Journal of the American Society of Brewing Chemists vol:67 issue:1 pages:44-57
A detailed study of a-acids isomerization kinetics was undertaken by performing pilot-scale brewing experiments with T90 hop pellets, nonisomerized hop extract, and nonisomerized hop extract plus hop residue. In addition, a brew was prepared by adding preisomerized hop extract at the onset of wort boiling. Compared with the use of nonisomerized hop extract, significantly higher isomerization yields were obtained when vegetative material was included in the hopping. However, when vegetative material was present final alpha-acids utilization was compromised by large losses of iso-alpha-acids postboiling. Evidence of reverse isomerization of iso-alpha-acids during wort boiling was not found when preisomerized hop extract was applied. The results obtained using quantitative HPLC profiling confirmed that both alpha-acids and iso-alpha-acids were not stable upon beer aging. In particular, trans-iso-alpha-acids underwent rapid degradation in finished beer. Therefore, beers prepared with preisomerized hop extract containing relatively less-sensitive traps-isomers, and thus more cis-isomers, show improved bitterness stability upon aging compared with conventionally hopped beers.