13,691
edits
Changes
Hops
,→Isomerization of Alpha Acids
[[File:Jester King aged hops.jpg|thumbnail|300|[https://web.archive.org/web/20180204131402/http://jesterkingbrewery.com/home-for-our-aged-hops Jester King Brewery in Austin, Texas aging their own hops in an old barn. Image provided by Jeffrey Stuffings.]]]
'''Hops''' are the flowers (also called seed cones or strobiles) of the female [https://www.reference.com/science/difference-between-monoecious-dioecious-6273de1768bd915f dioecious] (meaning that they have separate male and female plants) plant ''Humulus lupulus'' <ref>[https://en.wikipedia.org/wiki/Hops "Hops". Wikipedia. Retrieved 06/10/2017.]</ref>, and are used in brewing for flavor as well as for antimicrobial properties. Although bitterness from boiling hops is generally not desired in sour beers, sour and funky brewers can use hops to help regulate lactic acid bacteria and control acid production to desired levels, especially in aged [[Mixed-Fermentation Sour Beer|mixed-fermentation]] or [[Spontaneous Fermentation|spontaneous fermentation]] beers. Additionally, it may be argued that the earthy bitterness from aged hops is desired for lambic based styles (see [[Hops#Aged_Hops_in_Lambic|Hops in lambic]] below). Potentially other mixed fermentation styles can benefit from some degree of bitterness either from aged or fresh hops such as saisons, farmhouse ales, and experimental styles. So while the mantra for sour beer is that "bitterness and sour don't work together", there are certainly exceptions to this rule. Brewers who are interested in rapid acid production using quick/kettle souring techniques such as [[Wort Souring|wort souring]] may wish to limit or avoid hop use before acidifying so that sufficient acid is produced quickly.
===Acids===
'''Alpha acids''' (also called "humulones" and abbreviated as "α-acids") in hops mostly consist of humulone, cohumulone, and adhumulone. Trace amounts of other forms of humulones are also present but are difficult to quantify and currently have limited research: posthumulone, perhumulone, adprehumulone, and acetohumulone <ref name="Hao_2020">[https://www.tandfonline.com/doi/full/10.1080/03610470.2020.1712641 Junguang Hao, R.A. Speers, Heliang Fan, Yang Deng & Ziru Dai (2020) A Review of Cyclic and Oxidative Bitter Derivatives of Alpha, Iso-Alpha and Beta-Hop Acids, Journal of the American Society of Brewing Chemists, 78:2, 89-102, DOI: 10.1080/03610470.2020.1712641.]</ref><ref name="Leker_2022">[https://www.tandfonline.com/doi/abs/10.1080/03610470.2022.2079944 Jeremy Leker & John Paul Maye (2022) Discovery of Acetohumulone and Acetolupulone a New Hop Alpha Acid and Beta Acid, Journal of the American Society of Brewing Chemists, DOI: 10.1080/03610470.2022.2079944 ]</ref>. The ratio of these individual acids to each other can vary based on hop variety much like total iso-α-acid percent, though generally the primary acids are humulone and cohumulone. Cohumulone has been identified by some researchers as a source of a more harsh bitterness, although similar research contradicts this statement <ref>[http://www.scielo.br/scielo.php?pid=S0100-40422000000100019&script=sci_arttext&tlng=es Fundamentals of beer and hop chemistry. Denis De Keukeleire. 1999.]</ref>. While Being hydrophobic, alpha acids are mostly insoluble in wort at typical brewing pH (alpha acids become much more soluble as the pH rises towards 5.9 to 7, which is not typical for wort production <ref name="Bastgen_2019">[https://www.tandfonline.com/doi/full/10.1080/03610470.2019.1587734 Influencing Factors on Hop Isomerization Beyond the Conventional Range. Nele Bastgen, Tobias Becher & Jean Titze. 2019. DOI: https://doi.org/10.1080/03610470.2019.1587734.]</ref>). During boiling, the alpha acids are isomerized into iso-alpha acids (also called isohumulones) which are formed during boiling that are soluble. Isomerization leads to roughly a 70%/30% split between diastereomeric isomers called ''cis'' and ''trans'' iso-α-acids respectively, with ''cis'' iso-α-acids being more stable over time and more bitter<ref name="Schönberger and Kostelecky, 2012"> [http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2011.tb00471.x/abstract Schönberger and Kostelecky, 2012]</ref>. Alpha acids themselves do not taste bitter, but isomerized alpha acids (iso-α-acids/isohumulones) contribute to the bitterness of beer and have antimicrobial properties. Isocohumulone is often cited as being more harshly bitter than the other iso-α-acids, but studies of taste perception of individual iso-α-acids have not agreed with this. However isocohumolone , iso-cohumolone is slightly more soluble than the other acids and therefore a hop with a higher cohumulone composition may result in a beer with higher iso-α-acid for hops of equal iso-α-acid percent and use in brewing but different iso-α-acid breakdown<ref name="Schönberger and Kostelecky, 2012"/>. Alpha acids are susceptible to oxidation and the alpha acid content of a hop will decrease with storage.
===Oils===
* [http://brulosophy.com/other-projects/hop-chronicles/ Brulosophy's "The Hop Chronicles", an attempt to characterize hop flavor and aroma.]
* [https://www.hopsteiner.com/blog/blending-hops-to-match-target-hop-profile/ Hopsteiner presentation on specific blends of hops that match the flavor profile of another hop.]
===International Bitterness Unit===
Beer bitterness is often described in terms of International Bitterness Units (IBU), or more accurately, Bitterness Units (BU). The European Brewery Convention has adopted the [https://europeanbreweryconvention.eu/new-international-method-on-bitter-compounds-in-dry-hopped-beers/ “E.B.C. Bitterness Units,”] determined in a similar way and recently updated for dry hopped beers to account for humulinones, as a uniform method that best expresses the true bitter flavor value of beer <ref name="asbc_ibu">[https://www.asbcnet.org/Methods/BeerMethods/Pages/default.aspx ASBC Methods of Analysis website. Retrieved 02/11/2022.]</ref>. These measurements seek to measure the amount of iso-alpha acids, which contribute the majority of bitterness to beer. There are, however, other compounds that contribute to bitterness, such as oxidized alpha and beta acids (see [[Hops#Chemistry_and_Characteristics|Aged Hops]] below). These methods include using [https://en.wikipedia.org/wiki/Spectrophotometry spectrophotometry], [https://en.wikipedia.org/wiki/High-performance_liquid_chromatography High-Performance Liquid Chromatography-Ultraviolet (HPLC-UV)], and [https://en.wikipedia.org/wiki/Liquid_chromatography%E2%80%93mass_spectrometry liquid chromatography–mass spectroscopy (LC–MS)]. The ASBC describes these methods in depth on [https://www.asbcnet.org/Methods/BeerMethods/Pages/default.aspx their website] (see method 23; requires membership to read). While many brewers argue that the IBU measurement is not that helpful for communicating bitterness to consumers, it is generally agreed upon that IBU measurements are very useful to brewers who are seeking consistency in their products <ref>[https://beerandbrewing.com/dictionary/eej03p6ZUI/ The Oxford Companion to Beer definition of International Bitterness Units (IBUs). Retrieved 02/12/2022.]</ref>.
Compounds other than iso-alpha acids present several challenges to traditional methods of measuring BU's. Many hop compounds other than iso-alpha acids that have varying levels of perceived bitterness are detected at the same wavelength as iso-alpha acids using spectrophotometry. In addition, oxidized alpha acids are known to contribute to bitterness. As a result, updated methods of using High-Performance Liquid Chromatography-Ultraviolet (HPLC-UV) and liquid chromatography–mass spectroscopy (LC–MS) are generally recommended for measuring bitterness units in dry hopped beers (although models that account for bitterness contribution from compounds other than iso-alpha acids have not yet been completed). In addition to these challenges, iso-alpha acids and other hop compounds that contribute to bitterness degrade over time, thus the perceived bitterness of beer tends to become weaker as beer ages <ref>[https://www.chromatographyonline.com/view/liquid-chromatography-mass-spectrometry-analysis-of-hop-derived-humulone-and-isohumulone-constituents-in-beer-the-bitter-truth-of-hops-utilization-during-brewing Liquid Chromatography–Mass Spectrometry Analysis of Hop-Derived Humulone and Isohumulone Constituents in Beer: The Bitter Truth of Hops Utilization During Brewing. Bruce C. Hamper, Nicholas Viriyasiri, Aaron Boland, Lorna Espinosa, Hunter J. Campbell, Kurt Driesner, Michael McKeever. January 1, 2022. LCGC Europe, January 2022, Volume 35, Issue 01. Pages: 32–37.]</ref><ref name="asbc_ibu"/>.
Measuring BU's directly requires costly laboratory equipment, and so researchers and enthusiasts have made some progress in creating models that attempt to estimate bitterness units. These models are generally based off of boil time, alpha acid percent of the hops used, weight of the hops used, post-boil volume of wort, and gravity of the wort. These include the [http://www.realbeer.com/hops/research.html Tinseth] model, the Garetz model, and the [http://realbeer.com/hops/FAQ.html#units Rager] model. These models are often used in brewing software, such as [https://beersmith.com/blog/2021/09/23/hop-utilization-models-for-beer-brewing-compared/ BeerSmith™] and [https://www.brewersfriend.com/ibu-calculator/ Brewer's Friend]. [https://alchemyoverlord.wordpress.com/2021/11/10/ibus-and-the-smph-model/ Another model called the 'SMPH' model] has been proposed by John Paul Hosom as potential updated model that addresses their limitations in regard to newer hopping techniques such as whirlpool hopping and dry hopping, as well as accounting for IBU's from non-iso-alpha acid compounds Other limitations include differences in brewhouse size and efficiencies. For example, the Tinseth model was developed on a homebrew system using whole leaf hops (see the Experimental Brewing podcast interview with Glenn Tinseth link below).
See also:
* [http://scottjanish.com/dry-hopping-effect-bitterness-ibu-testing/ "Dry Hopping Effect on Bitterness and IBU Testing" by Scott Janish.]
* [https://www.experimentalbrew.com/podcast/episode-32-ibu-lie Experimental Brewing interview with Glenn Tinseth (52 minutes in).]
* [https://www.craftbeer.com/craft-beer-muses/beer-ibus-fact-fiction-misconceptions "Last Call for IBUs: Fact, Fiction and Their Impact on Your Beer," by Chris McClellan.]
