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Hops
,→Hop Derived Compounds In Beer and Biotransformations
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" />. In addition to biochemical changes to hop compounds, yeast derived flavor compounds that are independent of hops such as esters and phenols can affect the overall sensory characteristics of hop compounds in beer. Therefore, different strains of yeast can greatly impact the overall organoleptic experience of hop flavors and aromas in finished beers <ref>[https://www.mdpi.com/2304-8158/12/5/1064 Kumar, A.; Warburton, A.; Silcock, P.; Bremer, P.J.; Eyres, G.T. Yeast Strain Influences the Hop-Derived Sensory Properties and Volatile Composition of Beer. Foods 2023, 12, 1064. https://doi.org/10.3390/foods12051064.]</ref>.
===Terpenes===