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Hops
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'''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>. 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, alpha acids are isomerized into iso-alpha acids (also called isohumulones) 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, 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.
There is evidence to show that during wort boiling iso-humulone and perhaps also iso-cohumulone bind with the head forming proteins, Lipid Transfer Protein (LTP) and Protein Z, to help form foam-positive structures in beer. These iso-alpha acids bind less so with LTP than they do with Protein Z. The resulting bound structures have been described as "vesicles", which are protein "bubbles" (but with no gas in them) with thick surface layers <ref>[https://www.sciencedirect.com/science/article/pii/S0268005X19325391 Vesicular structures formed from barley wort proteins and iso-humulone. Yi Lu, Peter Osmark, Björn Bergenståhl, Lars Nilsson. 2020.]</ref><ref>[https://www.mdpi.com/2624-8549/7/2/65 Chen C, Lv C. Interaction Between Iso-α-Acid Extracted from Hops and Protein Z Improves Beer Foam Quality and Stability. Chemistry. 2025; 7(2):65. https://doi.org/10.3390/chemistry7020065.]</ref>. See also [https://www.youtube.com/watch?v=5F8vmuTV5Mg Escarpment Labs presentation on the science of beer foam].
'''Beta Acids''' (lupulones) are similar in structure to alpha acids and have the analogous individual beta acids (lupulone, colupulone, adlupulone, prelupulone, postlupulone, adprelupukone, and acetolupulone <ref name="Dušek_2014">[http://pubs.acs.org/doi/abs/10.1021/jf501852r Qualitative Determination of β‑Acids and Their Transformation Products in Beer and Hop Using HR/AM-LC-MS/MS. Martin Dušek, Jana Olšovská, Karel Krofta, Marie Jurková, and Alexandr Mikyška. 2014.]</ref><ref name="Hao_2020" /><ref name="Leker_2022" />) to individual alpha acids. In their original form, beta acids do not contribute to the flavor of beer because they are not soluble in beer unless the pH of the boiling wort is significantly raised to around 7 pH (which is not typical in brewing conditions) and the original gravity is relatively low (2-8°P) <ref name="Bastgen_2019" />. They are also not able to isomerize during wort boiling. Beta acids do not become soluble in wort or beer unless they are chemically modified by a process such as oxidation <ref name="Algazzali_2014" />, nor are they soluble in beer when dry hopping <ref name="Maye_EBC2017">John Paul Maye. EBC 2017 Presentation. 2017.]</ref>. Oxidized beta acids are soluble and can contribute to bitterness in beer. Oxidized beta acids are discussed more under [[Hops#Acids_2|aged hops]].
====Isomerization of Alpha Acids====
The isomerization of alpha acids into iso-alpha acids is mostly dependent on alpha acid content of the hops, time (to a certain extent), temperature, original gravity, hop rate (hop weight), and IBU saturation. Other variables also affect isomerization to a lesser extent such as pH and calcium concentration <ref>[https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1964.tb06356.x CHANGES IN HOP ACIDS CONCENTRATIONS ON HEATING IN AQUEOUS SOLUTIONS AND UNHOPPED WORTS. H. O. Askew. 1964.]</ref><ref name="Malowicki_2005">[http://pubs.acs.org/doi/abs/10.1021/jf0481296 Isomerization and Degradation Kinetics of Hop (Humulus lupulus) Acids in a Model Wort-Boiling System. Mark G. Malowicki and Thomas H. Shellhammer. 2005.]</ref><ref name="justice_2018">[https://www.mbaa.com/publications/tq/tqPastIssues/2018/Pages/TQ-55-3-1205-01.aspx Tracking IBU Through the Brewing Process: The Quest for Consistency. Aaron Justus. Director of R&D and Specialty Brewing, Ballast Point Brewing. MBAA TQ 2018; vol. 55, no.3. https://doi.org/10.1094/TQ-55-3-1205-01.]</ref>. The higher the gravity of wort above 1.050 SG, the more proteins coagulate and drop iso-alpha acids out of solution (lower gravity worts are not affected by this). During fermentation, yeast cells can absorb iso-alpha acids, which results in further loss of iso-alpha acids in the finished beer <ref name="Bastgen_2019" />. Lower flocculating yeast strains tend to reduce the IBU in finished beer more than high flocculating yeast <ref name="justice_2018" />. Significant isomerization of alpha acids can occur in water without sugar at all (temperatures around boiling are still required), which is relevant in the production of [http://www.garshol.priv.no/blog/331.html "hop tea" in traditional farmhouse brewing] where hops are steeped in hot water for some time, and this is said to extract bitterness from the hops <ref name="Malowicki_2005" /><ref>[http://www.ijbbb.org/papers/161-E005.pdf Kinetic Modeling of Hop Acids during Wort Boiling. Yarong Huang, Johannes Tippmann, and Thomas Becker. 2013.]</ref><ref>[http://www.garshol.priv.no/blog/331.html Lars Marius Garshol. "Raw ale". Larsblog. 05/06/2015. Retrieved 12/17/2018.]</ref>. Aside from boiling hops in wort, [https://www.frontiersin.org/articles/10.3389/fnut.2022.843808/full Hydrodynamic Cavitation] at a 90°C temperature is another method that results in the isomerization of alpha acids into iso-alpha acids; although an additional 10 minutes of boiling at 100°C was needed to remove DMS and achieve hot break. This method reportedly had a 33% savings in energy costs in [https://www.sciencedirect.com/science/article/abs/pii/S0960308524000166 one study].
