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O-40. Direct oxidation of amino acids—An unrevealed pathway leading to the formation of staling aldehydes in bottled beer?

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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