* [http://thebrulab.libsyn.com/episode-076-a-modern-method-for-predicting-ibu-w-john-paul-hosom The Brü Lab Podcast Episode 076 | A Modern Method For Predicting IBU w/ John Paul Hosom]
==Antimicrobial Properties==
Hops are known to have antimicrobial properties against Gram-positive bacteria. This includes bacteria which that can be present in beer both as spoilage organisms and as intentionally added in sour and mixed fermentation beer such as ''[[Lactobacillus]]'' and ''[[Pediococcus]]''. Certain other Gram-negative bacteria found in beer , such as ''Acetobacteraciae'' are Gram-negative , and are not susceptible to the antimicrobial properties of hops<ref name="Hough_1957">[https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1957.tb06267.x J. S. Hough, B.Sc, Ph.D., G. A. Howard, M.Sc., Ph.D., and C. A. Slater, Ph.D. 1957.]</ref><ref name="Macrae_1964">[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.1964.tb02001.x SIGNIFICANCE OF THE USE OF HOPS IN REGARD TO THE BIOLOGICAL STABILITY OF BEER: I. REVIEW AND PRELIMINARY STUDIES. R. M. Macrae. 1964.]</ref>. Certain Gram-positive bacteria strains that have adapted to the brewing environment, such as some strains of ''Lactobacillus brevis'' and ''L. paracasei'' <ref>[https://proceedings.science/slacan-2023/papers/prospects-for-the-development-of-a-new-hopped-and-functional-sour-beer-survival?lang=en Marcos Edgar Herkenhoff; Susana Marta Isay Saad. PROSPECTS FOR THE DEVELOPMENT OF A NEW HOPPED AND FUNCTIONAL SOUR BEER: SURVIVAL OF PROBIOTIC STRAINS OF LACTICASEIBACILLUS PARACASEI SUBSP. PARACASEI IN HIGH HOPPED BEERS (HUMULUS LUPULUS L.).. In: CADERNO DE RESUMOS DO 15° SLACAN - SIMPóSIO LATINO AMERICANO DE CIêNCIA DE ALIMENTOS E NUTRIçãO, 2023, Campinas. Anais eletrônicos... Campinas, Galoá, 2023. Disponível em: <https://proceedings.science/slacan-2023/trabalhos/prospects-for-the-development-of-a-new-hopped-and-functional-sour-beer-survival?lang=en> Acesso em: 23 nov. 2023.]</ref>, are known to be more resistant to the antimicrobial effects of hops. Multiple mechanisms The antimicrobial effect is characterized as inhibiting the growth and lactic acid production of lactic acid bacteria, however, this does not always also include cell death as ''Lactobacillus'' that has been inhibited by hops can later be revived <ref name="Macrae_1964" />. The effectiveness of hops to inhibit Gram-positive bacteria is also dependent on pH; at a lower pH, hops have a greater effect on inhibiting bacteria <ref name="Almaguer_2015" />. Hop extracts have also been proposed demonstrated to explain why hops are antimicrobially activebe antimicrobial <ref>[https://www.sciencedirect.com/science/article/abs/pii/S0963996923003770 Yan Li, Sevim Dalabasmaz, Sabrina Gensberger-Reigl, Marie-Louise Heymich, Karel Krofta, Monika Pischetsrieder. Identification of colupulone and lupulone as the main contributors to the antibacterial activity of hop extracts using activity-guided fractionation and metabolome analysis. Food Research International. 2023.]</ref>.
Multiple mechanisms have been proposed to explain why hops are antimicrobially active. One mechanism of the antimicrobial activity of hops is due to the role of iso-alpha alpha acids and possibly similar hop acids (such beta acids and iso-α-oxidized hop acids) as ionophores, or compounds which can transport ions across cell membranes. While their antimicrobial properties are strong, alpha and beta acids in beer and wort and their effects on brewing are generally disregarded because they do not solubilize <ref name="Fernandez and Simpson, 1993"> [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.1993.tb02782.x/full Fernandez and Simpson (1993)] </ref> <ref name="Sakamoto and Konings, 2003"> [http://www.sciencedirect.com/science/article/pii/S0168160503001533 Sakamoto and Konings (2003)] </ref>. The protonated iso-α-acid (the form of the acid with an associated H+ ion, an H+ ion is a proton) is the antimicrobially active form. This means that for a beer with a given iso-α-acid concentration, the antimicrobial effects will be stronger at lower pH values because a greater percentage of the acid will be protonated. Protonated iso-α-acids act against bacteria by crossing into the cell and dissociating (releasing H+ ions from the iso-α-acidand decreasing the pH within the cell <ref name="zhao_1027">[https://www.frontiersin.org/articles/10.3389/fmicb.2017.00239/full#B28 Heterogeneity between and within Strains of Lactobacillus brevis Exposed to Beer Compounds. Yu Zhao, Susanne Knøchel and Henrik Siegumfeldt. 2017. DOI: https://doi.org/10.3389/fmicb.2017.00239.]</ref>), therefore disrupting the cellular proton gradient which is necessary for cells to function, before binding an equal charge in metal ions and crossing back out of the cell. Cells with a resistance to hop bitter acids are better able to eject disassociated iso-α-acids from the cell and therefore preserve their proton gradients. The mechanism to expel iso-α-acids appears to be specific toward this type of compound rather than by a more general antimicrobial resistance mechanism such as multi-drug resistant bacteria possess<ref name="Sakamoto and Konings, 2003"/>. The anti-microbial power of iso-α-acids is pH dependent. At a higher pH (5.6) iso-α-acids begin to lose their anti-microbial properties, but at a typical beer pH (4.3) iso-α-acids inhibited a sample of 6 strains of ''L. brevis'' that exhibited a range of general hop tolerance in one study <ref name="zhao_1027" />. Hop resistant bacteria cultured in the absence of hop acids can lose their resistance if grown in an environment without antibacterial hop compounds<ref name="Fernandez and Simpson, 1993"/> and some hop resistant microbes need to be acclimated to hop acids by growth in sub-limiting levels of antibacterial acids before they are able to resist higher levels<ref name="Sakamoto and Konings, 2003"/>.
Another antimicrobial mechanism resulting from oxidative stress has been attributed to both iso-α-acids and humulinic acids<ref name="Schurr et al, 2015"> [http://www.sciencedirect.com/science/article/pii/S0740002014002470 Benjamin C. Schurr et al, Hannes Hahne, Bernhard Kuster, Jürgen Behr, Rudi F. Vogel. Molecular mechanisms behind the antimicrobial activity of hop iso-α-acids in Lactobacillus brevis.Food Microbiology, Volume 46, (2015), Pages 553-563, ISSN 0740-002., https://doi.org/10.1016/j.fm.2014.09.017.] </ref>. Humulinic acids are either not bitter tasting or much less bitter than iso-α-acids but are similar in structure to and are formed from the degradation of iso-α-acids as well as during the aging of hops<ref>[https://www.sciencedirect.com/science/article/abs/pii/S0040402001981992 The absolute configuration of the isohumulones and the humulinic acids. D.De Keukeleire, M.Verzele. 1971. https://doi.org/10.1016/S0040-4020(01)98199-2.]</ref>. Humulinic acids consist of the "cis-" and" trans-" forms of humulinic acid, cohumulinic acid and adhumulinic acid <ref>[https://www.sciencedirect.com/science/article/pii/B9781855734906500088 "8 - The chemistry of hop constituents," Editor(s): Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes, Roger Stevens, In Woodhead Publishing Series in Food Science, Technology and Nutrition, Brewing. Woodhead Publishing, 2004. Pages 255-305. ISBN 9781855734906. https://doi.org/10.1533/9781855739062.255.]</ref>. This oxidative stress-driven antimicrobial activity is due to the potential for oxidation-reduction (redox) reactions within bacterial cells between Mn2+ ions and these specific hop acids. The redox potential is due to different conditions inside (higher pH, higher Mn2+) and outside (lower pH, lower Mn2+) of the bacterial cell<ref name="Behr and Vogel, 2010"> [http://aem.asm.org/content/76/1/142.short Behr and Vogel, (2010)] </ref><ref name="Schurr et al, 2015"/>. Iso-α-acids or humulinic acids passing into the cell, form complexes with Mn2+ and transfer electrons out of the cell<ref name="Behr and Vogel, 2010"/>. By targeted molecular modifications [http://www.sciencedirect.com/science/article/pii/S0740002014002470 Schurr et al. (2015)] determined that the Mn oxidative stress-driven antimicrobial effect of iso-α-acids was more important than the antimicrobial effect of the ionophore proton transfer discussed above in the overall antimicrobial activity of hops. Thus, the antimicrobial effects of humulinic acids have been found to be even stronger than iso-alpha acids, suggesting that aged hops retain at least some antimicrobial properties at least partially from humulinic acids <ref name="Schurr et al, 2015"/>. One study found that adding Mn2+ to a lager beer minimized this effect and slightly increased the cell count of 6 strains of ''L. brevis'' that had a range of hop tolerances <ref name="zhao_1027" />. The oxidized forms of hop acids have been shown to have a limited inhibitory effect on Gram-positive bacteria. This might explain the anecdotal experiences of some brewers that have tried using aged hops that were high alpha varieties and produced beer that wasn't sour. Stevens et al. (1961) reported that a strain of ''Lactobacillus'' that was cultured from infected beer was inhibited by alpha acids at 40 ppm, beta acids at 10 ppm, iso-alpha acids at 160 ppm, and oxidized beta acids (cohulupone) at 200 ppm. So, while the oxidized beta acids had the least inhibitory power, a high concentration was still inhibitory <ref name="Stevens_1961">[https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1961.tb01830.x R. Stevens, Ph.D., F.R.I.C., and D. Wright, Ph.D. 1961.]</ref>. Oxidized alpha acids (humulinones) have only been tested for antibacterial properties at a concentration of 50 ppm or less. At 50 ppm, oxidized alpha acids were not able to inhibit two strains of ''Lactobacillus'' that were isolated from infected beer, as reported by Hough et al. (1957) <ref name="Hough_1957" />. See [[Hops#Acids_2|oxidized hop acids]] for more information on oxidized hop acids.
Dry hopping has also been demonstrated to inhibit lactic acid bacteria. See [[Hops#Dry_Hopping|Dry Hopping]] below.
===Bacterial Resistance to Hop Compounds===
Due to the multiple mechanisms for hop antimicrobial activity, multiple resistance mechanisms are necessary for a Gram-positive bacterial cell to successfully be hop-tolerant<ref name="Behr and Vogel, 2010"> [http://aem.asm.org/content/76/1/142.short Behr and Vogel, (2010)] </ref>. Hop resistance of bacteria will vary by species as well as within a species with individual strains. The environment in which strains are cultured and maintained may also influence their hop tolerance. It is possible for a small subpopulation of individual cells that have a higher tolerance to hops to eventually dominate the overall population <ref name="zhao_1027" />. The hop tolerance of lactic acid bacteria strains decreases when they are cultured in hop-free environments and strains cultured in media with increasing concentrations of hop compounds show an increase in hop tolerance<ref name="Sakamoto and Konings, 2003"/>. The stability of hop resistance, or the rate at which it is lost when bacteria are cultured in unhopped wort, varies by strain. It can take up to 1 year for maximum loss of hop resistance, suggesting that in some strains have a relatively stable hop resistance<ref name="Sakamoto and Konings, 2003"/>. Because of this intra-species variability and dependence on how the strains were cultured, it is difficult to give specific advice about the hop-tolerance of a wide range of strains offered from different sources. As a general rule, some common lactic acid bacteria species used in sour beer and found as beer spoilage organisms like ''Lactobacillus brevis'', ''Lactobacillus lindneri'' and ''Pediococcus delbrueckii'' have some resistance to hops<ref name="Sakamoto and Konings, 2003"/>. Brewers seeking to make acidic beers with higher doses of hops may wish to seek out one of these species. Some hop-tolerant species benefit from pre-culturing in media with below-limiting concentrations of compounds before being used in more highly hopped wort or beer<ref name="Simpson and Fernandez, 1992"> [http://onlinelibrary.wiley.com/doi/10.1111/j.1472-765X.1992.tb00636.x/abstract Simpson and Fernandez, 1992]</ref>.
See also [[Pediococcus#Hop_Resistance|''Pediococcus'' hop resistance]] and [[Lactobacillus#Hop_Tolerance|''Lactobacillus'' hop tolerance]].
