Making sense of the oxygen paradox and flavour management on beer fermentations

This paper proposes that redox balance is the most important determinant of yeast fermentation outcomes. Consequently redox balance can be used to predict flavour outcomes, to link raw materials and process to flavour. Fermentation involves oxidation steps that need to be balanced by corresponding reduction reactions such as sulfate reduction, ergosterol, glycerol, and ethanol production and higher alcohol and ester production. Disruption or loss of one of these options and consequently starve another route for reducing power. Anaerobically grown yeast is especially sensitive to external oxidative stress compared to aerobically grown cells. This can be overcome by a brief aeration period. This regulates anti-oxidative genes; eg. TRX2, TRR1, GPX2, SOD1, TSA1 and YAP1 which are needed to make thioredoxin, glutaredoxin, and glutathione, all strong reductants. These are also mobile electron carriers that cycle from in- to outside. So the oxygen paradox - 'necessary but dangerous' - means that under stress electrons are redirected from inside yeast cells to outside. Clearly this can change the end-product accumulation and distribution in the final beer. Studies with mutants show that manipulated changes in one pathway affect the flux and accumulation of products in another. The anti-oxidant glutathione and thioredoxin levels are lower in anaerobic compared to aerobic cultures. But after aeration the levels are comparable. Thus strict redox balance rules.