Influence of different hop products on the cis/trans ratio of iso-α-acids in beer and changes in key aroma and bitter taste molecules during beer ageing

Isomerization of α-acids from hops can result in different ratios of cisto trans-iso-α-acids. In contrast to oxidative degradation processes, the conversion of iso-α-acids to tri- and tetracyclic compounds during beer ageing is only possible in the trans form. Therefore, it might be an advantage to use pre-isomerized hop products in order to maximize the cis form in beer. For this study, beers conventionally hopped with regular pellets were compared with beers produced with isomerized pellets and isomerized extracts. In addition, inline pre-isomerization was investigated as another alternative. The four variants were produced in a pilot brewery. Bottled beers were stored at 5 and 28°C. Analysis of fresh and aged samples after 4, 8, and 12 months was focused on bitter compounds (HPLC-MS/MS) and on volatile aroma compounds (GC-MS/MS). Beers produced with isomerized hop products showed higher ratios of cis-to transiso-α-acids, and their amounts of degradation products of trans-iso-α-acids were accordingly lower after ageing. However, all of the four types of beers hopped in different ways showed very similar profiles of volatile aroma compounds even during ageing. Moreover, sensory evaluation of aged beers gave no preference for the variations with pre-isomerized hop products.