Oxidative reactions in beer influence the flavor stability by decreasing flavor notes associated with a freshly brewed beer and by increasing stale flavors. Free thiols are suggested to work as secondary antioxidants, able to remove oxidants in beer and hereby be involved in the prevention of formation of off-flavors. The overall purpose of this PhD thesis was to develop new ways to protect beer from oxidation and increase flavor stability during storage by elucidating the effects derived from brewing yeast.
The level of free thiol was found to increase during fermentation, explained by either a reducing effect of fermentation on the pool of oxidized thiols or a secretion of thiol-containing compounds by yeast, dependent on the yeast strain. At high concentrations, sulfite were able to regenerate thiols in boiled wort, increasing thiol regeneration with increasing sulfite concentration and increasing incubation time. Additionally, thioredoxin was found to have an efficient reducing capacity towards oxidized thiols in wort, in the presence of thioredoxin reductase and NADPH. However, thioredoxin is suggested to be inactive in beer, as sulfite inactivated thioredoxin and fresh beer contained no or non-detectable levels of NADPH.
The influence of yeast strain on beer flavor stability during storage was investigated. Beer was brewed in duplicates with lager yeast strain KVL001 and KVL018 resulting in four independent beers stored for up to 6 months at 25 oC and 35 oC. Sensory analysis showed a slightly higher flavor stability in beer brewed with yeast strain KVL001, containing less amino acids and less sulfite.
Development of off-flavors in beer brewed with different yeast strains, were influenced more by the concentration of amino acids, than the concentration of iron, sulfite and free thiols. Formation of Maillard-derived volatile compounds were observed to increase during storage dependent on temperature, but with no significant difference between beer brewed with different yeast strains. However, the level of higher alcohols during storage was found to be highly dependent on the applied yeast strain. In conclusion, to obtain increased flavor stability of lager beer during storage, it is suggested to avoid elevated concentrations of free amino acids, iron and copper during storage, by applying a yeast strain with a high capacity to take up amino acids and iron, and produce high levels of sulfite.