THOMAS KUNZ (1), Philip Wietstock (1), Wiebke Hense (1), Frank-Jürgen Methner (1)
(1) Berlin Institute of Technology (TU Berlin), Department of Biotechnology, Chair of Brewing Sciences, Berlin, Germany
The influence of specific hop bitter acids like α-, β-, and
iso-α-acids as well as the hop dosage regime on the oxidative stability
of wort and beer were evaluated using ESR-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
were 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. For compensating
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 wort’s
oxidative stability 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 wort’s oxidative stability can be
found during the whirlpool rest. Considering all results, the fractional
later hop dosages, especially during the whirlpool rest, make sense to
increase 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 as a certified technician in preservation
engineering (1991–1993), Thomas Kunz completed his basic studies in
chemistry at the University of Applied Sciences, Isny (1994–1995), and
his basic studies in food chemistry at Wuppertal University (1995–1998),
before starting to study food technology at the University of Applied
Sciences, Trier (1998–2002). After graduating, he worked as a chartered
engineer in the area of ESR spectroscopy at the Institute of Biophysics
at Saarland University (2002–2004). Since 2005, he has been employed as a
Ph.D. student at the Research Institute of Brewing Sciences, Berlin
Institute of Technology (Technische Universität Berlin). His main
research focus lies in analyzing radical reaction mechanisms in beer and
other beverages using ESR spectroscopy.
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