Papers by Anne Nordmark Murmann
Journal of Agricultural and Food Chemistry, Nov 8, 2017
Free thiol-containing proteins are suggested to work as antioxidants in beer, but the majority of... more Free thiol-containing proteins are suggested to work as antioxidants in beer, but the majority of thiols in wort are present in their oxidized form as disulfides and are therefore not active as antioxidants. Thioredoxin, a disulfide-reducing protein, is released into the wort from some yeast strains during fermentation. The capacity of the thioredoxin enzyme system (thioredoxin, thioredoxin reductase, NADPH) to reduce oxidized thiols in boiled wort under fermentation-like conditions were studied. Free thiols were quantitated in boiled wort samples by derivatization with ThioGlo®1 and fluorescence detection of thiol-derivatives. When boiled wort was incubated with all components of the thioredoxin system at pH 7.0 and 25 °C for 60 min under anaerobic conditions, the free thiol concentration increased from 25 to 224 µM. At pH values similar to wort (pH 5.7) and beer (pH 4.5), the thioredoxin system was also capable of increasing the free thiol concentration, although with lower efficiency to 187 and 170 µM, respectively. The presence of sulfite, an important antioxidant in beer secreted by the yeast during fermentation, was found to inactivate thioredoxin by sulfitolysis. Reduction of oxidized thiols by the thioredoxin system was therefore only found to be efficient in the absence of sulfite.
Journal of the American Society of Brewing Chemists, 2016
An atmosphere-controlled brewing system was built to study thiol oxidation during brewing in labo... more 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.
Journal of the American Society of Brewing Chemists, 2016
Extraction of protein-derived thiols by protease treatment during mashing for improvement of flav... more Extraction of protein-derived thiols by protease treatment during mashing for improvement of flavor stability in beer has previously been shown to cause concomitant increase in free amino acid concentrations and thereby increased levels of unwanted Maillard reaction products during aging. The previous methodology, with addition of protease dosed at 50 mg of enzyme/kg of malt at the beginning of the protein rest during mashing, resulted in increased amino acid concentrations of approximately 40% and increased free thiol concentrations of approximately 50% in wort compared with a control without protease addition. The objective of this study was to reduce the amount of amino acids in wort obtained by protease treatment but still obtain a significant increase in protein-derived thiols. In this paper we demonstrate that by omitting the protein rest during mashing in combination with addition of a protease with a higher temperature optimum dosed at only 3 mg of enzyme/kg of malt, it is possible to increase thiol concentrations in wort by 30% and with only a maximum 10% increase in amino acid concentration compared with a control. Pilot brewing showed that beer brewed with addition of protease resulted in a 30% increase in total thiol concentration, along with a decrease in or no effect on the concentration of some amino acids but also an increase in the concentration of other amino acids. Sulfite concentration was increased by 37%, so the effect of increasing free thiol concentration on flavor stability during storage could not be evaluated. Overall, similar brewing and sensory characteristics were obtained compared with a control beer brewed without addition of protease. Foam stability was decreased by protease treatment, and formation of haze was reduced by protease treatment.
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Papers by Anne Nordmark Murmann