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Thiols during production and storage of beer

Technical Session 20: Finishing and Stability Session
Marianne Lund, University of Copenhagen
Co-author(s): Signe Hoff, René Lametsch, and Mogens Andersen, University of Copenhagen, Denmark

ABSTRACT: Thiol-containing proteins have been suggested to play a role together with sulfite in the antioxidative mechanism controlling the oxidative stability of beer. Thiols may react with H₂O₂, which is hereby removed. The resulting thiol-containing oxidation products may be regenerated to thiols if they are reduced and may then again react with H₂O₂ for further removal. A method for quantification of sulfite and free thiols in beer was developed based on derivatization with the fluorescent reagent ThioGlo1, separation of sulfite- and thiol-ThioGlo derivatives by high-performance liquid chromatography (HPLC), and subsequent fluorescent detection. Quantification of sulfite and free thiols was performed by preparing standard addition curves in each beer sample with sulfite and glutathione (GSH), which is a tripeptide containing cysteine. The obtained method was used to quantify sulfite and free thiols in wort and beer samples from different storage experiments in order to investigate the correlation of sulfite and thiols with oxidative stability (determined by lag phase measurements using electron spin resonance [ESR] spectroscopy), volatile compounds (analyzed by gas chromatography-mass spectrometry [GC-MS]), protein concentration and profile (determined by the Bradford method and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis [SDS-PAGE] with subsequent MS identification of relevant protein bands), and content of transition metals (determined by inductively coupled plasma [ICP]-MS). Results showed that sweet wort did not contain any detectable thiols. In fact, addition of GSH to sweet worts during the standard addition protocol used for thiol determination caused consumption of GSH resulting in a “thiol-removing capacity.” Contrary to sweet wort, detectable amounts of thiols were quantified in beer, indicating that thiols are either secreted by the yeast or that oxidized thiols are reduced during fermentation. A positive correlation between content of sulfite and oxidative stability (length of ESR lag phase) was confirmed, but the content of thiols was also found to be positively correlated with oxidative stability. Pasteurization improved the oxidative stability of beer during storage for one year at room temperature compared to unpasteurized beer, and pasteurized beers also contained more soluble protein after one year of storage than unpasteurized beers, indicating that an increased concentration of proteins positively influences the oxidative stability of beer. Furthermore, the thiol-containing protein, LTP1, was still present in beers with good oxidative stability after storage for one year at room temperature, while beers with poor oxidative stability did not contain any LTP1, suggesting that LTP1 is either involved in oxidative changes during storage of beer or is a marker for oxidation. Determining the content of transition metals did not provide any conclusive results in relation to the effect of pasteurization on the oxidative stability of beer, but the profile of volatile compounds showed that the content of Maillard-derived compounds, such as certain Strecker aldehydes, increased when beer was pasteurized, which is likely due to the introduction of heat during pasteurization.

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, Denmark (UCPH), where she also obtained her Ph.D. degree in 2007 based on studies of protein oxidation in meat, with a focus on characterization of radical formation in meat proteins and the consequences 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 under the age of 40 engaged in research on red meat. In 2008 she began work as a post-doc in the Department of Food Science at UCPH, working with oxidation in beer and the influence of thiol groups on the oxidative stability of beer (with pauses for 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 16 peer-reviewed scientific papers.