13. Impact of malt-specific pro- and antioxidants on the formation of dimethyl sulfoxide

Matthias Baldus (1), Sarah Majetschak (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

Technical Session 4: Barley & Malt I
Sunday, August 14  •  2:00–3:15 p.m.
Tower Building, Second Level, Grand Ballroom

Dimethyl sulfide (DMS) is one of the most investigated undesirable aroma compounds in beer. Malt contains significant amounts of its oxidized form dimethyl sulfoxide (DMSO). DMSO cannot be removed via evaporation and survives the brewhouse, acting as DMS precursor during fermentation. The actual source of DMSO in malt and wort has not been investigated in detail. In this study different species of reactive oxygen in combination with the transition metal ions iron (Fe(II)) and copper (Cu(II)) were investigated for their oxidation ability and quantity of DMS in model solutions. Moreover the impact of malt endogenous reducing substances (sulfite, thiols, polyphenols and reductones) was tested. The results show that molecular oxygen hardly produced any DMSO during 5 hr of incubation at 80°C. DMS oxidation and concomitant DMSO formation increases with the following composition: Fe(II)+O₂ < Cu(II)+O₂ < H₂O₂+Fe(II) < H₂O₂. The added reducing substances showed an overall prooxidative behavior that could eliminated by EDTA overshoot for every substance except for polyphenols and sulfite. This indicates an direct reduction of molecular oxygen by the latter substances, whereas the other substances seem to recycle metal ions into their reduced state, keeping them available for hydrogen peroxide formation via the Fenton reaction. The observations could be further confirmed by comparison of hydrogen peroxide formation. Peroxidic oxygen is proposed as the most likely source of DMS oxidation. The pro-and antioxidative effect of certain reducing substances is further discussed based on redox kinetics and recycling ability of transition metal ions. The results of this work are of fundamental importance for initial approaches to DMSO minimization in malt, wort and beer.

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.