Malowicki and Shellhammer determined a calculation that predicts the isomerization rates of alpha acids into iso-alpha acids at different temperatures. Beginning at the boiling temperature of 100°C/212°F, which could be considered a rate of 100%, at 96°C/205°F the rate is 72%, and at 90°C/194°F the rate is 43%. This rate continues to drop significantly as the temperature of the wort decreases. At 82°C/180°F isomerization occurs at a rate of 17%. At a temperature of 50°C/122°F, the isomerization rate is at 1%, and finally 0% at 45°C/113°F (note that [https://www.tandfonline.com/doi/full/10.1080/03610470.2021.1878684 Kishimoto et al. (2021)] reported that isomerization begins somewhere between 60-70°C <ref name="Kishimoto_2021">[https://www.tandfonline.com/doi/full/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.]</ref>). This fact has several impacts on brewing processes. For example, when brewing at higher altitudes where the boiling point of wort is less than 100°C/212°F, the isomerization of alpha acids into iso-alpha acids will be reduced to whatever the rate is at that lower temperature. "Hop stands" or "whirlpool additions" where hops are left in contact with hot wort that is less than boiling temperature will continue to isomerize alpha acids <ref name="Malowicki_2005" /><ref>[https://www.mbaa.com/publications/tq/tqPastIssues/2017/Pages/TQ-54-3-0806-01.aspx A Look at Isomerization Reduction Due to Altitude. John Palmer. MBAA TQ 2017 http://dx.doi.org/10.1094/TQ-54-3-0806-01.]</ref>.
==Antimicrobial Properties==
Hops are known to have antimicrobial properties against Gram-positive bacteria. This includes bacteria 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]]''. Gram-negative bacteria found in beer, such as ''Acetobacteraciae'', 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. 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 demonstrated to be antimicrobial , with colupulone and lupulone being suggested as the strongest inhibitors <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><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, Volume 169, 2023. 112832. ISSN 0963-9969. https://doi.org/10.1016/j.foodres.2023.112832.]</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 <ref>[https://kuleuven.limo.libis.be/discovery/search?query=any,contains,LIRIAS4199525&tab=LIRIAS&search_scope=lirias_profile&vid=32KUL_KUL:Lirias&offset=0 Goedseels, M., & Michiels, C. (2024). Mechanisms of resistance against hop beta acids in Listeria monocytogenes.]</ref> and 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-acid and 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 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, 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" />.
For techniques and usage amounts of aged hops, see [[Hops#Aged_Hops_in_Lambic_and_Other_Spontaneous_Fermentation_Beer|Aged Hops in Lambic]].
See also:
* [https://byo.com/article/the-effects-of-aged-hops-on-beer-quality/ "The Effects of Aged Hops on Beer Quality," by Dr. Pattie Aron.]
===Aging Hops===
====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><ref>[https://www.tandfonline.com/doi/abs/10.1080/03610470.2025.2497645 Chenot, C., & Shellhammer, T. H. (2025). Hop Storage and HSI – Past, Present and Future. Journal of the American Society of Brewing Chemists, 1–9. https://doi.org/10.1080/03610470.2025.2497645.]</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>.
Aging hops while exposed to oxygen develops a cheesy aroma due to [[Isovaleric Acid|isovaleric acid]], isobutyric acid, and 2-methylbutyric acid. These acids are produced by the oxidative cleavage of acyl side chains of the hop resins <ref name="Briggs_2004" />. These cheesy oxidation compounds can be esterified to form wine-like and fruity tasting compounds (see [[Hops#Esters|Esters]] below and [[Aging_and_Storage#Acids_and_Esters|Aging and Storage]]) <ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>.
[https://www.mdpi.com/2311-5637/7/2/66/html Bruner et al (2021)] investigated whether or not yeast strain selection has an impact on hop creep with the goal of finding if any strains would reduce the potential for dry-hop creep. They examined 30 different ''Saccharomyces'' yeasts from different yeast labs, and measured the [https://beerandbrewing.com/dictionary/gaKDNn0yxE/ real degrees of fermentation (RDF)] that occurred after dry-hopping post fermentation. With the exception of two strains, all of the yeasts re-fermented the beer after dry hopping, indicating that yeast strain selection won't reduce the chances for dry hop creep. There were two exceptions to this. The first was a diastatic strain of ''Saccharomyces cerevisiae'' (SafAle™ BE-134), which was able to break down starches in the beer prior to dry hopping due to the [[Saccharomyces#Diastatic_strains_of_Saccharomyces_cerevisiae|diastatic properties]] of this yeast strain. The other exception was a strain of ''Saccharomyces mikatae'' that is used as a co-fermenter in wine fermentations and is a poor attenuater of wort. The study also found no correlation between flocculation and attenuation from dry-hop creep, but suggested that further research be done to investigate the belief that higher flocculating strains could reduce the negative effects of dry hop creep <ref>[https://www.mdpi.com/2311-5637/7/2/66/html Bruner J, Marcus A, Fox G. Dry-Hop Creep Potential of Various Saccharomyces Yeast Species and Strains. Fermentation. 2021; 7(2):66. https://doi.org/10.3390/fermentation7020066.]</ref>.
