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 the solid particles of grape musts) are sterols assimilated by Saccharomyces cerevisiae yeasts during oenological fermentation (under anaerobic conditions). However, excessive clarification of grape musts results in a loss of phytosterols, leading to sluggish or stuck fermentations. Little is known about the physiological impact of phytosterols uptake compared to ergosterol and the influence of sterol type on fermentation kinetics parameters. The objective of this thesis was to better understand the role of sterols and to study the impact of sterol type on oenological fermentation. First, we compared the impact of ergosterol versus phytosterols on fermentation parameters and on the synthesis of key fermentation 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 better maintenance of viability at the end of fermentation and, consequently, to finish the fermentation earlier, while phytosterols led to a reduction of acetate and glycerol quantities in the two conditions tested. Phenotypic diversity was revealed, indicating that S. cerevisiae strains are not equally resistant to stress conditions. Evaluation of the impact of sterol dose and type on the enological fermentation of 10 S. cerevisiae strains revealed that the differences between ergosterol and phytosterols on the fermentative performance of the strains were greater when sterols were the limiting nutrient. The effect of the timing of sterol addition, evaluated for two S. cerevisiae strains in a synthetic must deficient in sterols, showed a better efficiency at the beginning of fermentation compared to an addition in the stationary phase: faster fermentation, increase in the production of fermentative aromas. Finally, transcriptomic analyses showed that the ability of certain strains to resist sterol deficiency 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 with synthetic must in natural must.