Nils W Rettberg, TU Berlin / VLB Berlin
Co-author(s): Leif Garbe, TU Berlin/VLB Berlin, Germany
ABSTRACT: For decades fatty acids (FA) and their oxidation products have attracted the attention of brewers. They strongly influence beer foam and fermentation, are closely linked to beer flavor (in)stability, and may promote beer gushing. Data from free and oxygenated hydroxy fatty acids (HFA) are known from the literature. However, analytical approaches differ considerably in time and chemicals required. In the current paper we present two user-friendly sample preparation procedures for low-level quantification of C12-C20 fatty acids and their abundant oxidation products. The aim of the study was a significant reduction in the amount of sample, chemicals, and costs required. We compare a modified and miniaturized Bligh and Dyer extraction with a solid phase extraction (SPE) assay. Additionally, we introduced a mild and efficient derivatization procedure that enables simultaneous methylation of all free organic acids. Quantification of FA and HFA can be done by GC-FID/internal standard methods. Here, we present a convenient GC-MS route with stable isotope dilution techniques (SIDA). Lab synthesis of deuterium labeled FA and O-18 labeled HFA was performed. Selected ion monitoring mass spectrometry (SIM-MS) strongly increased the sensitivity and selectivity of our analysis. We analyzed wort, beer, and several intermediates to validate our experimental setup and to underline its advantages. High recovery rates and excellent repeatability prove that both sample preparation procedures are very suitable and useful. Compared to assays that have been presented in the past we could strongly reduce solvents, waste, and harmful reagents.
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.