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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]].
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]]===