Global analysis of the interaction between saccharomyces cerevisiae and other saccharomyces species of enological interest

Résumé

Global warming has provoked an increase in the content of sugars in grape that implies higher levels of ethanol in wines as well as unbalanced acidity, which affects the consumer perception. In addition, wine market is becoming more and more sophisticated, demanding for products with new and richer aroma profiles. Here, the solution we propose involves the use of alternative species of the Saccharomyces. Those are characterized by lower yields of ethanol than S. cerevisiae, and distinct aroma profiles potentially interesting for industry. The main problem with these yeasts is that they are easily outcompeted from the fermentative media by the action of the naturally present S. cerevisiae. This is why the first of our goals was to characterize the behavior of this species in competition with S. cerevisiae at different temperatures. We observed that S. cerevisiae was able to dominate the culture at high temperatures; however, at low temperature the cryophilic species S. kudriavzevii and S. uvarum remained in the culture in high proportions. That came also with a notable decrease of ethanol concentration in the final wines. Once we achieved our goal at low temperature, we designed different approaches for getting S. kudriavzevii to survive in fermentations at regular red wine temperature fermentation: increasing the proportions of S. kudriavzevii with respect to S. cerevisiae in the inoculum; sequential inoculation of both of our yeast of S. kudriavzevii 24 hours before S. cerevisiae; and feeding air into our fermentations with a controlled flowrate, so that we allowed S. kudriavzevii to breath and therefore obtaining energy in a more efficient way. We achieved profitable results in the case of the combination of two of our techniques: inoculation with higher proportions of S. kudriavzevii plus airflow rate. Ethanol was reduced in 2 percent degrees, with the counterpart of dramatically increasing acetic acid, which gives astringent taste to the wine. This is why although this approach could be valid for industry, fine tuning of the system and strain selection will be needed in order to obtain a quality product. Finally, to understand the interactions between S. cerevisiae and its close relative S. kudriavzevii at the molecular level in fermentation, we made a transcriptomic analysis of both species during the competition in fermentation. We found that S. cerevisiae responded strong and efficiently to the presence of S. kudriavzevii at the beginning of the fermentation, whereas the latest did not. This response by S. cerevisiae showed that competition for nutrients was central. Indeed, HPLC analysis proved that S. cerevisiae was leading nitrogen sources consumption and accelerating sugar uptake rates in competition. Moreover, we proved that for that response to competition was dependent on cell-to-cell contact. Finally, we studied the strain dependency of this phenomenon, which would be linked to human driven evolution.