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Hops
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The primary '''alpha acids''' (also called "humulones" and abbreviated as "α-acids") in hops are humulone, cohumulone, and adhumulone. 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 alpha acids are mostly insoluble in wort, the isomerized alpha acids (also called isohumulones) which are formed during boiling are soluble. Isomerization leads to roughly a 70%/30% split between ''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 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.
The isomerization of alpha acids into isoalpha acids is mostly dependent on temperature, and not other variables such as sugar concentration, 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>[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>. Malowicki and Shellhammer determined a calculation that predicts the isomerization rates of alpha acids into isoalpha 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. 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 isoalpha 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>[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>.
'''Beta Acids''' (lupulones) are similar in structure to alpha acids and have the analogous individual beta acids (lupulone, colupulone, adlupulone, prelupulone, and postlupulone <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>) 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. 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]].