==Hop Derived Compounds In Beer and Biotransformations==
[[File:Svedlund 2022.jpg|thumb|400px|An overview of the biotransformation reactions occurring in certain yeast with the required genetic/enzymatic capability. Abbreviations: 3MH 3-mercaptohexanol, 3MHA 3-mercaptohexyl acetate, Cys cysteine, GSH glutathione, TPA terpene alcohol. Credit: Henrik Svedlund.
Source: [https://link.springer.com/article/10.1007/s00253-022-12068-w Svedlund, N., Evering, S., Gibson, B. et al. Fruits of their labour: biotransformation reactions of yeasts during brewery fermentation. Appl Microbiol Biotechnol 106, 4929–4944 (2022). https://doi.org/10.1007/s00253-022-12068-w]]]
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=== 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 evidence for glycosidic activity in ''S. cerevisiae'' in regards to hop derived compounds is very weak; see [[Hops#Glycosides|Hop Glycosides]]). Post-fermentation dry hopping preserves linalool and alpha-terpineol, and limits citronellol to trace levels <ref name="Praet_2012" />. [https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.2010.tb00428.x Takoi et al. (2012)] used Citra hops with a high content of geraniol added late in the boil, and reported a steep decline on geraniol during the first three days of fermentation with a lager yeast strain. Linalool had a gradual decline but ended up at higher levels than geraniol in the finished beer. Citronellol had a sharp increase during the first three days of fermentation and then remained at a stable level until the end of fermentation. However, after storing the beer at 15°C (59°F) for 1 week, the amount of citronellol more than doubled. This indicated that active fermentation may not be required for the transformation of geraniol into citronellol (the yeast was filtered before packaging the finished beer, after a storage time of 6-8 days at 13–15°C and then at 0°C for 2–3 weeks). The enzyme NADPH dehydrogenase 2, encoded by the OYE<sub>2</sub> gene in some yeast strains, was proposed as the mechanism for this transformation. Interestingly, Takoi et al. (2012) also showed that coriander seeds, which also have high levels of linalool and geraniol, have a nearly exact same effect on beer, with a beer made with 0.5 g/L of coriander seed resulting in 20 ppb of citronellol and a beer made with 0.75 g/L of coriander seed resulting in 30 ppb of citronellol. The Citra beer had a citrus and "green" aroma, while the coriander beers had a very floral aroma with a slight citrus impression. They also conducted a sensory experiment with different levels of geraniol and citronellol added to linalool to see if small amounts of these would affect the flavor of a large dosage of linalool, and the results confirmed that small increases of geraniol and citronellol increased flowery and fruity flavors even in the presence of high dosages of linalool <ref>[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.2010.tb00428.x The Contribution of Geraniol Metabolism to the Citrus Flavour of Beer: Synergy of Geraniol and β‐Citronellol Under Coexistence with Excess Linalool. Kiyoshi Takoi, Yutaka Itoga, Koichiro Koie, Takayuki Kosugi, Masayuki Shimase, Yuta Katayama, Yasuyuki Nakayama, Junji Watari. 2012. DOI: https://doi.org/10.1002/j.2050-0416.2010.tb00428.x.]</ref>. The data for the Citra beer is shown below:
[[File:Biotransformation Takoi 2012.png|[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.2010.tb00428.x Takoi et al. 2012]]]
[[File:Biotransformation Takoi 2014.png|[https://www.researchgate.net/publication/261475199_Screening_of_Geraniol-rich_Flavor_Hop_and_Interesting_Behavior_of_beta-Citronellol_During_Fermentation_under_Various_Hop-Addition_Timings Takoi et al. 2014]]]
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 Colomer et al. (2020) measured the monoterpenes in two experiments before and after inoculating with different strains of ''thiolsBrettanomyces'' have also been shown to be produced by yeast fermentation from hop derived precursors (suspected to be Sthat had varying degrees of beta-glutathione)glucosidase activity. So far, science has They found that these include the volatile thiols 3strains with the least beta-sulfanyl-4-methylpentan-1-ol (3S4MP; grapefruit) and 3-sulfanyl-4-methylpentyl acetate (3S4MPA; passionfruitglucosidase activity had the least impact on biotransformation, grapefruit). These thiols were found but the increase in beers dry hopped separately with Amarillo, Hallertau Blanc, and Mosaic hop varieties. The amounts of these two thiols were beta-citronellol was higher than expected based on the content of these thiols what has been reported in the hops alone biotransformation studies with ''Saccharomyces'' <ref name="Cibaka_2016" />. See also this [https://wwwonlinelibrary.facebookwiley.com/groupsdoi/full/10.1002/jib.610 Biotransformation of hop derived compounds by Brettanomyces yeast strains. Marc Serra Colomer, Birgitte Funch, Natalia Solodovnikova, Timothy John Hobley, Jochen Förster. 2020. DOI: https://MilkTheFunkdoi.org/permalink10.1002/1373899592638251jib.610.]</ MTF thread speculating on how ref>. See [[Brettanomyces#Hop_Biotransformation|''Brettanomyces'' might produce thiolshop biotransformation]]for more information.
In general, different yeast strains have a large impact on how hops are perceived in the final beer, including both perceived bitterness and flavors. For example, POF+ (phenolic positive) strains of ''[[Saccharomyces|Saccharomyces cerevisiae]]'' tends to mask the hop-derived aromas in dry hopped beers <ref name="Sharp_Presentation" />. A beer hopped with the Tradition hop variety produced fruit flavors when fermented with Abbaye ale yeast, and woody/spicy flavors when fermented with US-05. When the beer was brewed with Citra hops, with US-05 the beer had sweet fruits/citrus flavors and more bitterness, but when fermented with the Abbaye ale strain the beer had a more one dimensional sweet fruit/floral flavor and less bitterness <ref>"Influence of yeast strain on hop aroma development in dry hopped beers." Christina Schönberger, Elisabeth Wiesen, Benedikt Matsche, Barth Innovations Yves Gosselin, Stephan Meulemans, Fermentis. Presentation slides at 35th Congress EBC.</ref>.
See also:
:<youtube height="200" width="300">GCQ22HSDDUQ</youtube> <youtube height="200" width="300">r09eb46k97I</youtube>* [https://www.crowdcast.io/c/itcl8mhhsdsa Escarpment Labs presentation by Richard Preiss on biotransformation.]* [https://www.masterbrewerspodcast.com/282 MBAA Podcast #282: New Belgium's tests with mid-fermentation dry hopping.]
====Glycosides====Hops contain In addition to terpenes and monoterpene alcohols being found in hops in a free form, they are also present in the form of glycosides, which are flavor compounds that are bound to a sugar molecule. Plants use glycosides for a range of metabolic purposes. The amounts and types of glycosides in hops varies by hop variety, and can be affected by plant fertilization and harvest timing <ref name="Svedlund_2022">[https://link.springer.com/article/10.1007/s00253-022-12068-w Svedlund, N., Evering, S., Gibson, B. et al. Fruits of their labour: biotransformation reactions of yeasts during brewery fermentation. Appl Microbiol Biotechnol 106, 4929–4944 (2022). https://doi.org/10.1007/s00253-022-120.]</ref>. In their bound form, glycosides are flavorless. Studies on hop compounds elude to the possibility of compounds being produced by the glycosidic activity of ''S. cerevisiae'', however direct evidence of glucosidic activity in ''S. cerevisiae'' is lacking. Daenen (2008) reviewed the glycosidic activity of many strains of ''S. cerevisiae'', and found that only a few strains expressed any real glucosidic activity and none that exhibited exo-beta-glucosidase which would be required to break glycosidic bonds in the beer/wort. Daenen did find that enzymatic activity from some strains of ''Brettanomyces'' can efficiently release these bound compounds and release their flavor and aromatic potential <ref name="Praet_2012" />. Beta-glucosidase enzyme can also be added to beer to enhance the breakdown of glycosides and intensify hop-derived flavors and aromas. For example, one study showed an increase in citrus, orange, grapefruit, and tropical pineapple in a Cascade dry hopped beer that had beta-glucosidase enzymes added to it <ref>"Optimizing hop aroma in beer dry hopped with Cascade utilizing glycosidic enzymes (presentation slides)." Kaylyn Kirkpatrick from New Belgium Brewing Co. Young Scientist Symposium, Chico, CA 2016.</ref>. There is also some evidence to support that there is higher glucosidase activity in seeded hops, which are generally not used in the brewing industry <ref>"Seeded and "Unseeded Hops - a Quality Comparison (presentation slides)." Martin Zarnkow. EBC 2015.</ref>. Hops also contain polyhenols that are bound in glycosidic form that could contribute a small amount of bitterness <ref>[https://www.tandfonline.com/doi/abs/10.1080/03610470.2021.2024112?journalCode=ujbc20 Martin Biendl, Stefanie Ritter & Christina Schmidt (2022) Monitoring of Glycosidically Bound Polyphenols in Hops and Hop Products Using LC-MS/MS Technique, Journal of the American Society of Brewing Chemists, DOI: 10.1080/03610470.2021.2024112.]</ref> (see also [https://traffic.libsyn.com/secure/forcedn/thebrulab/056_Applying_the_Science_-_High_Hop_Loads_w_Jordan_Folks.mp3 Bru Lab Podcast Episode 055; Hop Bitterness And Polyphenols w/ Dr. Martin Biendl]).
See [[Glycosides]] for more information on glycosides.
===Thiols===
Sulfur-based compounds bound to a hydrogen atom known as ''thiols'' have also been shown to be produced by yeast fermentation from the hop derived precursors cysteine or glutathione via yeast β-lyase activity. This β-lyase activity is expressed by strains of ''Saccharomyces cerevisiae'' that have the ''IRC7'' gene. This activity takes place within the yeast cells which uptake the bound thiol precursors and then expel the free thiols. Most industrial brewing and wine strains of ''S. cerevisiae'' do not have a functional version of this gene due to a lack of selective pressure. Even in strains with a functional version of the ''IRC7'' gene, the expression of the gene is repressed in nitrogen rich substrates such as wort <ref>[https://www.masterbrewerspodcast.com/227 Dr. Laura Burns and Lance Shaner. MBAA Podcast Episode 227 Interview. 10/18/2021. Retrieved 02/10/2023.]</ref>(~4 mins in). The cysteine and glutathione precursors are also found in malt, hops, certain varieties of wine grapes (Sauvignon blanc, Gewürztraminer, Semillon, Chardonnay and Riesling grapes), and rice, with levels varying greatly depending on varieties of the plants. In addition to some strains of ''Saccharomyces cerevisiae'' that express the genes to produce thiols, strains of the yeasts ''Metschnikowia pulcherrima'', ''Torulaspora delbrueckii'', ''Lachancea thermotolerans'', ''Candida zemplinina'', and the bacteria ''Lactobacillus plantarum'' have also been found to produce thiols <ref name="Svedlund_2022" />. So far, science has found that these include the volatile thiols:
* Grape and hop based thiols:
** 3-mercaptohexan-1-ol (3MH; also referred to as 3-sulfanylhexan-1-ol, 3SH; ''tropical fruit'')
** 4-mercapto-4-methylpentan-2-one (4MMP; also referred to as 4-methyl-4-sulfanylpentan-2-one, 4MSP; ''blackcurrant'')
* Hop based thiols (Nelson Sauvin hops):
** 3-sulfanyl-4-methylpentan-1-ol (3S4MP; ''grapefruit'')
** 3-mercaptopentanol (3MP; also referred to as 3-sulfanylpentan-1-ol, 3SP)
Acetylation (acetate) variations:
* 3-sulfanyl-4-methylpentyl acetate (3S4MPA; ''passionfruit'', ''grapefruit'')
* 3-mercaptohexyl acetate (3MHA; ''passionfruit'')
Typically, in beer and wine, the amount of free thiols that are formed from these precursors is less than 1%, perhaps due to poor activity of β-lyase activity in acidic media and inhibition by polyphenols. However, 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" />. Dry hopping temperature plays a role, with with 18-24°C being optimum for 3Mh and 3MP, and 28°C being optimum for 3S4MP. Mash hopping can potentially increase thiols, while using copper in the brewing or winemaking process can reduce them <ref name="Svedlund_2022" />. In order to get around the nitrogen caused suppression of the IRC7 gene in brewing yeast, [[Omega Yeast Labs]] has bioengineered a yeast strain called [https://omegayeast.com/news/cosmic-punch-new-thiol-boosting-strain Cosmic Punch™ (British V OYL-011)] to produce significant amounts of thiols from hops and malted grains. [https://berkeleyyeast.com/available-yeast-strains/ Berkeley Yeast] also offers bioengineered yeast strains that produce thiols. See also this [https://www.facebook.com/groups/MilkTheFunk/permalink/1373899592638251/ MTF thread speculating on how ''Brettanomyces'' might produce thiols].
