Hops (Humulus lupulus) provide a pleasant bitterness in beer from the isomerization of humulone congeners (alpha acids) into more soluble isohumulone congeners (iso-alpha acids) through acyloin-type ring contraction in the brewing process (see figure). The percentage of available alpha acids that end up as iso-alpha acids in finished beer is affected by the efficiency of the isomerization step. The utilization of iso-alpha acids in commercial brewing has been greatly improved through the addition of pre-isomerized hop extracts to the brew kettle. In some cases, the alpha acids or iso-alpha acids are hydrogenated (see figure) to provide greater flavor stability or foam quality to the finished beer. In this study we report that the isomerization of tetrahydro-alpha acids proceeded by first order kinetics and was 3-4 times faster than the rate of isomerization of alpha acids between 80 � 100 �C and pH 5.2 or 7.3. The rate of cis-isomer formation was about 2 times faster than that of trans-isomers for the iso-alpha acids and about 1.5 times faster for the tetrahydro-iso-alpha acids. The activation energy was between 90-100 kcal/mol for the isomerization of alpha acids and 70-90 kcal/mol for tetrahydro-alpha acids.

Comparative kinetic study of the isomerization of the beer bittering precursors humulone and tetrahydrohumulone from hops (Humulus lupulus)

246th National Meeting (2013)

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