Changes

'''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.

'''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====

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==

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" />.

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>.

===Terpenes===

See also:

:<youtube height="200" width="300">GCQ22HSDDUQ</youtube> <youtube height="200" width="300">r09eb46k97I</youtube>

====Glycosides====

* [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.]

===Lightstruck===

| 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.

|-

|-

| 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.

|}

'''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 believed that hulupones have a greater impact on beer flavor and bitterness than these compounds <ref name="Dušek_2014" />.

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" />.

* [http://scottjanish.com/increasing-bitterness-dry-hopping/ "Increasing Bitterness By Dry Hopping", article by Scott Janish on oxidized alpha acids.]

===Dry Hopping===

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 gallon or 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" />.

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) also reported that the exposure time of the beer to the dry hop material played a significant role in the breakdown of dextrins. Most of the breakdown of dextrins occurs within 5 days (+0.7% ABV), but continued slowly up until at least 40 days (+1.3%). They also tested removing the hops via centrifuge and storing the beer at 10°C or 20°C. Their results suggested that the effect of the enzymatic breakdown of dextrins by hops appears to only be active when in contact with the hops and that once the beer is removed then this breakdown of dextrins stops. The authors suggest that to avoid as much breakdown of starches and over-attenuation from dry hops as possible, brewers can limit the amount of time sits on the hops and reduce the temperature, however, it is also important to consider how this might impact the product's flavor and careful measures should be taken to balance the over-attenuation problem and overall beer quality <ref name="Kirkpatrick_2018" />. After removing the beer from the hops, a second diacetyl rest has been suggested as a way to clean up any diacetyl or off-flavors that the yeast produces from the additional fermentation during dry hopping <ref>[https://www.rockstarbrewer.com/how-dry-hop-creep-causes-diacetyl-in-beer-and-how-brewers-can-minimise-the-risk/ STEVE 'HENDO' HENDERSON. How “Dry Hop Creep” Causes Diacetyl In Beer and How Brewers Can Minimise The Risk. Rockstar Brewer Academy website. 09/03/2018. Retrieved 10/05/2018.]</ref>. Other recommended solutions to avoiding hop creep is pasteurizing, filtering, or cold crashing out the yeast before dry hopping, storing the beer cold so that the yeast remains inactive, reducing dry hopping amounts, and dry hopping before fermentation is finished <ref>[http://beersmith.com/blog/2019/03/31/dry-hop-creep-over-carbonation-and-diacetyl-in-beer/ Brad Smith. BeerSmith blog. 03/13/2019. Retrieved 07/23/2019.]</ref>.

===External Resources===

* [https://www.barthhaas.com/resources/hop-harvest-guide#!download BarthHaas Hop Harvest Guide with flavor wheels.]

* [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.]