M. BALDUS (1); (1) Technische Universität Berlin, Institute of Food Technology and Food Chemistry, Brewing and Beverage Technology, Berlin, Germany
Friday, October 9
Dimethyl sulfide (DMS) is of considerable importance to beer owing to its unpleasant flavor and aroma impressions. Packaged beer still contains considerable amounts of the DMS-precursor dimethyl sulfoxide (DMSO), as well as numerous reducing substances like sulfite or thiols. In this study we are able to demonstrate that bisulfite reduces DMSO, resulting in rising DMS levels. At first kinetic investigations were conducted in sodium acetate/acetic acid buffer solutions containing 5% (vol/vol) ethanol with varying pH and concentration ratios of reactants. DMS and DMSO levels were analyzed by GC-PFPD; sulfite concentrations were determined using continuous flow analysis. In beer representative excess of sulfite in relation to the DMSO reduction follows pseudo first-order kinetics. The reaction favors low pH, which is assumed to be related to higher DMSO protonation and easier nucleophilic attack at bisulfite anion. The mechanism was further verified in beer by storage trials at 28°C. Significant DMS formation and concomitant DMSO decrease was observed, likewise especially when the decline of sulfite was minimized using educt spiking under inert atmosphere and crown corks containing oxygen scavengers. Moreover, the thiols L-glutathione and L-cysteine reduced DMSO, resulting in DMS formation as well, whereas in a range lower than bisulfite reduction. In conclusion a reaction mechanism of the sulfite-mediated DMSO reduction and its relevance for DMS formation in beer is highlighted and discussed.
Matthias Baldus works as a scientific assistant at the Technical University of Berlin at the Chair of Brewing Science led by Professor Frank-Juergen Methner. He apprenticed as a brewer and maltster at a middle-sized brewery in western Germany before studying biotechnology and brewing technology. During that time Matthias 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 Ph.D. degree, 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.