Interactions between Torulaspora delbrueckii and Saccharomyces cerevisiae in wine fermentation: influence of inoculation and nitrogen content.

Alcoholic fermentation by an oenological strain of Torulaspora delbrueckii in association with an oenological strain of Saccharomyces cerevisiae was studied in mixed and sequential cultures. Experiments were performed in a synthetic grape must medium in a membrane bioreactor, a special tool designed to study indirect interactions between microorganisms. Results showed that the S. cerevisiae strain had a negative impact on the T. delbrueckii strain, leading to a viability decrease as soon as S. cerevisiae was inoculated. Even for high inoculation of T. delbrueckii (more than 20× S. cerevisiae) in mixed cultures, T. delbrueckii growth was inhibited. Substrate competition and cell-to-cell contact mechanism could be eliminated as explanations of the observed interaction, which was probably an inhibition by a metabolite produced by S. cerevisiae. S. cerevisiae should be inoculated 48 h after T. delbrueckii in order to ensure the growth of T. delbrueckii and consequently a decrease of volatile acidity and a higher isoamyl acetate production. In this case, in a medium with a high concentration of assimilable nitrogen (324 mg L(-1)), S. cerevisiae growth was not affected by T. delbrueckii. But in a sequential fermentation in a medium containing 176 mg L(-1) initial assimilable nitrogen, S. cerevisiae was not able to develop because of nitrogen exhaustion by T. delbrueckii growth during the first 48 h, leading to sluggish fermentation.

Influence of oxygen on alcoholic fermentation by a wine strain of Torulaspora delbrueckii: kinetics and carbon mass balance.

Torulaspora delbrueckii metabolism was assessed in a synthetic culture medium similar to grape must under various conditions: no aeration and three different oxygen feeds, in order to determine the effect of oxygen on metabolism. Carbon and nitrogen mass balances were calculated to quantify metabolic fluxes. The effect of oxygen was to decrease the flux of carbon going into the fermentation pathway in favor of growth. In the absence of aeration, higher amounts of glycerol were produced, probably to maintain the redox balance. The oxygen requirement of this strain was high, since even for the highest air supply oxygen became limiting after 24 h. Nevertheless, this strain developed well in the absence of oxygen and consumed 220 g/L of sugars (glucose/fructose) in 166 h at 20 °C, giving a good ethanol yield (0.50 g/g).