- Malolactic fermentation (MLF) is important in the production of wines as it reduces acidity through the conversion of malic acid to lactic acid. However, successful MLF can be difficult to achieve due to yeast antagonism of the bacterium responsible for
the secondary fermentation, Oenococcus oeni. In particular, yeast produced sulfur dioxide (SO₂) has been implicated in causing problematic MLFs. SO₂ can exist in a free or bound form with the free form thought to have the antimicrobial action. However,
some recent studies have demonstrated bacterial inhibition in wine when no free SO₂ was present indicating that bound SO₂ was responsible. Despite this, very little is known regarding the toxicity of the different forms of bound SO₂ and how they may impact the MLF. Therefore, the purpose of this research was to investigate the production of SO₂, and the major SO₂ binding compounds, acetaldehyde, pyruvic acid, and α-ketoglutaric
acid, by various commercial wine yeast strains and the impact these compounds have on the MLF.
Fermentations were conducted in a synthetic grape juice and Pinot gris must where viable yeast cell growth, SO₂, acetaldehyde, pyruvic acid, and α-ketoglutaric acid were measured during the alcoholic fermentation. At weekly intervals samples were
taken from the fermentations, sterile filtered, and inoculated with O. oeni strain VFO to induce MLF. Progress of MLF was monitored by measuring malic acid and bacterial viable cell counts. Results show that there were significant differences between the
amount of SO₂, acetaldehyde, and pyruvic acid produced by the various yeast strains but not α-ketoglutaric acid. Some yeast strains, such as FX10, S102, F15, and M69, produced
significantly higher SO₂ concentrations than other yeast strains and O. oeni viability decreased rapidly when inoculated into these wines. Very little if any free SO₂ was measured indicating that bound SO₂ and not free SO₂ was responsible for bacterial
inhibition. Acetaldehyde bound SO₂ was the dominant species of bound SO₂ found at almost all time points of the alcoholic fermentation indicating that inhibition of MLF by
bound SO₂ was due to acetaldehyde bound SO₂.
To further elucidate the role of bound SO₂ in the inhibition of wine lactic acid bacteria (LAB), growth studies on the impact of O. oeni strain VFO, Pediococcus parvulus, P. damnosus, and Lactobacillus hilgardii in media containing free SO₂ or
acetaldehyde and pyruvic acid bound SO₂ at two different pHs were performed. In general, inhibition was greater at pH 3.5 than at 3.7 and only P. damnosus demonstrated some tolerance to SO₂. Acetaldehyde bound SO₂ appeared to be more inhibitory than
either pyruvic acid bound SO₂ or a combination of acetaldehyde and pyruvic acid bound SO₂. Degradation of acetaldehyde appeared to stimulate the growth of O. oeni VFO and Lb. hilgardii at pH 3.5 but not at 3.7. Overall, a reduction in acetaldehyde and pyruvic
acid as well as SO₂ bound acetaldehyde and pyruvic acid was observed for all LAB tested, with the exception of O. oeni VFO which did not show a decrease in SO2 bound pyruvic acid at pH 3.5. Bacteria were inhibited in media containing acetaldehyde bound
SO2 and pyruvic acid bound SO₂ even though a decrease in SO₂ bound acetaldehyde and pyruvic acid was observed. This suggests that the decrease of the compound bound to SO₂ may have lead to inhibition by the subsequently released free SO₂.