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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.

'''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==

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 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"/>.