Transient acetaldehyde production by SO2-producing Saccharomyces cerevisiae promotes the survival of Oenococcus oeni during co-fermentation

Stuck or sluggish malolactic fermentation (MLF) can be problematic in stressful wine conditions, particularly white and sparkling base musts/wines. In these cases, knowledge of yeast-bacteria strain compatibility and the amount of sulfur dioxide (SO2) a yeast strain produces are important considerations for successful MLF. Here, laboratory- and pilot-scale co-fermentations in Chardonnay were used to investigate the effect of yeast-derived SO2 on Oenococcus oeni survival. Although yeast-derived SO2 is generally inhibitory, we show that SO2 production (to approximately 65 mg/L) can be uncoupled from O. oeni survival in the early stages of co-fermentation. Bacterial survival in the presence of specific SO2-producing yeast strains was correlated with the early, transient formation of high acetaldehyde concentrations. Oenococcus oeni survival coincided with molecular SO2 concentrations remaining below an extremely low threshold of inhibition, which exponentially increased from approximately 3–6 µg/L in the first three days of co-fermentation. Strain-dependent sensitivity of O. oeni to bound SO2 remains a possibility, although the extent and mechanism of such inhibition by the SO2 adduct during co-fermentation remain unclear. The choice of co-inoculation yeast strain also influenced wine diacetyl concentration, which was only detected in wines co-inoculated with high SO2-producing S. cerevisiae strains. The wines with high diacetyl concentrations were found to be distinct by a sensory panel, with comparatively high citation frequency for a buttery sensory attribute. Both the SO2- and acetaldehyde production capacity of yeasts are, therefore, seen as meaningful co-inoculation selection criteria. The range of yeast strains suitable for MLF induction by co-inoculation could be widened to include SO2-producing strains that transiently produce an early, high concentration of acetaldehyde. The effects of low, equilibrium concentrations of molecular SO2 should also be considered in conjunction with total SO2 as a measure of SO2 toxicity towards O. oeni following co-inoculation.