pH-dependent impact of metal ion complexes on haze formation and oxidative beer stability

THOMAS KUNZ (1), Stefanie Karrasch (2), Patricia Diniz (1), Frank-Jürgen Methner (1)
(1) Berlin Institute of Technology (TU Berlin), Department of Biotechnology, Chair of Brewing Sciences, Berlin, Germany; (2) University Hannover, Hannover, Germany

Our recent studies prove that the chill haze formation in stabilized beers is correlated to oxidative processes and generated reaction products of the Fenton-/Haber-Weiss reaction system after the consumption of the endogenous antioxidant potential (EAP). Our investigations clearly show that after the achievement of the EAP-zero value the reaction products of the Fenton-/Haber-Weiss system, such as Fe3+, Cu+, and OH•– radicals, start interacting and generating metal ion complexes with oxidized, haze-active polyphenol-protein-complexes. These complexes are significant for the visible chill haze formation and the formation depends on the temperature because of the low bonding forces. Experiments which have been carried out under addition of metal ions with specific oxidation steps such as Fe3+, Fe2+, and others to beer within different pH ranges clearly show an increase in haze, caused by these complexes. When facing different pH areas, the highest haze formation was observed to be around the pH value of 3.5. It seems that based on the stronger bonding forces of specific metal ions in beer at a lower pH value a higher haze formation can be observed. This phenomenon can be explained by more bonding sites which are stabilizing the generated chill haze. However, the EAP consumption proceeds more slowly, because there is a lower concentration of iron ions available for the acceleration of oxidative processes and the radical generation by the Fenton-/Haber-Weiss reaction system. This results in a higher oxidative stability but later and stronger chill haze formation. Contradictory experiments at higher pH areas resulted in an earlier but lower haze formation due to the weaker bonding power in the metal complexes with haze-active polyphenol-protein complexes and in a significantly faster consumption of the EAP. Furthermore, experiments with and without oxygen addition allow the conclusion that the oxygen content is the most important factor for the formation of oxidized polyphenol-protein complexes and their further complexation with specific metal ions. In both cases (with and without oxygen addition), the metal ions are responsible for a higher haze generation by metal ion protein-polyphenol complexes. In separated chill haze from beer, an agglomeration of stabilized organic radicals also could be detected by EPR-spectroscopy. During the progress of beer aging, the oxidized polyphenol-protein-metal ion complexes, including the stable organic radicals, can react in radical reactions and form covalent bonds, one way for converting chill haze to permanent haze. Additional filtration trials with kieselguhr, CMF, and other filter aids show a clear influence of different metal ion insertion on the haze formation and oxidative beer stability. It has been shown that this research work can give useful further knowledge about avoiding the undesired haze formation in beer and other beverages.

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 Biophysics at Saarland University (2002–2004). Since 2005, he has been employed as a Ph.D. student at the Research Institute of Brewing Sciences, Berlin Institute of Technology (Technische Universität Berlin). His main research focus lies in analyzing radical reaction mechanisms in beer and other beverages using ESR spectroscopy.