See also:
* [https://topcrop.co/collections/thiols "Thiols" articles on Top Crop from Omega Labs.]
* [http://scottjanish.com/the-locksmith-utilizing-bioengineered-yeast-and-high-bound-thiol-precersour-hops-and-phantasm-powder-to-thiol-drive-beer/ "The Locksmith: Utilizing Bioengineered Yeast and High Bound Thiol Precersour Hops and Phantasm Powder to Thiol Drive Beer," by Scott Janish.]
* [http://scottjanish.com/thiol-driver/ "Thiol-Driver," by Scott Janish.]
* [https://www.charlesfaram.co.uk/news/technical-article-introduction-to-thiols-in-hop-oils/ Charles Faram Blog; Introduction to Thiols in Hop Oils.]
* [https://www.masterbrewerspodcast.com/227 IRC7 + CRISPR = Cosmic Punch - MBAA podcast with Dr. Laura Burns and Lance Shaner from Omega Yeast.]
* [https://www.falsebottomedgirls.com/podcast/episode/212472ad/thiols-with-laura-burns-of-omega-yeast False Bottom Girls podcast interview with Dr. Laura Burns from Omega Yeast.]
* [https://www.masterbrewerspodcast.com/268 MBAA Podcast Episode 268 Free Thiol Release with Cécile Chenot.]
* [https://www.themadfermentationist.com/2023/05/are-thiols-scam-thiolized-yeast.html Practical advice from Michael Tonsmeire on The Mad Fermentationist blog.]
* [https://beerandbrewing.com/podcast-episode-306-ron-beatson-brand-ambassador-nz-hops/ Craft Beer & Brewing Magazine interview with New Zealand hop breeder Ron Beatson.]
===Lightstruck===
In [[lambic]] brewing, the term '''aged hops''' refers to hops (usually Noble varieties such as Tettnang, Saaz, Target, and Hallertau) which have been aged for 3-5 years in non-refrigerated conditions, and in burlap sacks or some other oxygen permeable bag <ref>[http://www.horscategoriebrewing.com/2016/04/hops-in-spontaneous-fermentation.html Dave Janssen. "Hops in spontaneous fermentation". Hors Catégorie Brewing blog. 04/28/2016. Retrieved 04/09/2018.]</ref><ref>[http://jesterkingbrewery.com/home-for-our-aged-hops "Home for Our Aged Hops". Jester King's blog. Retrieved 11/18/2016.]</ref>. It should be noted that the term "aged hops" can also refer to any sort of hop aging (especially in scientific literature), including short-term hop aging (1-6 months, for example) at refrigerated or non-refrigerated temperatures, and in oxygen-rich or vacuum sealed packaging. Much of the information below references hops that have been aged in warm conditions for shorter time periods than what hops are aged for in lambic brewing. The additional aging of hops that are used in lambic brewing or similar beers might have different effects than what has been studied in hops that are aged for shorter periods of time.
For techniques and usage amounts of aged hops, see [[Hops#Aged_Hops_in_LambicAged_Hops_in_Lambic_and_Other_Spontaneous_Fermentation_Beer|Aged Hops in Lambic]].
===Aging Hops===
===Chemistry and Characteristics===
During aging and if exposed to oxygen, compounds in hops oxidize into different compounds known collectively as "oxidative polar compounds" or OPC's. Hao et al. (2020) reported 39 different OPC's in aged hops, with 15 of them derived from alpha acids, 15 derived from isomerized alpha acids, and 9 from beta-acids <ref name="Hao_2020" />. Many of these oxidation derived compounds are considered "hard resins" (meaning they are soluable in alcohol) and contribute bitterness and antimicrobial properties to beer. Examples of "soft resins", which are not soluable in alcohol, are alpha and beta acids <ref name="Almaguer_2015">[https://onlinelibrary.wiley.com/doi/full/10.1002/jib.160 Almaguer, C., Schönberger, C., Gastl, M., Arendt, E. K. and Becker, T. (2014), Humulus lupulus – a story that begs to be told. A review, J. Inst. Brew., 120: 289– 314. DOI: 10.1002/jib.160.]</ref>.
====Summary of Oxidized Hop Acids====
{| class="wikitable sortable" style="width:75%;"
|-
| colspan="5" align="center" style="padding: 2em;" | '''Alpha Acid Derived Compounds <ref name="Hao_2020" />'''
|-
! Oxidized Compound
! Precursor
! Beer Soluble (Y/N/?)
! Bitterness Level
! Notes
|-
| Humulinones (cohumulinone, n-humulinone, and adhumulinone) || Alpha acids (cohumulone, humulone, and adhumulone) || Y || 66% as bitter as iso-alpha-acids || In hops, alpha-acids are degraded to humulinones in the presence of air, with higher temperatures leading to more rapid conversion.
|-
| tricyclooxyisohumulones A and B, deisopropyltricycloisohumulone, and tricycloperoxyisohumulone A || Alpha acids || ? || ? || Along with humulinones and 4'-hydroxy-allohumulinones, they are considered aging indicators of of hop oxidation.
|-
| Abeo-isohumulone || Humulinone || ? || Not reported; might contribute to bitterness of beers with high amounts of aged hops. || Derived from the oxidation of humulinone in hops. Found in naturally aged Saaz and Nugget hops over a long period of storage.
|-
| 4'-hydroxy-allohumulinones || Humulinones || ? || Not reported; might contribute to bitterness of beers with high amounts of aged hops. || Believed to be derived from the oxidation of humulinone in hops. They are more stable than humulinone over time and are thus thought to be useful indicators of the degree of oxidation in hops.
|-
| 4'-Hydroxyallo-cis-humulinones and cis-oxyhumulinic acids || 4'-hydroxy-allohumulinones || Y || Contributes to bitterness quality || These are formed during boiling where 4'-hydroxy-allohumulinones are isomerized into 4'-Hydroxyallo-cis-humulinones for the first 60 minutes of boiling, and then the 4'-Hydroxyallo-cis-humulinones are slowly degraded into cis-oxyhumulinic acids after 60 minutes of boiling. After 120 minutes of boiling, 40-50% of the 4'-hydroxy-allohumulinones is transformed into 4'-Hydroxyallo-cis-humulinones and cis-oxyhumulinic acids. These are thought to effect the bitterness quality of beer, and as hard resins they could help improve head retention <ref>[https://www.researchgate.net/publication/286063120_Contributions_of_hop_hard_resins_to_beer_quality Almaguer, Cynthia & Gastl, Martina & Arendt, Elke & Becker, Th. (2012). Contributions of hop hard resins to beer quality. BrewingScience. 65. 118-129.]</ref>.
|-
| Scorpiohumulinols A/B and dicyclohumulinols A/B || 4'-hydroxy-allohumulinones || ? || ? || Potentially makes up the majority (70%) of compounds derived from the degradation of 4'-Hydroxyallo-cis-humulinones in beer stored at 40°C over 12 weeks.
|-
| colspan="5" align="center" align="center" style="padding: 2em;" | '''Beta Acid Derived Compounds <ref name="Hao_2020" />'''
|-
! Oxidized Compound !! Precursor !! Beer Soluble (Y/N/?) !! Bitterness Level !! Notes
|-
| Hulupones (cohulupone, hulupone) || Beta acids (colupulone, lupulone) || Y || 84% as bitter as iso-alpha-acids (short-lasting alpha-acid-like bitterness); colupulone has a flavor threshold of 7.9 umol/L || Degradation rate of beta acids into hulupones increases with temperature. Has a relatively low absorption in wort (4.8–10.4%). May also degrade into other unknown compounds other than hulupinic acids.
|-
| Hulupinic Acids || Hulupones || Y || Contributes marginal bitterness to beer; flavor threshold of 69 umol/L || Has a relatively low degradation in wort.
|-
| Tricyclocolupone epimers A/B || Beta acids (colupulone) || Y || Contributes long-lasting lingering and harsh bitterness; flavor threshold of 38-54 umol/L || Found to survive the brewing process up to 50% in a Pilsner style beer.
|-
| Dehydrotricyclocolupones epimers A/B || Beta acids (colupulone) || Y || Contributes long-lasting lingering and harsh bitterness; flavor threshold of 40 umol/L || Found to not survive the brewing process in a Pilsner style beer, so it might not contribute to beer bitterness.
|-
| Hydroperoxy-tricyclocolupone epimers A/B || Beta acids (colupulone) || Y || Contributes long-lasting lingering and harsh bitterness; flavor threshold of 20 umol/L || Found to not survive the brewing process in a Pilsner style beer, so it might not contribute to beer bitterness.
|-
| Hydroxy-tricyclocolupone epimers A/B || Beta acids (colupulone) || Y || Contributes long-lasting lingering and harsh bitterness; flavor threshold of 14-17 umol/L || Found to not survive the brewing process in a Pilsner style beer, so it might not contribute to beer bitterness.
|-
| Nortricyclocolupone || Beta acids (colupulone) || Y || Contributes bitterness; flavor threshold of 90 umol/L || Found to survive the brewing process.
|-
| Epoxycohulupone and Epoxyhulupon || Beta acids (colupulone, lupulone) || ? || ? || Newly discovered; not much is known about them.
|-
| colspan="5" align="center" align="center" style="padding: 2em;" | '''Iso-Alpha Acid Derived Compounds <ref name="Hao_2020" />'''
|-
! Oxidized Compound !! Precursor !! Beer Soluble (Y/N/?) !! Bitterness Level !! Notes
|-
| ''cis''-alloisohumulone and ''trans''-alloisohumulone || Iso-alpha acids (''cis''-isohumulone and ''trans''-isohumulone) || Y || ? || ''cis''-alloisohumulone was found in trace levels in a Pilsner style beer at all different aging points, but ''trans''-alloisohumulone remained undetected.
|-
| ''trans''-humulinic acid and ''cis''-humulinic acid || Iso-alpha acids (''cis''-isohumulone and ''trans''-isohumulone) || Y || No bitterness || Minor levels in wort, but stable through the brewing process. Higher levels were detected in beers aged in oxygen permeable PET bottles versus glass bottles. Humulinic acid content increases over the storage of beer over many years as iso-alpha acids decrease. It has been suggested that the decomposition of iso-alpha acids into humulinic acids over time plays a role in the decrease in perceived bitterness of aged beer.
