Efficient Formation of Iso-Humulones in Aqueous Hop Solutions at Low Temperatures

In this work, new cognitions on the influence of common parameters for the isomerization of humulones, such as the temperature, pH, storage time, the concentration of humulones, microbial growth and pressure in especially prepared aqueous hop solutions were observed. The results provided an important background for future works regarding the enrichment of humulones and iso-humulones from aqueous solutions by using foam fractionation, in order to produce isomerized and non-isomerized marketable hop products. At low pHs between 2 and 3, higher isomerization rates were observed. Isomerization took place already at low temperatures between 8 and 10 °C, which previously has not been observed. At higher pHs, sufficient isomerization was also induced, but after a duration of ca. 72 hours.g..

Higher isomerization rates occurred at longer storage duration of up to, e., 240 hours. Regarding the concentration of humulones in the hop solutions, it was found that the ratio of solved humulones to the formation of iso-humulones remained nearly the same (mostly 50 %, but at least 25 %). The ratio observed over a period of 240 hours continued almost unchanged, except for 144 hours (more than 75 %). Isomerization under the applied conditions took place at normal pressure. A relationship between the concentration of humulones and iso-humulones towards microbial growth could not be established, because microorganisms grew in aqueous hop solutions spiked with a disinfectant, as well as in those without disinfectant.

BrewingScience - Monatsschrift für Brauwissenschaft, 61 (January/February) 2008, pp. 25-31