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RESEARCH PRODUCT

Study of yeast-yeast interactions in fermentative medium

subject

description

Alcoholic fermentation is the main step for winemaking, mainly performed by the yeast Saccharomyces cerevisiae. But other wine yeasts called non-Saccharomyces may contribute to alcoholic fermentation and improve the wine aroma complexity. The recurrent problem with the use of these non-Saccharomyces yeasts is their trend to die off prematurely during alcoholic fermentation, leading to a lack of their interesting aromatic properties searched in the desired wine. This phenomenon appears to be mainly due to interactions with S. cerevisiae. These interactions are most of the time negatives but remain unclear because of the species and strain specific response. That is why several studies focused on the comprehension of interactions, associating one or more non-Saccharomyces in co-fermentation with S. cerevisiae and by performing different analyses to try to elucidate the mecanisms involved. The aim of this PhD was to study interactions between two non-Saccharomyces with enological interest in sequential fermentations of grape must with S. cerevisiae (S. cerevisiae inoculated 24 to 48 h after the non-Saccharomyces). A flow cytometry protocol was developed to discriminate yeast populations and to monitor different physiological parameters (viability, lipid droplet and reactive oxygen species accumulation) of yeasts during fermentation. The focus was further done on the couple L. thermotolerans/S. cerevisiae by performing sequential fermentations with or without physical contact to study the cell-cell contact effect, which appeared to be involved in interactions between these two species as revealed by different analyses (competition for nutrients, volatile compound production or exo-metabolome). Thus, this PhD has confirmed the existence of interactions between S. cerevisiae and the two non-Saccharomyces studied by putting forward a negative effect of S. cerevisiae on L. thermotolerans but also, and for the first time, of L. thermotolerans on S. cerevisiae, especially about its biomass and fermentation kinetics of sequential fermentations.