|-
| Hydroperoxy-''cis''-alloisohumulones and Hydroperoxy-''trans''-alloisohumulones || Iso-alpha acids (''cis''-isohumulone and ''trans''-isohumulone) || Y || ? || Iso-alpha acids degrade into these in beer that is exposed to oxygen after just 1-2 hours. ''Cis''-isohumulone degrade into hydroperoxy-''cis''-alloisohumulones, while ''trans''-isohumulone degrade into Hydroperoxy-''trans''-alloisohumulones and tricyclohumol. They are also formed during the wort boiling process, but are reduced by 50% during the fermentation process. Hydroperoxy-''trans''-alloisocohumulone increases as beer is stored in bottles over many years (1, 4, and 10 years were measured in one study), while hydroperoxy-''cis''-alloisocohumulone is less stable and decreases. In PET bottles, both types of hydroperoxy-alloisohumulones decreased to zero after 4 years of storage at room temperature. While the degredation of iso-alpha acids into hydroperoxy-alloisohumulones occurs very quickly when in the presence of oxygen, the degradation of hydroperoxy-alloisohumulones occurs much more slowly during storage at room temperature.
|-
| Hydroxyl-''cis''-alloisohumulones and Hydroxyl-''trans''-alloisohumulones || Hydroperoxy-''cis''-alloisohumulones and Hydroperoxy-''trans''-alloisohumulones || Y || ? || Hydroperoxy-alloisohumulones are further degraded into their respective hydroxyl-alloisohumulones (''cis'' and ''trans'') via metal ions or UV light. Similar to hydroperoxy-alloisohumulones, hydroxyl-''trans''-alloisohumulones slowly increase during storage of bottles at room temperature over many years (1, 4, and 10 years), while hydroxyl-''cis''-alloisohumulones slowly decrease. In PET bottles, both types degrade to nearly 0 (from 0.39 umol/L to 0.08 umol/L) after 4 years at room temperature. When beer is intentionally oxidized, the hydroperoxy-alloisohumulones degrade very quickly over 24 hours into their hydroxyl-alloisohumulones derivatives.
|-
| Tricyclohumulactol || Hydroperoxy-alloisohumulones || Y || ? || Only detected in a model solution; not detected in fresh or aged beer possibly due to low concentrations and/or coelution with isobaric molecules, exposing a technical difficulty of detecting these compounds in beer.
|-
| Scorpiohumols || Hydroxyl-''trans''-alloisohumulone || Y || ? || Same as Tricyclohumulactol.
|-
| Tricyclocohumol, Tricyclocohumene, Isotricyclocohumene, Tetracyclocohumol, and Eptitetracyclocohumol || ''Trans''-isohumulone (iso-alpha acid) || Y || Bitterness thresholds in beer as umol/L: 30, 5, 10, 70, 70 (''Trans''-isohumulone is 20). Characterized as "lingering harsh bitterness". || The most abundant oxidation derived hop compounds in aged beer. All are derived from ''trans''-isohumulone, which is the iso-alpha acid that is less stable than ''cis''-isohumulone. ''Trans''-isohumulone degrades into these faster at warmer temperatures and lower pH (no degradation at a pH of 6, slower degradation at a pH of 3, and very fast degradation at a pH of 1). Even differences as small as 0.1 pH (4.2, 4.3, and 4.4 pH values were tested) can make a significant difference in the degradation rate of ''trans''-isohumulone into tricyclocohumol, with lower pH values leading to faster degradation. After storing beer in brown bottles at 28°C for 8 months, these degradation products were detected at high amounts. Tricyclohumol remains relatively stable, even in the presence of oxygen.
|-
|}
====Acids====
During aging, both alpha and beta acids oxidize and degrade with warmer temperatures and more oxygen exposure having a greater impact. The generally accepted theory is that oxygen interacts directly with hop acids. This event is called "autooxidation". An alternative theory to this is that oxygen indirectly oxidizes acids by first oxidizing the hop oils and turning them into pro-oxidants, which then oxidize the hop acids which are mixed in with the oils within the lupulin glands <ref name="Algazzali_2014" />. The oxidation of hop acids corresponds with an increase in the [http://methods.asbcnet.org/summaries/hops-12.aspx Hop Storage Index (HSI)], which is a practical way of measuring the overall freshness of hops. As the oxidation of hop oils rises, the measured HSI number on a lot of hops increases <ref name="Lam et al., 1986"/><ref name="Maye_2016">[http://www.hopsteiner.com/wp-content/uploads/2016/07/TQ-53-1-0227-01.pdf Humulinone Formation in Hops and Hop Pellets and Its Implications for Dry Hopped Beers. John Paul Maye, Robert Smith, and Jeremy Leker. 2016.]</ref>. These oxidized compounds lead to a higher amount of non-alpha-acid bitterness compounds in aged hops and have a remarkable effect on the bitterness of the beer. The bitterness from oxidized hop compounds has been described as more earthy, harsh, and astringent than the sharper, cleaner bitterness from iso-alpha acids <ref name="Mikyška_2012">[http://onlinelibrary.wiley.com/doi/10.1002/jib.40/abstract Assessment of changes in hop resins and polyphenols during long-term storage. Alexandr Mikyška and Karel Krofta. 2012.]</ref>.
{| class="wikitable sortable"
|-
! Storage !! Oil !! Hallertau Magnum (Pellet, 14% AA) <ref name="Kishimoto_2021" /> !! Saaz (pelletPellet) <ref name="Mikyška_2012" /> !! Sládek (pelletPellet) <ref name="Mikyška_2012" /> !! Premiant (pelletPellet) <ref name="Mikyška_2012" /> !! Agnus (pelletPellet) <ref name="Mikyška_2012" /> !! Saaz (leafLeaf) <ref name="krofta_2013">[http://www.actahort.org/books/1010/1010_26.htm Stability of Hop Beta Acids and Their Decomposition Products during Natural Ageing. K. Krofta, S. Vrabcová, A. Mikyska, M. Jurková, T. Cajka , J. Hajslová. 2013.]</ref> !! Vital (leafLeaf) <ref name="krofta_2013" /> !! Pure Beta Acid <ref name="krofta_2013" /> |-| colspan="10" | '''Open air at 20°C for 12 months'''|- align="right"| || Alpha acids || || -80% || -88.3% || -64.3% || -78.2% || || |- align="right"| || Beta acids || || -60.5% || -83% || -53.7% || -51% || -50% || -77.5% || -99%|- | colspan="10" | '''Open air at 40°C for 3 months'''|- align="right"| || Alpha acids || -100% || || || || || || |||- | colspan="10" | '''Vacuum sealed at 20°C for 12 months'''|- align="right"| || Alpha acids || || -20.6% || -24.9% || -22.2% || -21.7% || || |||- align="right"| || Beta acids || || -2.7% || -1.7% || -2.1% || -1.2% || || ||
|-
| colspan="910" | '''Open air Vacuum sealed at 20°C 2°C for 12 months'''|- align="right"| || Alpha acids || || -1.1% || -5.5% || -0.3% || -1.4% || || |||- align="right"| || Beta acids || || -1.7% || -2.3% || -0.4% || -0.5% || || ||
|-
| || Alpha } It has been established that many factors influence the initial content of alpha and beta acids, such as cultivar (variety), cultivation area, climate and environmental conditions, harvest year, and harvest date. For example, early harvest dates result in hop cones that are not fully developed and contain less alpha/beta acids, and late harvest dates can produce hop cones with lower alpha/beta acids as these acids begin to degrade. Thus, optimal harvest dates occur when the hop cones reach their maximum size but before they become brittle and hop acid degradation begins. [https://www.pja.iung.pulawy.pl/stability-of-the-hop-bitter-acids || -80% || during-long-term-storage-of-cones/ Skomra and Koziara-88Ciupa (2020)] found that harvest date also affects how alpha and beta acids degrade over time during storage. They looked at alpha and beta acid degradation differences for 4 varieties of hops grown at the same farm, but harvest at 3% || -644 different dates (7-10 days between harvest dates) <ref name="Skomra_2020">[https://www.pja.iung.pulawy.pl/stability-of-the-hop-bitter-acids-during-long-term-storage-of-cones/ Stability of the hop bitter acids during long-term storage of cones with different maturity degree. Urszula Skomra, Marta Koziara-Ciupa. Polish Journal of Agronomy 2020, 40, 16–24. DOI: 10.26114/pja.iung.406.2020.40.03.]</ref>. The hops were stored at 5°C and 20°C in aerobic conditions for 12 months <ref>Private correspondence with Urszula Skomra by Dan Pixley. 07/21/2020.]</ref>. As expected, the lower storage temperature of 5°C preserved the alpha and beta acids better than the warmer 20°C storage temperature. In general, an earlier harvest date also resulted in greater degradation of alpha and beta acids, but there were some exceptions to this. For example, Sybilla Harvest Date II produced the most stable alpha acid content (harvesting too early or too late resulted in less stable alpha acid content), while for Magnet Harvest Date II resulted in the most degradation of alpha and beta acids and Harvest Date III was preferable for alpha acid stability. In general, the stability of beta acids depended more on the harvest date than the stability of alpha acids <ref name="Skomra_2020" />. See the table below. Skomra and Koziara-Ciupa (2020) also reported that for the first 3% || to 6 months, the alpha and beta acids were relatively stable. The cooler storage had a longer period of initial stability than the warmer storage. After the initial period of stability, the alpha and beta acids began a rapid drop around 9 months of total storage. From 9-7812 months, the rate of decline in alpha and beta acids was much slower and seemed to somewhat stabilize <ref name="Skomra_2020" />.2% || | {| class="wikitable sortable"
|-
|-
| colspan="9" | '''Vacuum sealed Open air at 20°C 5°C for 12 months'''(Harvest dates I, II, II, and only for Magnat/Magnum IV)
|-
| || Alpha acids || -2010.68%, -4.5%, -7.9% || -2411.90%, -6.4%, -6.8% || -2225.8%, -40.8%, -2.3%, -9.3% || -2117.7% || || ||, -4.5%, -12.9%, -11.2%
|-
| || Beta acids || -27.6%, -6.3%, -8.75% || -28.1%, -10.79%, -17% || -33.3%, -37.2%, -21.1%, -22.3% || -28.1%, -7.6%, -3.9%, -5.2% || || ||
|-
| colspan="9" | '''Vacuum sealed Open air at 2°C 20°C for 12 months'''(Harvest dates I, II, II, and only for Magnat/Magnum IV)
|-
| || Alpha acids || -128.5%, -11.3%, -27.1% || -518.56% || , -035.3%, -37.6% || -135.4%, -56.8%, -31.7%, -36% || || ||-38.3%, -25.7%, -28.9%, -24.4%
|-
| || Beta acids || -126.4%, -38.6%, -49.75% || -243.3%, 45.7%, 54.1% || -046.3%, -66.46%, -59.8%, -59.2% || -053.7%, -28.51%, -16.8%, -19.9% || || ||
|-
|}
'''Oxidized beta acids''' produce some compounds that also contribute to the perception of bitterness, specifically hulupones. Unlike humulinones which form relatively quickly from the oxidation of alpha acids, hulupones form at a much slower rate <ref name="Dušek_2014" />. Also unlike humulinones, they survive boiling and fermentation. While some sensory analysis of beers containing oxidized beta acids describes the resulting bitterness as "harsh and clinging", another analysis by Krafta et al (2013) described the bitterness of oxidized beta acids in beer when added in their pure form at the beginning of the boil as "pleasant and not lingering". The more degradation of beta acids into oxidized beta acids that occurs in hops, the more bitter beers brewed with these hops will be <ref name="krofta_2013" />. Two other compounds other than hulupones have been identified as being produced by the oxidation of beta acids, epoxycohulupone and epoxyhulupone. Their effect on beer flavor is not yet known, ; however, it is thought believed that hulupones have the greatest a greater impact on beer flavor and bitterness than these compounds <ref name="Dušek_2014" />.
