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

EBC Symposium: Resources for the Future Session
Biendl Martin, Hopsteiner HHV m.b.H., Mainburg, Germany
Co-authors: Christina Schmidt and Andreas Stephan, Bitburger Braugruppe GmbH, Bitburg, Germany; Christian Vogt and Thomas Hofmann, Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Germany

ABSTRACT: Isomerization of alpha-acids from hops can result in different ratios of cis- to trans-iso-alpha-acids. Whereas wort boiling in the brewhouse usually gives a ratio of approx. 2.5:1, it can be considerably higher in the case of isomerization outside the brewery when using suitable catalysts. In contrast to oxidative degradation processes, the conversion of iso-alpha-acids to tri- and tetracyclic compounds during beer aging 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 to beers produced with isomerized pellets or isomerized extracts respectively (dosages at the beginning of wort boiling for each of the variations). In addition in-line pre-isomerization (isomerization of conventional hop products in the brewery prior to addition to the kettle) was investigated as another alternative. All hop products used were from the same variety (Hallertau Magnum, crop 2009). Their dosages were calculated in order to achieve identical bitter units in the final beers (target: 30 IBU). All of the four variations were produced in duplicate in a pilot brewery (20 hL scale). Bottled beers were stored at 5 and 28°C. Analysis of fresh and aged samples after 4, 8, and 12 months targeted bitter compounds (e.g., cis/trans-iso-alpha-acids, allo-isohumulones, allo-isohumulonhydroperoxides, allo-isohumulonhydroxides, tricyclohumols, tricyclohumenes, and tetracyclohumols by means of HPLC-MS/MS) and volatile aroma compounds (e.g., linalool and strecker aldehydes by means of GC-MS/MS). General wort and beer parameter analysis showed no significant differences between the various brews. Both the addition of regular pellets at the beginning of wort boiling and their in-line pre-isomerization in the brewery resulted in almost the same cis/trans ratio. As expected, beers produced with isomerized hop products showed higher ratios of cis- to trans-iso-alpha-acids, and their concentrations of tricyclohumols, tricyclohumenes, and tetracyclohumols were accordingly lower after aging. However, all of the four types of beers hopped in different ways showed very similar volatile aroma compound profiles, even during aging. Moreover sensory evaluation of aged beers showed no preference for the variations higher in cis/trans ratios of iso-alpha-acids.

Martin Biendl received a Ph.D. degree in organic chemistry from Regensburg University in 1990. He is head of the R&D/Analytical Department at the German branch of the Hopsteiner Group, one of the largest international hop growing, trading, and processing firms. His research experience is in the field of hop-related needs for the brewing industry and beyond. He is the representative of the International Hop Industry Cooperation in the EBC Analysis Committee and, since 2001, chair of the Hops Subcommittee. As an EBC representative he is also co-chair of the International Hop Standards Committee.