Thomas Kunz (1), Hans Fröhlich (1), Martina Gastl (2), Clemens Kanzler (3), Frank-Jürgen Methner (1); (1) Technische Universität Berlin, Chair of Brewing Science, Berlin, Germany; (2) Technische Universität München, Chair of Brewing and Beverage Technology, München, Germany; (3) Technische Universität Berlin, Chair of Food Chemistry, Berlin, Germany

Malt and Grains

The pro- and antioxidative effects of roasted malt during brewing and beer storage are controversially discussed. Our recent studies have shown that the use of roasted malt in general leads to a decrease in oxidative wort and beer stability. In consequence, a more rapid SO2-consumption rate and a stronger formation of specific aging components during brewing and beer storage are observable. The acceleration of prooxidative processes mainly arises from the strong reduction properties of specific Maillard intermediate reaction products with a reductone/endiol structure like alpha-dicarbonyls that are generated by the high temperatures during the roasting process. These reaction products rapidly reduce oxidized metal ions like Fe+3 and thereby intensify the Fenton-reaction system. In a chain of reactions an acceleration of oxygen activation by electron transfer and a stronger radical generation of very reactive radicals (e.g., OH·) are observable. Furthermore, a significant metallic ion release caused by roasting processes is responsible for the acceleration of the described prooxidative actions. The latest investigations have shown that, in correlation with the raw materials (barley, green malt, malt) and roasting conditions like temperature, roasting time, sprinkling, etc., a very different increase in the prooxidative acting alpha-dicarbonyls can be detected using the HPLC-DAD method. It should be pointed out that the described correlation is non-linear and cannot be linked with the color or extract yield. In fact, the results demonstrate a connection between generation and decomposition of the prooxidative acting roasting intermediates like alpha-dicarbonyls depending on the roasting conditions, whereby the following intermediates of the Maillard reaction show no prooxidative properties any more. They even have partial antioxidative capacities up to the time point where measurable over-roasting starts. At this point in the roasting time, the lowest prooxidative properties are temporarily reached in the time period of the maximum color yield. Beyond this reversal point, an increase in prooxidative acting radical generation is observable again using EPR-spectroscopy. These important and useful facts open an innovative possibility to influence the prooxidative properties of roasted malt by selective steering of roasting processes. However, in the range of high roasting temperatures (>220°C) the space in time including the advantageous reversal point is a very short period and makes process steering more difficult. All together, the results demonstrate the advantages of selective roasting process steering and can give a partial explanation for the controversially discussed pro- and antioxidative effects of roasted malt during brewing and beer storage in the literature.

After qualifying as a certified technician in preservation engineering (1991-1993), Thomas Kunz completed his basic studies in chemistry at the University of Applied Sciences, Isny (1994-1995) and his basic studies in food chemistry at Wuppertal University (1995-1998), before starting to study food technology at the University of Applied Sciences, Trier (1998-2002). After graduating, he worked as a chartered engineer in the area of ESR spectroscopy at the Institute of Bio Physics at Saarland University (2002-2004). Since 2005 he has been employed as a scientific assistant, Ph.D. student and since 2009 as head of the laboratory at the Institute of Food Technology and Food Chemistry, Chair of Brewing Science, Technische Universität Berlin. His main research focus lies in analyzing radical reaction mechanisms and oxidative processes in beer and other beverages using ESR spectroscopy. A further research focus consists of the optimization of filtration and stabilization processes.