The bitterness of hulupones has received some debate among researchers. In 1973, a researcher found that hulupones are about 50% as bitter as iso-alpha acids. Briggs et al stated the complete opposite, and that hulupones are twice as bitter as iso-alpha acids. More recent studies using modern analysis techniques found that on a weight for weight basis, hulupones are 35-40% as bitter as iso-alpha acids in one study, and another found that they were 84% (+/- 10%) as bitter as iso-alpha acids (note that this is an average; tasters vary widely in how much bitterness they perceived from different bitter compounds) <ref>[https://www.researchgate.net/publication/307796892_The_effect_of_hop_beta_acids_oxidation_products_on_beer_bitterness The effect of hop beta acids oxidation products on beer bitterness. Karel Krofta, Světlana VRABCOVÁ, Alexandr Mikyška, and Marie JURKOVÁ. 2013.]</ref><ref name="Algazzali_2014" /><ref name="Briggs_2004">[https://www.crcpress.com/Brewing-Science-and-Practice/Briggs-Boulton-Brookes-Stevens/p/book/9780849325472 Brewing Science and Practice. Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes, Roger Stevens. 2004.]</ref>. While the taste threshold of iso-alpha acids is 5-6 mg/L in light lager, the threshold for hulupones has been measured to be 7-8 mg/L in light lager <ref name="Algazzali_2014" />.
Other compounds have been associated with the oxidation of beta acids and are extracted during wort boiling. These are described as giving a long-lasting, lingering bitterness on the palate. They include hydroxytricyclo-lupulone, dehydrotricyklolupulone, and hydroperoxytricyklolupulone <ref>[http://www.sciencedirect.com/science/article/pii/S0308814609001770 Structure determination and sensory evaluation of novel bitter compounds formed from β-acids of hop (Humulus lupulus L.) upon wort boiling. Gesa Haseleu, Daniel Intelmann, Thomas Hofmann. 2009.]</ref>.
====IBU==== The overall effect of oxidized compounds in aged hops has been shown by Val Peacock, a former scientist at Anheuser-Busch. Peacock stored hops at four different temperatures for 18 months. His data showed that although the alpha acid content in the hops and the iso-alpha acid content in the beers brewed with them decreased the older the hops were stored, the measured IBU of the different beers was about the same. This is because the oxidized acids in hops show up in the same spectrum as iso-alpha acids when using the ASBC method of measuring IBUs with a spectrophotometer <ref name="Aron">[https://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars Dr. Patricia Aron. "Bitterness and the IBU: What’s It All About?" HomebrewCon 2017 Presentation. ~34 mins in. Retrieved 09/05/2017.]</ref>. This data is shown below. Caleb Buck's experiment [[Hops#Aged_Hops_in_Lambic|seen below]] supports this. Although it has not been shown that oxidized alpha and/or beta acids can inhibit lactic acid bacteria, if they do, then this might help explain reports <ref>[https://homebrewingfun.blogspot.com/2019/11/one-gallon-spontaneous-fermentation.html Adam Kielich. "One Gallon Spontaneous Fermentation Beer Batch 5 Recipe and Brewday". Brain Sparging on Brewing. 11/16/2019.]</ref> of using aged hops that originally had a high alpha acid content retaining a strong inhibitory effect towards lactic acid bacteria.
{| class="wikitable sortable"
| 70°F || 0.41% || 2.9 ppm || 11.0
|}
[https://www.tandfonline.com/doi/full/10.1080/03610470.2021.1878684 Kishimoto et al. (2021)] reported similar results when making beers with forcibly aged hops. They aged Magnum hop pellets (14% AA) at 40°C and exposed to air for a number of different days: 3, 5, 7, 10, 14, 21, 30, and 90 days. The hops aged for 14 days had 9.8% AA, 21 days had 4.4% AA, 30 days had 1.2% AA, and 90 days had 0% AA. The IBU (measured with a spectrophotometer using Method Beer-23A from the American Society of Brewing Chemists) for the different beers made with the aged hops was more or less the same despite how much alpha acids were left in the aged hops. However, beers brewed with the different aged hops reflected a rapid decrease in iso-alpha acids in the beers made with the aged hops. Perceived bitterness also decreased for the beers made with the aged hops, but not as much as the decrease in ppm of iso-alpha acids, again demonstrating the oxidized hop compounds carry some bitterness, but less bitterness than iso-alpha acids <ref name="Kishimoto_2021" />. See Figure 5 from Kishimoto et al. (2021):
[[File:Kishimoto 2021 Fig5.jpg|600px|[https://www.tandfonline.com/doi/abs/10.1080/03610470.2021.1878684 Toru Kishimoto, Satoko Teramoto, Akiko Fujita & Osamu Yamada (2021) Evaluation of Components Contributing to the International Bitterness Unit of Wort and Beer, Journal of the American Society of Brewing Chemists, DOI: 10.1080/03610470.2021.1878684.] Uploaded with permission from Toru Kishimoto.]]
It is therefore advised that brewers ask hop providers that offer aged hops what the original alpha acid percentage was when the hops were fresh, as well as the variety. This could be an important factor when determining how bitter the aged hops will taste, and potentially also how much they will negatively impact lactic acid bacteria growth. See also [[Hops#Aged_Hops_in_Lambic_and_Other_Spontaneous_Fermentation_Beer|"Aged Hops in Lambic" below]] and [https://www.facebook.com/groups/MilkTheFunk/permalink/2503097546385111 this MTF thread on IBU's from aged hops].
====Oils====
[[File:Lam Hop Oils.jpg|thumb|400|[http://pubs.acs.org/doi/pdf/10.1021/jf00070a043 Data from "Aging of hops and their contribution to beer flavor" by Kai C. Lam, Robert T. Foster and Max L. Deinzer.] '''Aged I''': 2 weeks at 90°F; '''Aged II''': 60 additional days at 90°F.]]
Hop oils also generally degrade over time, however, their degradation rates are more complex. [http://pubs.acs.org/doi/abs/10.1021/jf00070a043 Lam et al. (1986)] found that aging both cascade and North American grown Hallertauer Mittelfrueh resulted in an increase in grapefruit-like character, although the compound that caused this was not identified. In the case of Cascade the intensity of this flavor correlated with the age of the hops <ref name="Lam et al., 1986"> [http://pubs.acs.org/doi/abs/10.1021/jf00070a043 Aging of Hops and Their Contribution to Beer Flavor. Lam et al. 1986.] </ref>. In the Hallertauer hops, aging resulted in an increase in a spicy/herbal character <ref name="Lam et al., 1986"/>, which is in agreement with reports of oxidized sesquiterpenes (specifically humulenol II, humulene diepoxides, caryophyllene, and to a lesser extent humulene monoepoxides and alpha-humulene) contributing a spicy/herbal flavor to beer <ref name="Goiris et al., 2002">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2002.tb00129.x/abstract Goiris et al., 2002]</ref><ref name="Mikyška_2012" />. Many of the oils followed in the Lam et al. (1986) study which increased during a short accelerated aging period (2 weeks at 90°F) then decreased during extended aging (60 additional days at 90°F). The cascade hops lost more of the fruity/citrusy hop oils (myrecene, linalool, and geranial) than Hallertauer, suggesting that different strains of hops can withstand aging better than others. The concentration of hop oils are affected by the brewing process and fermentation (see the table) <ref name="Lam et al., 1986"/>. Another study found that beta-ionone (classified as a ketone, and characterized as "floral" and "woody" <ref>[http://www.thegoodscentscompany.com/data/rw1006632.html Beta-ionone. Good Scents Company. Retrieved 11/22/2016.]</ref>) increased in beers brewed with hops that were aged for 30 days at 40°C versus beers brewed with aged hops <ref name="kishimoto_2007" />.
A recent 2017 study at the Shellhammer lab looked at how trained panelists and consumers perceived a lager beer dry hopped with slightly oxidized Hallertau Mittelfrüh hops (exposed to oxygen once, then stored at 38°C for two weeks) versus highly oxidized (daily exposure to oxygen and stored at 38°C for two weeks). They found that the trained panelists detected more characteristics that are associated with noble hops; e.g. more woody, earthy, and herbal characteristics in the lager beers dry hopped with oxidized hops. They also found the oxidized hopped beers to be more bitter (probably due to oxidized alpha and beta acids). Consumers were not statistically able to tell the difference. The study determined that oxidized hops might serve to provide nuanced increases in noble hop character <ref>[http://www.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-1287-01.aspx Aroma Properties of Lager Beer Dry-Hopped with Oxidized Hops. Daniel M. Vollmer, Victor Algazzali, and Thomas H. Shellhammer. 2017.]</ref>. In a similar study by Hengyuan et al. (2023), aging Saaz hops for a short amount of time (aged at 30 ℃ in a ventilated environment for 5 days) reduced the grassy and resinous character and improved the spicy and woody character of single hopped beers (hopped at three points in the boil and dry hopped). Consumers also preferred this beer over the other samples, including fresh Saaz. However, over-aging Saaz hops resulted in less preference with consumers (30 ℃ for 10 days, 40 ℃ for 5 days, or 40 ℃ for 10 days). This effect was not demonstrated by Simcoe hops, however, indicating that short aging is only beneficial for some hop varieties. Under the same aging condition with Simcoe hops, consumers preferred the fresh Simcoe, and any degree of aged Simcoe was less preferred. The beers with aged Simcoe hops were described as having less fruity and floral character and more dry, thin bitterness <ref>[https://www.bio-conferences.org/articles/bioconf/abs/2023/04/bioconf_icbb2023_01016/bioconf_icbb2023_01016.html Aging of Hops and Their Effects on India Pale Ale Flavor. Hengyuan Xu, Shaokang Sun, Xiaochen Wang, Haojun Zhang and Cong Nie. BIO Web Conf., 59 (2023) 01016. DOI: https://doi.org/10.1051/bioconf/20235901016.]</ref>.
* [http://scottjanish.com/increasing-bitterness-dry-hopping/ "Increasing Bitterness By Dry Hopping", article by Scott Janish on oxidized alpha acids.]
====See Also====
* [https://www.morebeer.com/articles/storing_hops_properly "Hop Storage", MoreBeer blog. 07/15/2015.]
* [https://www.balmyjuicebrewing.com/brewing-science/hop-freshness-1-hop-supply-chain/ Hop Freshness – Part 1: Hop Degradation and the Supply Chain] and [https://www.balmyjuicebrewing.com/brewing-science/hop-freshness-2-hop-bitter-acids Hop Freshness – Part 2: Hop Bitter Acids] by Balmy Juice Brewing.
===Aged Hop Suppliers===
* [https://www.mainiacalyeastyakimachief.com/commercial/hop-wire/introducing-the-ych-aged-hops/ Mainiacal Yeast -program YCH offers 1-2 oz bags for homebrewers and 44 lb bags of aged hops (small lots ; contact for homebrewers)more information.]
