Frank-Jürgen Methner, Technische Universität Berlin, Department of Biotechnology, Chair of Brewing Sciences, Berlin, Germany
Co-author(s): Thomas Kunz and Christian Müller, Technische Universität Berlin, Department of Biotechnology, Chair of Brewing Sciences, Berlin, Germany
ABSTRACT: For the prognosis of the flavor stability of beer, electron spin resonance (ESR) spectroscopy has been used for determination of the so-called lag time. Our former investigations demonstrated that the lag-time measurement used until now falsifies the results of oxidative flavor stability due to an increasing pH value during the analysis caused by the spin-trap reagent (PBN). The developed EAP determination excludes the falsifications due to the use of a different spin-trap reagent (POBN) in lower concentrations, which results in a beer matrix dependent, linear correlation between the SO2 content and the EAP value. For this reason, the EAP determination offers a new beneficial index number for the evaluation of flavor stability, the beverage antioxidative index (BAX), which provides additional information about the anti- and pro-oxidative properties of the beer matrix independent of SO2 content. BAX is affected by the content of metallic ions, pH value, polyphenols, proteins, intermediate Maillard reaction products, etc. and gives information about the consumption rate of the existing antioxidative potential during storage. In addition, EPR spectroscopy standard analyses performed according to MEBAK and CFA (continuous flow analyzer) (SO2, polyphenols, etc.) were used to obtain additional information about the influences of the different beer ingredients on the radical generation and oxidative stability by application of the described EAP and BAX determination. The investigations clearly demonstrate that lower pH values improve oxidative beer stability, which is reflected by higher EAP, lower radical generation, and higher BAX values. Iron entry caused by raw materials and kieselguhr filtration deteriorates oxidative stability, although polyphenols do not change the EAP and BAX significantly. Furthermore, it could be illustrated that hop ingredients like alpha- and beta-acids can act as chelating agents and can significantly influence radical generation and reduce oxidative processes. The latest results showed that specific intermediate Maillard reaction products with reductones/endiol structure formed during kilning of the malt and wort boiling decreased the oxidative stability by the acceleration of the Fenton-Haber-Weiss reaction system. In this context, brewing with raw barley leads to improved oxidative stability, as indicated by a higher BAX and decreasing radical generation. The optimized lag-time measurement, called EAP determination, makes an unbiased examination of flavor stability possible. In combination with BAX, based on a beer matrix dependent linear correlation of the EAP value and the SO2 content of a beer, it is possible to obtain a deeper insight into the influences of different beer ingredients on flavor stability. An additional advantage of the BAX determination is the indirect determination of the SO2 content.
Frank-Jürgen Methner conducted studies in brewing science at Berlin Institute of Technology (TU) from 1975 to 1981. After the studies, he began working as an operating supervisor at the Schlösser Brauerei, Düsseldorf. From 1982 to 1986, he was a scientific assistant with teaching duties. Research projects and Ph.D. thesis, “Aroma Formation of Berliner Weissbier with Special Focus on Acids and Esters,” were further tasks. For 18 years, starting in 1987, he held a leading position as a director at the Bitburger Brauerei, Bitburg, Germany, with responsibilities in fields such as research and technology, as well as quality assurance. Beginning with the winter semester 2004/2005 he took over the Chair of Brewing Science within the Department of Biotechnology at Berlin Institute of Technology (TU Berlin).