CONICET | Buscador de Institutos y Recursos Humanos

In the winemaking process the alcoholic fermentation led by Saccharomyces cerevisiae, is occasionally followed by malolactic fermentation (MLF) carried out mainly by Oenococcus oeni. The MLF which consists of the enzymatic decarboxylation of L-malic acid into L-lactic acid is required during the vinification of most red wines and certain white wines. Also, this secondary fermentation can improve microbiological stability, taste and flavour of wine. However, the MLF is difficult to accomplish due to the physico-chemical conditions of wine such as a low pH, temperature and nutrient depletion as well as some inhibitory metabolites from yeasts such as ethanol, SO2, medium chain fatty acids and antibacterial proteins/peptides. We determined by the double-layer plate method that S. cerevisiae mc2 wine strain showed a strong inhibitory activity on the growth of O. oeni X2L. The aim of this work was to evaluate the inhibitory effect of S. cerevisiae mc2 on O. oeni X2L during sequential inoculation in liquid media and to establish the possible correlation between inhibitory activity and the concentrations of ethanol and SO2 produced by the wine yeast. S. cerevisiae was cultured in grape juice medium for 6 days at 25ºC and the supernatant was used as fermented medium for sequential inoculation of O. oeni. Unfermented grape juice medium was used as control. To elucidate the possible role of fermentation metabolites in the inhibitory effect on O. oeni, ethanol and/or SO2 were added to the control medium at the concentrations produced in the medium fermented by S. cerevisiae. Subsequently, all media were added with MRS components, adjusted at pH 4.5 with citrate-phosphate buffer and sterilized by filtration. The bacterial enumeration was determined by viable plate count. In the medium fermented by S. cerevisiae, the growth rate of O. oeni decreased 62% and the cell viability declined from 108 cfu/ml to105 cfu/ml in regards to control medium. An inhibition of 10, 15 and 24% in the bacterial growth rate was observed by the addition to the unfermented medium of 4% ethanol, 30 mg/l SO2 or both compounds, respectively. The fermentation compounds did not show a noticeable inhibitory effect on the final cell population of O. oeni, only the combined effect of ethanol and SO2 produced a diminution of half log cycle of viable cells. The results highlight that in the conditions tested neither ethanol nor SO2 were only factors responsible for the inhibitory activity exerted by S. cerevisiae mc2 on the growth of O. oeni X2L. Probably a synergistic effect with a proteic factor produced by the yeast could be the responsible of this antagonistic action, since this effect is partially reverted by protease treatment of the yeast fermented medium.