* <s>[http://www.hopsdirect.com/choice-debittered-aged-hops-leaf/ Hops Direct "Choice Debittered/Aged Hops" (Leaf - Cascade).]</s>
* <s>[https://hopsdirect.com/products/choice-debittered-pellets Hops Direct "Choice Debittered/Aged Hops" (Pellet - Columbus).]</s>* [https://www.freshops.com/shop/hop/aroma-hopproduct/lambic-hops/ Freshhops "Lambic Hops" (Leaf - Willamette)".]* [httphttps://www.yakimavalleyhops.com/Lambic2oz_pproducts/hopslambichops3.htm lambic-hop-pellets Yakima Valley Hops "Lambic / Aged Hops" (Pellet).]
* [http://www.farmhousebrewingsupply.com/lambic-hop-blend-4-oz-2015/ Farmhouse Brewing Supply "Lambic Hop Blend" (Pellet - Blend of varieties that are aged for ~5 years and then pelletized <ref>Private correspondence with Dustin Carver by Farmhouse Brewing Supply. 03/22/2016.</ref>).]
* [httphttps://www.themaltmiller.co.uk/index.php?_a=viewProd&productId=592 The Malt Miller (UK).]* [http:/product/www.brewstore.co.uk/specially-aged-fuggles-hops-100-grams Brew Store UK / The Malt Miller (Leaf - Fuggles).]* [http://www.brewstore.co.uk/specially-aged-hallertau-hops-100-grams Brew Store UK (Leaf - Hallertau).]
* [https://northwesthopfarms.com/products/lambic-aged-hops Northwest Hop Farms (BC, Canada).]
* [https://www.yakimachiefbsgcraftbrewing.com/ YCH lambic-hop-blend BSG sometimes offers 44 lb bags of aged pellet hopsfor commercial brewers; contact for more informationdetails.] See [https://www.facebook.com/groups/MilkTheFunk/permalink/4326634927364688 this MTF thread]on experiences with these hops.* [https://www.ebay.com/itm/Lambic-Hops-pellet-or-whole-cone/172249202875?hash=item281ada40bb:m:mX2CSrKvWb0-3WIIuBh0aLA&var=471070249004 Ted from Hop Heaven on eBay sells 8 oz and 1 lb bags of aged pellet and leaf hops.] See this [https://www.facebook.com/groups/MilkTheFunk/?comment_tracking=%7B%22tn%22%3A%22R%22%7D reviews on permalink/4338527352842112/ MTFthread]on reviews of this product.* [https://shop.humle.se/humle/lambichumle/ Humlegårdens (Sweden); several varieties of aged whole leaf hops, 100 gram quantities.]
===Cryo Hops® and Debittered/"American Noble" Hops===
See also:
* [https://byo.com/stories/issue/item/3500-hoppy-sour-beers "Hoppy Sour Beers", Dec 2016 BYO article by Michael Tonsmeire.]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/3238859366142255/ MTF thread on barrel-aged NEIPA/dry hopping before barrel aging; inspired by Sour Hour Podcast #117 with Kevin Osbourne.]
===Whirlpool Hopping===
On commercial systems, adding hops during the whirlpool has become a common technique. The idea is that hopping during the whirlpool will decrease the amount of isomerization of alpha acids in the final beer, while providing flavor and aroma from the hops.
Aaron Justice reported that a considerable amount of isomerization occurs on both a 150 BBL system (75-90 minutes of total steeping time), a 50 BBL system (65-80 minutes of total steeping time), and a 5 BBL pilot system (35-40 minutes of total steeping time). Justice reported an average of 30% utilization (the amount of iso-alpha acids from the potential alpha acids), with a 12.2% deviation. The deviations were based on the gravity of the wort and the amount of hops added before the whirlpool. With lower gravity worts and worts with less kettle additions, a high increase in utilization was observed. Most of the IBU's were gained within the first 10 minutes of the whirlpool, with only very small increases in IBU (~3 IBU) after 10. This data indicates that the total whirlpool steeping time and thus brewhouse size does not necessarily have a large impact in the amount of isomerization that occurs during commercial whirlpooling. The temperature of the whirlpool was not reported <ref name="justice_2018" />.
===Dry Hopping===
[https://beerandbrewing.com/dictionary/96jvGQTSdT/ Dry hopping] is the technique of adding hops to beer that has finished fermenting or is in the process of fermenting. Historically, dry hopping is associated with English pale ales of the 1800's, and it was re-discovered by Fritz Maytag of Anchor Brewing Company in 1975 when the company brewed the first American IPA, Liberty Ale <ref>[https://beerandbrewing.com/dictionary/96jvGQTSdT/ "The Oxford Companion to Beer definition of dry hopping," Garrett Oliver. Craft beer & Brewing Magazine. Retrieved 11/16/2023.]</ref><ref>[https://northamericanbrewers.org/liberty-ale/ "Liberty Ale". North American Brewers Association. April 28, 2022. Retrieved 11/16/2023.]</ref><ref>[https://www.youtube.com/watch?v=4ijHO5TFG3I Scott Ungermann, Brewmaster of Anchor Brewing Company. The Doug Piper Gourmet Brewing Podcast. July 14th, 2023.]</ref>(20 mins in). Brewers have had positive and interesting results dry -hopping sour and funky beer. Often fresh American or New Zealand varieties that compliment complement fruit flavors are chosen, however, other varieties have been used as well, including English and German hops. Just as in dry hopping normal beers, dry hopping sour/funky should be done after the beer has matured. Dry hopping for around 1-3 days before [[packaging]] the beer is adequate for extraction, depending on whether or not the beer is recirculated or agitated (agitation of the beer while on contact with the dry hops attains full extraction in 24 hours) <ref>[http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf?sequence=1 A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer (master thesis). Peter Harold Wolfe. 2012.]</ref>. Hopping rates generally range from 0.5-1 ounces per 5 gallons 1 gallon of beer (1-2 pounds per bblor 3.7-7.5 grams per liter) to achieve hop-forward flavors, although lesser rates can be used to achieve a more subtle character (see the threads below) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1693639013997639/?comment_id=1693660390662168&comment_tracking=%7B%22tn%22%3A%22R4%22%7D Nate Walter and Dan Pixley. Milk The Funk Facebook group. 05/21/2017.]</ref>.
Dry hopping can contribute to bitterness in beer through oxidized alpha acids and oxidized beta acids. Oxidized alpha acids can also reduce iso-alpha acids in beers that begin with more than 25 IBU from iso-alpha acids, potentially reducing percieved bitterness after dry hopping (see [[Hops#Acids_2|Oxidized Alpha Acids]] above). Some alpha acids will also dissolve into the beer, which are estimated as being 10% as bitter as iso-alpha acids. Dry hopping also has a linear impact on the pH of beer regardless of the starting IBU or pH: the pH rises by 0.14 per pound of hop pellets per barrel of beer in a beer that started with a pH of 4.2 (~0.5 ounces per gallonor 3.7 grams per liter) <ref name="Maye_2016" /><ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>. This rise in pH might be less in more acidic beers that are dry hopped since pH is a logarithmic scale. Dry hopping can also reduce head retention in beers, although this is variety dependent (one study found that dry hopping with Eureka and Apollo hops increased head retention, while dry hopping with Bravo, Centennial, and Cascade decreased head retention). Extended dry hopping times (after 3 days) can also reduce head retention <ref name="Maye_EBC2017" />.
* [https://www.facebook.com/groups/MilkTheFunk/permalink/1416506428377567/?comment_id=1458425540852322&comment_tracking=%7B%22tn%22%3A%22R%22%7D Caroline Whalen Taggart's data point on the effects of dry hopping on ''L. plantarum'' (GoodBelly). No hops finalized at a pH of 3.53, and the dry hopped version finalized at a pH of 4.35.] She used around 4 grams per gallon of 10-15 AA hops <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1746176672077206/?comment_id=1746211618740378&comment_tracking=%7B%22tn%22%3A%22R0%22%7D Carloine Whalen Taggart. Milk The Funk Facebook group. 07/06/2017.]</ref>.
* Per Buer's experiment on the effects of dry hopping on ''Lactobacillus'':
: <youtube width="300" height="200">J2g5P7ZlGn4</youtube>
* [https://www.facebookratebeer.com/groupsforums/MilkTheFunk/permalink/2358742877487246/ MTF thread by Dave Janssen on experiences with doing longlab-and-term dry hopping with noble hops in sour beershops_289071.] See also [http://www.horscategoriebrewinghtm "CLevar" on Ratebeer.com/2018/11/saison-from-1911-finishing-data point on ''Lactobacillus'' being inhibited by hops, but not as much by iso-andalpha acid hop extract.html Dave's research on a 1911 saison that was dry hopped long term].
====The Freshening Power of the Hop (Hop Creep)====
Also known as "dry hop creep", it was first discovered in 1893 by Brown and Morris that dry hopping increases the ABV of beers and dries them out. Dry hop creep, along with the secondary fermentation of ''Brettanomyces'' in long aged British ales of the late 19th and early 20th centuries, were once characterized as the potential mechanisms by which long aged ales could retain a high level of carbonation in casks <ref>[https://archive.org/details/principlespracti00syke "The principles and practice of brewing" Sykes, Walter John. London, C. Griffin and Company, limited, 1907. Pgs 384-388.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/4709953772366133 Gareth Young. Milk The Funk Facebook group thread about English brewers historically relying on Brettanomyces and dry hop creep for carbonation in long aged ales. 06/17/2021.]</ref>. It was proposed that the likely cause is the release of glycolytic enzymes that break down starches into sugars which viable yeast can then ferment. Brewers normally aim to control the final alcohol percentage in a beer through brewhouse operations rather than postfermentation dilutions with lower/higher alcohol beers or water. This approach to brewing is called "brewing to final gravity." Due to the need to have a predictable ABV for government regulatory reasons, unexpected fermentation is, therefore, a concern for many breweries <ref name="Kirkendall_2018">[https://www.tandfonline.com/doi/abs/10.1080/03610470.2018.1469081?journalCode=ujbc20 The Freshening Power of Centennial Hops. Jacob A. Kirkendall, Carter A. Mitchell & Lucas R. Chadwick. 2018. DOI: https://doi.org/10.1080/03610470.2018.1469081.]</ref>. Hop creep can also result in additional attenuation and higher carbonation after packaging, as well as diacetyl production.
Historically, there have been two studies published on the phenomenon of hops releasing glycolytic enzymes that break down starches during dry hopping: [http://barclayperkins.blogspot.com/2018/03/why-dry-hop.html Brown and Morris (1893)] and [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1941.tb06070.x Janicki et al. (1941)]. More recently, several researchers and brewers have revisited this phenomenon. Brown and Morris (1893) discovered that hops could break down maltodextrin, but failed to identify the enzymes from the hop plant material and hypothesized (probably incorrectly) that tannins were inhibiting the enzymes. Janicki et al. (1941) came to similar conclusions regarding the enzymes and tannin inhibitors, and they also concluded that the enzyme activity was independent of hop variety, geography, age, storage conditions, pH values between 4.1 and 4.8, and that one or more additional unknown factors were at play <ref name="Kirkendall_2018" />.
Kirkendall et al. (2018) found that hop varieties also have a varying ability to ferment dextrins. They reported the following ABV increases when dry hopped in a pale ale at one pound per barrel: Centennial hops (+0.27%), Citra (+0.12%), Simcoe (+0.33%), Cascade (+0.49%) and Amarillo (+0.49%). Prolonged contact with Centennial hops (42 days) increased the ABV even more so and resulted in a nearly 1% ABV increase. Rousing the hops into suspension hastened the increase in ABV compared to samples that were left still. From their results, it appears as though contact with hops during dry hopping continues the breakdown of starches and dextrins into fermentable sugars. They also concluded that dry hopping at a temperature that is too cold for the yeast strain in the beer to ferment resulted in no change in ABV. They compared the enzymatic activity of Centennial hops that were stored at -20°C versus room temperature storage and found that there was no significant difference, indicating that the unidentified enzymes are relatively stable <ref name="Kirkendall_2018" />.
