M. LUND (1), N. E. de Almeida (2), D. R. Cardoso (2), M. B. Sørensen (3), M. L. Andersen (1); (1) University of Copenhagen, Frederiksberg C, Denmark; (2) University of Sao Paulo, Sao Carlos, Brazil; (3) Novozymes A/S, Bagsværd, Denmark

Stability
Wednesday, June 4 - 3:00 p.m.-4:45 p.m.
Level 3, Crystal Room

Radical driven redox reactions are important contributors to beer aging processes. The 1-hydroxyethyl radical arising from oxidation of ethanol is one of the most abundant radicals, and it can potentially react with many different beer components, including bitter acids from hops, phenolic compounds, and thiol-containing peptides and proteins, eventually resulting in changes in flavor and colloidal stability. Thiol-containing proteins have been suggested to play a role together with sulfite in the antioxidative mechanism controlling the oxidative stability of beer. In this study we have evaluated the ability of endogenous thiols in beer to react with and thereby quench 1-hydroxyethyl radicals in competition with other major beer components. The fate and significance of thiols as well as the stability of beer were evaluated during a storage study. Beers with increased levels of thiols were produced using a new approach based on enzyme treatment during mashing. Second-order rate constants for the reactions of beer components toward the 1-hydroxyethyl radical were determined in beer model solutions based on a competitive kinetic approach using spin trap alpha-(4-pyridyl N-oxide)-N-tert-butylnitrone (4-POBN) and electron paramagnetic resonance (EPR) detection. The content of free thiols in beers was determined by detection of ThioGlo 1 derivatized thiols by reverse-phase liquid chromatography and fluorescence detection. The total level of thiols in beer (the sum of free thiols and reducible disulfides) was determined by the same method after treating the beer with the disulfide reducing agent 3,3’,3”-phosphanetriyl-tripropanoic acid (TCEP). The distribution of reactions of 1-hydroxyethyl radical with different beer components can be calculated using concentrations of the components and the respective rate constants. This distribution provides a valuable tool for evaluating the antioxidative potential of different groups of compounds. In a standard pilsner beer the calculated distribution shows that the majority of 1-hydroxyethyl radical reacts with hops bitter acids. However, the rates of scavenging 1-hydroxyethyl radical by thiols were comparable with the rate of reaction of hop bitter acids with the 1-hydroxyethyl radical, indicating that thiols are likely to quench a significant part of the 1-hydroxyethyl radicals in beer. Results obtained from a beer storage experiment showed that both the free thiols and the reducible disulfides were consumed during storage. This result shows that even though the thiols must be present in the free form in order to remove reactive oxygen species, the reducible disulfides are also involved in the redox processes of beer and should be included in thiol quantification when discussing antioxidant capacity. In a pilsner with enhanced thiol levels, obtained by enzyme treatment during mashing, the calculated distribution of reactions changes so that a larger proportion of the 1-hydroxyethyl radical reacts with thiols compared with hop bitter acids. Increased oxidative stability of thiol-enhanced beer is predicted, therefore, and has been tested by evaluation of flavor stability in a storage experiment.

Marianne Lund (Lametsch) is an associate professor. She received an M.S. degree in food science and technology in 2003 from the University of Copenhagen (UCPH), Denmark, where she also obtained her Ph.D. degree in 2007 based on studies of protein oxidation in meat with focus on characterization of radical formation in meat proteins and the impact of protein oxidation on meat quality. In 2007 she was awarded the IMS Prize at the International Congress for Meat Science and Technology for scientific excellence among those (<40 years of age) engaged in research on red meat. In 2008 she began work as a post-doc in the Department of Food Science at UCPH, focusing on oxidation in beer and the influence of thiol groups on the oxidative stability of beer (paused by two maternity leaves) and was appointed associate professor in 2012. She now works on a collaborative project between UCPH and Novozymes A/S on flavor stability of beer. She has published 27 peer-reviewed scientific papers.