Abstract
steps of the winemaking process. These yeasts, poorly adapted to high ethanol concentration and limited oxygen availability, are later overcome by S. cerevisiae, required to complete the fermentation. Nevertheless, non-Saccharomyces yeasts may positively contribute to the organoleptic quality of wines. The main objective of this project is to provide a new knowledge on the physiology of Torulaspora delbrueckii and Metschnikowiapulcherrima, two non-Saccharomyces yeasts distributed by Lallemand SAS, to better exploit their potentialities during wine fermentation in particular for the production of aromas. The project was first focused on investigating T. delbrueckii and M. pulcherrima metabolisms, determining their fermentation performances and aroma production using various nitrogen sources. Strain-specific properties in terms of capability to consume the different nitrogen compounds and to synthetize volatile compounds were revealed, although the overall incidence of the nature of the nitrogen source was close to that of S. cerevisiae. The fate of some individual amino acids throughout the metabolic network has been then elucidated using isotopic tracer experiments, showing a different management of the nitrogen resource between the two non-Saccharomyces species: towardthe protein biosynthesis for T. delbrueckii and toward the synthesis of higher alcohols for M. pulcherrima. The second part was dedicated to the analysis of the non-Saccharomyces yeasts in sequential inoculation with S. cerevisiae, according to nutrient availability. Substantial variations were found in the response to sugar, nitrogen and lipids content of the production of fermentative and varietal (thiols) aromas between the two modalities of sequential fermentation and S. cerevisiae pure culture, related to interactions between S. cerevisiae and the non-Saccharomyces yeasts and their metabolic specificities. Finally, the benefit of the use of non-Saccharomyces yeasts was assessed in natural must, revealing the positive impact of non-Saccharomyces yeasts on the aroma profile of wines. This knowledge will allow to design fermentation management strategies making best use of T. delbrueckii and M. pulcherrima properties for the production of wines with a strong aromatic character.