Influence of malt roasting on the oxidative stability of sweet wort was evaluated based on radical intensity, volatile profile, content of transition metals (Fe and Cu) and thiols. Malt roasting had a large influence on the oxidative stability of sweet wort. Light sweet worts were more stable with low radical intensity, low Fe content, and ability to retain volatile compounds when heated. At mild roasting, the Fe content in the wort increased but remained close to constant with further roasting. Dark sweet worts were less stable with high radical intensities, high Fe content, and a decreased ability to retain volatiles. Results suggested that the Maillard reaction compounds formed during the roasting of malt are prooxidants in sweet wort. A thiol-removing capacity was observed in sweet wort, and it was gradually inhibited by malt roasting. It is possibly caused by thiol oxidizing enzymes present in the fresh malt.

The impacts of pasteurization of a lager beer on protein composition and the oxidative stability were studied during storage at 22 °C for 426 days in the dark. Pasteurization clearly improved the oxidative stability of beer determined by ESR spectroscopy, whereas it had a minor negative effect on the volatile profile by increasing volatile compounds that is generally associated with heat treatment and a loss of fruity ester aroma. A faster rate of radical formation in unpasteurized beer was consistent with a faster consumption of sulfite. Beer proteins in the unpasteurized beer were more degraded, most likely due to proteolytic enzyme activity of yeast remnants and more precipitation of proteins was also observed. The differences in soluble protein content and composition are suggested to result in differences in the contents of prooxidative metals as a consequence of the proteins ability to bind metals. This also contributes to the differences in oxidative stabilities of the beers.

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A method for quantification of total soluble protein-derived thiols in beer was developed based on the formation of fluorescent adducts with the maleimide compound ThioGlo 1. The problem of interference from fluorescent adducts of sulfite and ThioGlo 1 was solved by HPLC separation of the adducts followed by fluorescence detection. Using standard addition of GSH, a detection limit of 0.028 μM thiols was achieved. The application and validation of the method was demonstrated for beers with different color intensities, and the application range is in principle for any biological system containing thiols. However, the quantification of cysteine was complicated by a lower fluorescence response of its ThioGlo 1 adducts. Based on the studies of the responses of a series of cysteine-derived thiols and (1)H NMR studies of the structures of ThioGlo 1 adducts with GSH and cysteine, it was concluded that thiols with a neighboring free amino group yield ThioGlo 1 adducts with a reduced fluorescence intensity.

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