Monitoring flavor active epoxydecenals during beer storage at ppt levels

Technical Session 02: Analytical I Session
Nils W Rettberg, TU Berlin / VLB Berlin
Co-author(s): Konrad Neumann and Leif Garbe, TU Berlin/VLB Berlin, Germany

ABSTRACT: Flavor active cis- and trans-4,5-epoxy-2E-decenal isomers (epoxydecenals) are important (off)-flavor compounds in wheat bread, popcorn, oils, beef meat, fruit juices, tomatoes, tea, etc. They originate from linoleic acid oxidation; therefore, their presence in wort and beer is likely. trans-4,5-Epoxy-2E-decenal has a very intense metallic taste and smell. Its flavor threshold is reported at 0.6 pg/L in air and 20 ng/L in water, respectively. In the literature, the aroma of cis-4,5-epoxy-2E-decenal is described as citrus-like, sweet, fatty, and malty. So far there are no odor and flavor thresholds published. The concentration of epoxydecenals in a foodstuff varies widely. Fresh tomatoes contain up to 600 µg trans-epoxydecenal per kg in fruit juices, and black tea concentrations are considerably lower. In fresh grapefruit juice 3 µg/L trans-epoxydecenal were traced; in black tea the sum of both isomers is in the 1 µg/L range. Even at these comparatively low concentrations, epoxydecenal isomers were identified as key odorants of these products. In fresh beer we analyzed epoxydecenal concentrations at 20 pg/L. In wort we quantified 3–4 µg/L of both isomers in sum. Determination of this ultra-trace compound requires sophisticated analytical techniques. To quantify epoxydecenals from beer, mash, and wort, we established a rapid and effective solid phase extraction (SPE) procedure. A stable isotope dilution assay (SIDA) was established for their quantification. Negative chemical ionization–selected ion monitoring mass spectrometry (NCI-SIM-MS) was used for analysis. This method increased the sensitivity/selectivity and resulted in a reliable and ultra-trace quantification. Epoxydecenals were quantified in mash and wort, as well as in fresh and aged beers. We observed an increasing epoxydecenal concentration during beer storage. The storage temperature, light, or linoleic acid addition showed remarkable effects on their concentration and their cis/trans ratio.

Nils W. Rettberg is a trained brewer and maltster from Radeberger Gruppe, Germany. In 2011, he received a diploma in biotechnology from the Berlin Institute of Technology (TUB) and started as a Ph.D. student at the TUB Chair of Bioanalytics. In addition, Nils is employed at the Research and Teaching Institute for Brewing in Berlin (VLB), Department for Special Analyses. His work includes courses for students of biotechnology and brewing science ranging from basic chemical-technical analysis to more sophisticated modern analytical techniques. As a member of Leif-Alexander Garbe’s research group his scientific work focuses on brewing-relevant special analyses using mass spectrometry and stable isotope dilution assays. Initiated by his diploma thesis on “Flavor Active Epoxydecenals,” he has developed a deep interest in lipid oxidation, beer staling, and trace analysis in brewing.