Presenter: Philip Wietstock, Berlin Institute of Technology, Department of Biotechnology, Chair of Brewing Sciences, Berlin, Germany. Coauthor(s): Thomas Kunz, Jakob Frenzel, Wiebke Hense, and Frank-Jürgen Methner, Berlin Institute of Technology, Department of Biotechnology, Chair of Brewing Sciences, Berlin, Germany.
The influence of specific hop bitter acids, like α-, β-, iso-α-acids, as well as the hop dosage regime on the oxidative stability of wort and beer were evaluated using EPR spectroscopy, GC-MS, HPLC, and sensory analyses. The addition of hops resulted in significantly higher oxidative stabilities of wort and beer compared to brews where no hops was added. Furthermore, hop α- and β-acids showed similar radical quenching abilities, while iso-α-acids displayed a negligible effect. Consequently, the isomerization of α-acids to iso-α-acids significantly reduced the antioxidant capacity of wort. Compared to a single hop dosage at the beginning of wort boiling, it was possible to increase the concentration of strongly antioxidative α-acids in wort by applying fractional hop dosage regimes while achieving comparable hop bitter yields. The radical generation could be decreased by 15–28%, yielding significantly higher oxidative wort stabilities. Based on these results, further investigations were carried out, whereby fractional hop dosage regimes were also applied during the whirlpool rest. To compensate for lower hop bitter yields, it was necessary to partially preisomerize the used hop extract before adding it. The results clearly demonstrate that the fractional addition of preisomerized hop extracts in the whirlpool leads to a higher content of antioxidative α-acids in the pitching wort. Hence, a lower radical generation can be detected, and the oxidative stability of wort increases significantly, while comparable bitter units can be achieved. One explanation for this phenomenon may lie in diminished precipitations of hop ingredients during wort boiling and hot trub formation. In conclusion, the ideal stage for adding α-acids to the wort in order to increase the oxidative stability of wort is during the whirlpool rest. Considering all results, the fractional later hop dosages, especially during the whirlpool rest, increased the antioxidant properties of wort and beer. Additionally, in a lot of cases, the higher amount of α-acids in the pitching wort resulted in slightly higher SO2 contents after fermentation. The enhanced SO2 formation may be caused by higher α-acid contents which, in turn, lead to a lower consumption of SO2 by oxidative processes during fermentation.
After qualifying for the final secondary-school examinations (2000), in 2002 Philip Wietstock started his biotechnology studies for qualification as a graduate engineer at the Technische Universität Berlin, which he successfully completed in 2009. During his studies, he worked as a student research assistant at the VLB/Technische Universität Berlin. Currently, Philip is working on his dissertation, for which he is investigating the influence of hops on oxidative stability of beer.