Presenter: Philip C. Wietstock, Technische Universität Berlin,
Berlin, Germany
Coauthors: Thomas Kunz and Frank-Jürgen Methner,
Technische Universität Berlin, Berlin, Germany
Off-flavor in lager beer is often associated with the appearance
of staling aldehydes such as 3-methylbutanal, 2-methylbutanal,
methional, phenylethanal, and benzaldehyde. Previous storage
experiments revealed that elevated concentrations of Fe and
oxygen promote the rate of beer flavor staling. Known mechanisms,
such as Strecker degradation of amino acids, alone give
no explanation for this phenomenon. Additional storage trials
were conducted to further assess this observation. A commercially
available lager beer was dosed with varying level of
amino acids and Fe, respectively, and stored over a period of 6
months (dark, 28°C, with and without oxygen in headspace).
Bottles without additions served as references. Staling aldehyde
levels were measured during storage via SAFE-GC/MS, and
oxidative beer stability was determined using electron spin
resonance (ESR) spectroscopy. The experiment confirmed that
staling aldehyde levels in bottled beer positively correlate with
both amino acid and Fe levels and are increased by the presence
of oxygen. Bench trials using model solutions (acetate buffer,
pH 4.3, 5 vol.-% EtOH) revealed that oxidation of leucine,
isoleucine, and phenylalanine via Fe-H2O2 yields 3-methylbutanal,
2- methylbutanal, phenylethanal, and benzaldehyde,
respectively. Furthermore, radical concentration as measured
using ESR spectroscopy directly correlated with the formation
of these aldehydes. A so far unrevealed staling mechanism via
direct oxidation of amino acids by hydroxyl radicals in bottled
beer is proposed.
Philip Wietstock is a scientific assistant at the Technische
Universität Berlin, Germany. After graduating with a Dipl.-Ing.
degree in biotechnology from the Technische Universität Berlin
(2009), he worked for one year as an intern at the Department
of Food Science and Technology at Oregon State University,
Corvallis, OR. In 2011, he transferred to his present profession,
where he is working on his dissertation, for which he is
investigating parameters influencing oxidative beer stability.
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