A Comparison of the Kinetics of Hop Alpha Acids and Their Reduced Forms
Marlatt, Craig W.
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Two separate samples of hop alpha acids (humulones) were purified from a CO2 extract; one of these samples was further hydrogenated to produce TH-alpha-acid (tetrahydrohumulones). Partitions of these acids underwent acyloin ring contraction isomerization reactions at 100, 90, and 80 °C under acidic (pH 5.2) and basic (pH 7.3) conditions (0.1% sample concentration) in a 3,3-dimethylglutamic acid and sodium hydroxide aqueous buffer solution. Samples were taken at fixed intervals during each reaction, quenched, and were analyzed by HPLC for amounts of starting material and product (isohumulones or tetrahydroisohumulones) present. Based on these data it was found that both alpha- and TH-alpha-acids follow first order kinetics and Arrhenius behavior in this reaction; data for isohumulone formation resemble literature values for that reaction. Additionally, TH-alpha-acids were found to have higher rate constants and lower activation energies when compared to alpha-acids isomerized under the same conditions, meaning that they react more readily under boiling-wort conditions present in beer brewing. It was also found that the ratio between the rates of formation of cis vs. trans tetrahydroisohumulone is more even in this case, another advantage that species has over its non-reduced counterpart.