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

Stuck or sluggish malolactic fermentation (MLF) can be problematic in
limiting 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, the effects of
yeast-derived SO2 production on O. oeni survival was investigated in
laboratory- and pilot-scale co-fermentations in Chardonnay. Further to the
S. cerevisiae strain affecting O. oeni survival and MLF, we show
that SO2 production by yeast (to approximately 65 mg/L) can be
uncoupled from O. oeni survival in early stages of co-fermentation.
Bacterial survival with certain SO2-producing yeast strains was correlated
with early, transient formation of a high concentration of
acetaldehyde.  Upon co-inoculation, an extremely low
concentration range (approximately 1–44 µg/L) of calculated molecular SO2
is indicated to regulate O. oeni survival. Possible strain-dependent
sensitivity of O. oeni to bound SO2 may also occur, although the
extent and nature of such inhibition by the SO2 adduct itself
during co-fermentation remains unclear. 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. These wines also had 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 which
transiently produce an early, high concentration of acetaldehyde. Reliance
on total / bound SO2 concentration may also potentially provide an
inaccurate measure of SO2 toxicity towards O. oeni, particularly
for co-inoculation, and the impact of low, equilibrium concentrations of
molecular SO2 should also be considered.