140. Response surface and kinetic modeling of the oxidation of dimethyl sulfide in malt model solutions

Matthias Baldus (1), Satoka Tsushima (2), Xi De (2), Frank-Jürgen Methner (2); (1) Technische Universität Berlin, Berlin, Germany; (2) Technische Universität Berlin, Institute of Food Technology and Food Chemistry, Lab of Brewing Science, Berlin, Germany

Malt and Grains

Dimethyl sulfide (DMS) is one of the most important flavor compounds in beer. During kilning of the germinated grain significant amounts of DMS oxidize leading to the formation of dimethyl sulfoxide (DMSO), the precursor of DMS during fermentation. For minimization of DMSO in wort, determining the reaction mechanism is the required first step. In this work the oxidation of DMS by hydrogen peroxide as a potential oxidant is investigated in model solutions using response surface and kinetic modeling. The oxidation of DMS was investigated in 0.1 M sodium acetate/acetic acid buffer solutions. A 23 factorial central composite design with 6 central points and 6 axial points was used to assess the impact of the independent factors temperature, pH and hydrogen peroxide concentration on DMS oxidation. Each trial was conducted in 50 mL Schott bottles containing 16.95 µmol of DMS. The bottles were incubated under the respective conditions for 4 hr before they were cooled to 1°C and analyzed for DMS and DMSO. The kinetic trial was carried out at 95°C at a ratio of H2O2/DMS = 3 for 5 hr. The trials show significant DMSO formation by H2O2-mediated oxidation of DMS in a range between 0 and 31%. The multivariate data analysis led to a 2-factor interaction model (2FI) for DMS and DMSO, revealing temperature and hydrogen peroxide concentration as significant impact factors for DMSO formation (P ≤ 0.003), whereas pH was insignificant (P ≥ 0.4). Significant interactions were found between temperature and H2O2 concentration. Similar results were obtained by linear modeling. 2FI was chosen owing to the higher correlation coefficient (R2lin = 0.756, R22FI = 0.9491) revealing an adequate representation of the experimental results. In each trial the sum of DMS and DMSO remained constant, demonstrating no further oxidation of DMSO. The kinetics of DMS oxidation were shown as an overall bimolecular reaction. Hydrogen peroxide was found to be a potential source of DMSO in malt. Response surface modeling showed that the reaction was independent of the pH (3.3-6.7) and was dominated by temperature and H2O2 concentration. The rate of oxidative consumption of DMS was found to follow a pseudo first-order reaction even in small overshoot of H2O2, whereas the overall reaction can be seen as a second-order process. For the first time DMS oxidation by H2O2 was investigated with respect to thermal food processing parameters. For malt and beer production the results can serve as basics for the minimization of DMSO in wort.

Matthias works as scientific assistant at the Technische Universität Berlin at the Chair of Brewing Science. He apprenticed as a brewer and maltster at a middle-sized brewery in Germany before he studied biotechnology and brewing technology. He graduated from his studies as a Diplom-Ingenieur. During this time he investigated grist fractionation methods to optimize the lautering process. Moreover, he evaluated thermal desorption processes to optimize volatilization of undesired aroma compounds. Matthias is currently working on his dissertation, which is focused on sulfuric substances in the brewing process, especially on DMS and its precursors. In addition, he is working on optimization techniques for the brewhouse process.