Aims:  To investigate the impact of acetaldehyde‐ and pyruvic acid‐bound sulphur dioxide on wine lactic acid bacteria (LAB). Methods and Results:  Growth studies were performed where Oenococcus oeni, Pediococcus parvulus, Ped. damnosus and Lactobacillus hilgardii were inoculated into media containing various concentrations of acetaldehyde or pyruvic acid and an equimolar concentration of SO2 at pH 3·50 and 3·70. Low concentrations of acetaldehyde‐ and pyruvic acid‐bound SO2 were inhibitory to the growth of all bacteria although acetaldehyde‐bound SO2 was generally more inhibitory than pyruvic acid‐bound SO2. Inhibition was greater at pH 3·50 than 3·70, and Lact. hilgardii was the most sensitive to acetaldehyde‐bound SO2, while O. oeni was the most sensitive to pyruvic acid‐bound SO2. Degradation of SO2‐bound acetaldehyde was observed for all LAB, and aside from O. oeni, there was also complete degradation of SO2‐bound pyruvic acid at both pH values. O. oeni only degraded pyruvic acid at pH 3·70. Degradation of SO2‐bound acetaldehyde or pyruvic acid did not correlate with bacterial growth as inhibition was always observed in media containing bound SO2. Conclusions:  Acetaldehyde‐ and pyruvic acid‐bound SO2 were inhibitory to wine LAB growth at concentrations as low as 5 mg l−1. Despite this inhibition, all wine LAB degraded SO2‐bound acetaldehyde and pyruvic acid suggesting that bound SO2 may have a bacteriostatic rather than bacteriocidal action. Significance and Impact of the Study:  Sulphur dioxide bound to acetaldehyde or pyruvic acid is inhibitory to growth of wine LAB and must be considered when conducting the malolactic fermentation or controlling the growth of spoilage bacteria such as Pediococcus and Lactobacillus.

The objective of this study was to investigate the production of SO 2 and SO 2 binding compounds by wine yeast and the impact of the production of these compounds on the MLF at various time points during alcoholic fermentation. Fermentations were observed for a number of commercial wine yeasts in a synthetic grape juice and Pinot gris juice and SO 2 , acetaldehyde, pyruvic acid and α-ketoglutarate. Measurements were taken at multiple time points during the fermentation. Samples were taken from the fermentations at weekly intervals, sterile filtered, and inoculated with O. oeni strain VFO to induce MLF. Significant differences between the amount of SO 2 , acetaldehyde and pyruvic acid produced by the various yeast strains were noted. Some yeast strains such as FX10, CK S102, F15 and M69, produced significantly higher SO 2 concentrations than other yeast strains and MLF was inhibited in these wines. Insignificant free SO 2 was measured, indicating that bound SO 2 rather than free SO 2 was responsible for inhibition. At almost all time points of the alcoholic fermentation, acetaldehyde bound SO 2 was determined to be the dominant species of bound SO 2 present, suggesting that MLF inhibition by bound SO 2 was due to acetaldehyde bound SO 2 .