Abstract
Sterols are part of the yeast lipidome and are essential for their growth, metabolism and viability,particularly under stress conditions encountered by the yeast during wine alcoholic fermentation.Ergosterol is the yeast sterol, synthesized in the presence of oxygen, while phytosterols (present in thesolid particles of grape musts) are sterols assimilated by Saccharomyces cerevisiae yeasts duringoenological fermentation (under anaerobic conditions). However, excessive clarification of grapemusts results in a loss of phytosterols, leading to sluggish or stuck fermentations. Little is known aboutthe physiological impact of phytosterols uptake compared to ergosterol and the influence of steroltype on fermentation kinetics parameters. The objective of this thesis was to better understand therole of sterols and to study the impact of sterol type on oenological fermentation. First, we comparedthe impact of ergosterol versus phytosterols on fermentation parameters and on the synthesis of keyfermentation metabolites for 27 S. cerevisiae strains under two stress conditions found in enology:sterol deficiency or osmotic stress (excess of sugars). Ergosterol is the sterol that allows a bettermaintenance of viability at the end of fermentation and, consequently, to finish the fermentationearlier, while phytosterols led to a reduction of acetate and glycerol quantities in the two conditionstested. Phenotypic diversity was revealed, indicating that S. cerevisiae strains are not equally resistantto stress conditions. Evaluation of the impact of sterol dose and type on the enological fermentationof 10 S. cerevisiae strains revealed that the differences between ergosterol and phytosterols on thefermentative performance of the strains were greater when sterols were the limiting nutrient. Theeffect of the timing of sterol addition, evaluated for two S. cerevisiae strains in a synthetic mustdeficient in sterols, showed a better efficiency at the beginning of fermentation compared to anaddition in the stationary phase: faster fermentation, increase in the production of fermentativearomas. Finally, transcriptomic analyses showed that the ability of certain strains to resist steroldeficiency was linked to their capacity to develop a "genetic" response specific to the type of sterol(ergosterol or phytosterols). Finally, the last part of this study validated the results obtained withsynthetic must in natural must.