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An atmosphere-controlled brewing system was built to study thiol oxidation during brewing in laboratory scale under conditions with limited oxygen exposure. Quantification of free and total thiols and protein showed that thiols were lost during wort boiling possibly owing to protein precipitation. Fermentation caused an increase in free thiols, and the balance between free and total thiols was shifted toward a higher degree of free thiols. This was explained by either a reducing effect of fermentation or secretion of thiol-containing compounds from yeast. The efficiency of sulfite to reduce reversibly oxidized thiols was determined by incubating boiled wort obtained from a pilot scale trial with various concentrations of sulfite (0–2,000 µM) in an anaerobic chamber for up to 7 days. Addition of sulfite increased the concentration of free thiols in a concentration-dependent manner, and the effect was more pronounced at longer incubation times. However, the reduction of the pool of oxidized thiols by sulfite was inefficient for sulfite concentrations typically found in beer, and the reaction was found to be relatively slow compared with reduction by tris(carboxyethyl)phosphine. Keywords: Antioxidants, Oxygen, Mashing, Protein thiols, Sulfite, Reducing capacity