Kirkpatrick and Shellhammer (2018) found that the enzymes responsible for the conversion of dextrins into sugars include amyloglucosidase (removes glucose from non-reducing ends of α-1,4 and branching α-1,6 linkages, with a preference for α-1,4 linkages and longer chain oligosaccharides), α-amylase (hydrolyzes randomly along glucopolysaccharides to produce maltose, maltotriose, maltopentaose, and maltohexaose products from amylose as well as maltose, glucose, and branched dextrins from amylopectin), β-amylase (saccharifiying enzyme, cleaving maltose in small amounts from nonreducing ends of glucopolysaccharides, and to a minor extent, maltotriose), and limit dextrinase (debranches limit dextrins at α-1,6 linkages, producing linear α-1,4 chains which can further be degraded by the combined action of amylases). They were able to successfully extract them from Cascade pellet hops using commercially available assays (enzyme specific para-nitrophenyl blocked oligosaccharide substrates). The amount of α and β-amylase found in Cascade hops was well below that of malted barley, but within the range reported in other plant leaves. These enzymes are denatured by high temperatures, and as such would be denatured when boiling hops. They reported a similar increase in ABV of 1.3% after 40 days when dry hopping a beer with Cascade hops (and a decrease of 1.9°P) at a rate of 10 g/L. They also found that the hops contained glucose and a small amount of fructose, which accounted for a sugar increase of 0.02−0.03 °P per gram of hops. More studies on whether or not the amount of dry hopping has a large effect needs to be done, and whether or not warmer temperatures speed up the enzymatic breakdown of dextrins, and the authors hypothesized that the rate of dextrin break down could be slowed by dry hopping at lower temperatures <ref name="Kirkpatrick_2018">[https://pubs.acs.org/doi/pdf/10.1021/acs.jafc.8b03563 Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops (Humulus lupulus). Kaylyn R. Kirkpatrick and Thomas H. Shellhammer. 2018. DOI: DOI: 10.1021/acs.jafc.8b03563.]</ref>. Rubottom and Shellhamer (2023) later demonstrated that drying hops at 150°F reduced the amount of enzymes in dried pellitized hops versus drying them at 130°F <ref>[https://www.tandfonline.com/doi/abs/10.1080/03610470.2023.2194838 Evaluating the Impact of High and Low Kilning Temperatures on Popular American Aroma Hops. Lindsey N. Rubottom, Thomas H. Shellhammer. Received 07 Feb 2023, Accepted 21 Mar 2023, Published online: 28 Apr 2023. https://doi.org/10.1080/03610470.2023.2194838.]</ref>.
See also:
* [https://www.thebrewingnetwork.com/hop-and-brew-school-ep8-hop-creep/ Methods for avoiding hop creep and diacetyl production from hop creep explained by Nick Zeigler ("Hop and Brew School" podcast).]
* [http://masterbrewerspodcast.com/098-dry-hop-creep MBAA podcast episode 98 on dry hop creep.]
* [https://brulosophy.com/podcasts/the-bru-lab/ Episode 008 | The Freshening Power Of Hops with Jake Kirkendall on the The Brü Lab podcast.]
* [https://www.onedropbrewingco.com.au/cleanfusion One Drop Brewing Co's "Clean Fusion" method of using a centrifuge to remove yeast before dry hopping in order to avoid hop creep.]
===Aged Hops in [[Lambic]] and Other [[Spontaneous Fermentation|Spontaneous Fermentation Beer]]===
[[File:Cantillon aging hops.jpg|thumbnail|right|Brasserie-Brouwerij Cantillon aging their hops; image provided by Dave Janssen.]]
Modern lambic traditionally uses aged hops at a moderate rate to help limit and select for microbes and regulate acid production. Modern Lambic brewers cite rates in the range of roughly 450 grams of hops per Hl hectoliter of finished beer (0.6 ounces per gallon) <ref name="Jean Van Roy on Basic Brewing Radio"> [http://hwcdn.libsyn.com/p/e/a/2/ea26e00136fe1638/bbr05-30-13cantillon.mp3?c_id=5723890&expiration=1443888327&hwt=8dd886677defabdd73669cdc262ef446 Jean van Roy on Basic Brewing Radio] </ref> (~43 min in) (see also the notes pertaining hopping rates on the [[Cantillon]] page), with some brewers possibly going above this range. The age of hops used depends on the producer and their preferences/stock. Cantillon uses hops that are roughly 3 years old<ref>D. Janssen personal communication with Jean Van Roy, 9-Nov-2013</ref>, while 3 Fonteinen reports using hops that are over 10 years old<ref name="Drie Fonteinen on Belgian Smaak"> [http://www.belgiansmaak.com/armand-debelder-michael-blancquaert-drie-fonteinen/ Drie Fonteinen on Belgian Smaak] </ref> (~48 minutes in). Jester King reported using 0.66 - 0.75 pounds of whole leaf aged hops per BBL (0.34-0.39 ounces per gallon) in their spontaneously fermented ales <ref>Averie Swanson. "Sour Power! A Pro Brewer Spontaneous Fermentation Roundtable". HomebrewCon seminar. 2018.</ref> (~31:00 mins in). Lambic brewers either add their hops while still collecting wort, sometime before the wort comes to a boil<ref>[https://www.facebook.com/groups/Lambic.Info/permalink/1831338433787524/ Video of Cantillon wort reaching a boil from Bill on Lambic.info]</ref> (also known as "first wort hopping"), or shortly after boil is reached<ref name="Drie Fonteinen on Belgian Smaak"/> (~48 min in). The hops are then boiled with the wort for essentially the full length of the boil <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1593059604055581/?comment_id=1593928187302056&reply_comment_id=1593938693967672&comment_tracking=%7B%22tn%22%3A%22R9%22%7D Conversation with Dave Janssen on MTF. 02/24/2017.]</ref><ref>[http://www.lambic.info/Brewing_Lambic#Hopping "Brewing Lambic", section "Hopping". Lambic.info website. Retrieved 02/24/2017.]</ref>. The resulting lambic beers are often surprisingly bitter, especially when young. Historically, there is [http://www.horscategoriebrewing.com/2016/04/hops-in-spontaneous-fermentation.html some evidence] that lambic brewers used a combination of aged hops and fresh dried hops. Not all aged hops are the same; different varieties/sources result in different levels of residual alpha/beta acids (probably not zero), oxidized acids, IBU's, perceived bitterness, and inhibition of lactic acid bacteria. Varieties with high acids and hop oils probably have more residual acids and oils, and aging times/conditions may not be ideal enough to completely age high alpha/beta/oil hop varieties. Therefore, it is impossible to give a blanket statement on how much aged hops to use given a specific lot of aged hops. Andrew Holzhauer from Funk Factory Geuzeria suggests tasting aged hops for bitterness and adjusting the amount of hops depending on how bitter they taste <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2723907944304069/?comment_id=2723956477632549&reply_comment_id=2724104977617699&comment_tracking=%7B%22tn%22%3A%22R%22%7D Andrew Holzhauer. Milk The Funk Facebook group on how much aged hops to use. 06/13/2019.]</ref>, while James Howat from Black Project suggests making a small batch and having the wort/beer analyzed for IBU's and adjusting accordingly.
See also:
* [http://www.horscategoriebrewing.com/2017/03/lambic-characteristics-fg-and-ibus-in.html "Lambic characteristics - FG and IBUs in Geuze" on Hors Categorie blog.]
* [http://www.horscategoriebrewing.com/2016/04/hops-in-spontaneous-fermentation.html "Hops in spontaneous fermentation" on Hors Categorie blog.]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/2811991652162364/ MTF discussion on methods of aging leaf and pellet hops.]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/1760069707354569/ MTF discussion on the general benefits of using aged hops in sour beers.]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/2503097546385111/ MTF discussion on IBU's, alpha acids, oxidized bittering compounds, etc. might play a role in aged pellet hops.]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/1923361737692031/ MTF discussion on using aged hops in other styles of beers, including historical references and tips on producing lambic-like character from aged hops and commercial cultures.]
* [https://www.facebook.com/groups/MilkTheFunk/permalink/2534320159929516/ MTF discussion on leaving hops beer fermented with ''Brettanomyces'' long term; inspired by historical English brewing methods.]
===Spent Hops===
There has been some research and experimentation on using spent hops (mostly spent dry hops) in a second beer.
* [https://www.facebook.com/groups/MilkTheFunk/permalink/3280694625292062/ MTF thread containing summaries of science and historical use of re-using spent hops.]
===Hop Extract Products===
* [https://www.stitcher.com/show/craft-beer-brewing-magazine-podcast/episode/episode-202-brandon-capps-of-new-image-brews-better-ipas-through-chemistry-86471398 Craft Beer & Brewing Magazine Podcast Episode 202: Brandon Capps of New Image Brews Better IPAs Through Chemistry.]
==See Also==
===External Resources===
* [https://www.tandfonline.com/doi/suppl/10.1080/03610470.2023.2232267/suppl_file/ujbc_a_2232267_sm7384.pdf "Free Exhaustive Literature Review on Hops (Humulus lupulus L.)," Keven Bélanger Harbour; Journal of the American Society of Brewing Chemists.]
* [https://www.barthhaas.com/resources/hop-harvest-guide#!download BarthHaas Hop Harvest Guide with flavor wheels.]
* [https://appellationbeer.com/blog/hop-queries/ "Hop Queries"; Stan Hieronymus's hops newsletter.]
* [https://www.youtube.com/channel/UCYi3RcKUPk9hCOxaN4TCoHQ/videos Hop Growers of America video presentations (YouTube).]
* [https://ext.vt.edu/agriculture/commercial-horticulture/hops.html Virginia Tech Virginia Cooperative Extension "Hops" webpage: growing and agriculture resources.]
* [https://lisameoli.wordpress.com/2015/11/04/the-hop-plant-dissected/ "The Hop Plant Dissected" by Lisa Meoli, 2015.]
* [https://patspints.com/2019/01/16/the-surprising-science-of-dry-hopping-lessons-from-tom-shellhammer/?fbclid=IwAR1QOAFBjzXLGeSftpgYuuwaWkjqiEZL41_cLogTWwWmoCbpzowRgAYCmJ4 Summary of a Shellhammer presentation on dry hop saturation, bitterness from dry hopping, and hop creep (Pat's Pints blog, Jan 2019).]
* [http://www.thebrewingnetwork.com/hop-and-brew-school-ep11-hops-and-sour-beer/ Hop and Brew School podcast interview with Vinnie Cilurzo from Russian River Brewing, Jay Goodwin from The Rare Barrel and Charlie Johnson from the Ronin Fermentation Project on using fresh and aged hops in sour beer.]
* [https://pubs.acs.org/doi/10.1021/acs.jafc.7b04055 Overview of scientific evidence of health benefits of polyphenols.]
* [https://cryopopblend.com/wp-content/uploads/2021/10/Survivable-Compounds-Handbook.pdf Survivable Compounds Handbook.]
* [https://www.youtube.com/watch?v=52nl1Fh6qnQ The hops that built craft beer – a documentary | The Craft Beer Channel.]
==References==