts of these two thiols were higher than expected based on the content of these thiols in the hops alone <ref name="Cibaka_2016" />.