Metabolism of SO₂ binding compounds by Oenococcus oeni during and after malolactic fermentation in white wine

Sulfur dioxide SO₂ is the key additive for the preservation of wines. Carbonyl and keto compounds in wine can bind to SO₂ and decrease its efficacy, resulting in higher total SO₂ requirements. Increased consumer demand for low sulfite and organic wines pose production challenges if SO₂ binders have not been properly managed during vinification. Malolactic fermentation (MLF) has been known to reduce bound SO₂ levels but detailed time course studies are not available. In this work, the kinetics of major SO₂ binding compounds and malic acid were followed during MLF in wine with 12 commercially available strains of Oenococcus oeni. Pyruvic acid, acetaldehyde and α-ketoglutaric acid were degraded to various degrees by O. oeni, but galacturonic acid was not. At the time of malic acid depletion, percent degradation of pyruvate, α-ketoglutaric acid and acetaldehyde was 49%, 14% and 30%, respectively. During MLF, the decrease in average bound SO₂ levels, as calculated from carbonyl metabolism, was 22%. The largest reduction in wine carbonyl content occurred in the week after completion of MLF and was 53% (107 mg/L to 34 mg/L) calculated as bound SO₂. Prolonged activity of bacteria in the wines (up to 3 weeks post malic acid depletion) resulted only in reduced additional reductions in bound SO₂ levels. The results suggest that microbiological wine stabilization one week after malic acid depletion is an effective strategy for maximum removal of SO₂ binders while reducing the risk of possible post-ML spoilage by O. oeni leading to the production acetic acid and biogenic amines.