A Masters thesis by Tanriverdi (2024) demonstrated that that longer contact times of hops in beer significantly promoted enzymatic activities involved in glucose production, maltose degradation and isoamyl acetate hydrolysis. The same observations were evidenced at higher temperatures. It was noted that a dry heat treatment of hops prior to incubation revealed to reduce the release of glucose and the hydrolysis of isoamyl acetate. Surprisingly, only a high dosage of 10% v/v ethanol in beer resulted in inhibition of amylolytic enzymes. For this dosage, esterase activity remained intact. Concerning the influence of hop variety, only amylolytic enzymes of Simcoe hops exhibited pronounced activities compared to other varieties. Finally, cryo pellets exhibited lower enzymatic activities compared to T90 pellets and whole cones, which indicates that enzymes could be mainly found in vegetative material <ref>[https://dial.uclouvain.be/memoire/ucl/en/object/thesis%3A48802 Tanriverdi, Yudum. Influence of the residual enzymatic power of hops added during fermentation and/or maturation on beer stability. Faculté des bioingénieurs, Université catholique de Louvain, 2024. Prom. : Collin, Sonia ; Willemart, Guillaume.]</ref>. A second study by Tanriverdi et al. (2025) reported that op T90 pellets that were were baked in an oven at 100°C for 40 min had no significant enzymatic activity while hops that did not undergo this treatment exhibited significant enzymatic activity from the dry hops (both hydrolysis of isoamyl acetate via esterases and an increase in glucose and maltose via α-amylase, β-amylase, limit dextrinase, and α-glucosidase). In addition, they confirmed that longer contact time with dry hops (1-3 weeks) increased the enzymatic activity. Warmer conditions also increased this activity (24°C vs 50°C). Again, they saw limited enzymatic activity when ethanol was at 10%, indicating that high ABV beers should be less affected by dry hop creep <ref>[https://www.tandfonline.com/doi/full/10.1080/03610470.2024.2432146 Willemart, G., Tanriverdi, Y., & Collin, S. (2025). Impact of Contact Time, Temperature, and Ethanol Content on Hop Creep-Related Enzymatic Activities in Beer. Journal of the American Society of Brewing Chemists, 1–7. https://doi.org/10.1080/03610470.2024.2432146.]</ref>.
[https://www.tandfonline.com/doi/full/10.1080/03610470.2024.2388430 Hrabia et al. (2024)] demonstrated several conditions that encourage of discourage dry hop creep. They demonstrated that pasteurized beer does not demonstrate hop creep due to yeast being killed during the pasteurization process. It was also demonstrated that separating the hops plant material from the beer after 3 days reduced the effect of dry hop creep. Microbiological analysis shows that if there were any microorganisms on the hops, no beer spoilers such as ''Brettanomyces'' were found that could have over-attenuated the beers that suffered from dry hop creep. The duration of the hop creep in hazy IPA's was faster than in a cold IPA; this was hypothesized to be caused by lower fermentation temperatures of the cold IPA, mashing regiment, or being a bottom fermented beer (lager). They also demonstrated that fermenting under head pressure slows the effect of dry hop creep, but given enough time, non-pressure and pressure fermentation will reach the same final gravities <ref>[https://www.tandfonline.com/doi/full/10.1080/03610470.2024.2388430 Hrabia, O., Poręba, P., Ciosek, A., & Poreda, A. (2024). Effect of Dry Hopping Conditions on the Hop Creep Potential of Beer. Journal of the American Society of Brewing Chemists, 82(4), 412–421. https://doi.org/10.1080/03610470.2024.2388430.]</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://www.masterbrewerspodcast.com/327 MBAA Podcast 327 "Creep Control".]
* [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.]
* [https://www.youtube.com/watch?v=4g8cyAz3jM4 The Brü Lab podcast 169: What We Know About Hop Creep w/ Dr. Tom Shellhammer (Aug 27, 2024).]
===Aged Hops in [[Lambic]] and Other [[Spontaneous Fermentation|Spontaneous Fermentation Beer]]===
* [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://byo.com/article/the-effects-of-aged-hops-on-beer-quality/ "The Effects of Aged Hops on Beer Quality," by Dr. Pattie Aron.]
* [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.]