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====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><refname="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>.
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. 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>[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>.