Abstract
In a wine, the aroma consists of thousand volatile compounds, some of which being synthesised by yeasts during fermentation. The most abundant fermentation aromas are higher alcohols, their acetate esters, medium-chain acids and ethyl esters. Other molecules are also formed at low concentrations during fermentation which however can contribute significantly to the sensory quality of the wines due to their organoleptic properties. Thus, the presence of these unconventional aromas, as terpenes with floral notes, benzene derivatives with vanilla or almond notes, and lactones with fruity aromas, is sought in wines and spirits, to stand out in an increasingly competitive world market. Little information is available on the intrinsic capacities of oenological yeasts to synthesize these molecules. The general objective of this work was to assess and exploit the unconventional aroma production capacities of yeasts during wine fermentation. The first part consisted in developing and validating an analytical method specific for these compounds, based on Dispersive Liquid-Liquid Micro Extraction (DLLME) extraction coupled with GC-MS analysis. This method was applied to provide an overview of the unconventional flavour production capacities by a collection of 40 wine yeast strains during fermentation on synthetic medium and natural must (2 musts with different composition). This work highlighted, in particular, Starmerella bacillaris 3800, for its ability to produce large quantities of terpenes and benzene derivatives via its metabolism, and Saccharomyces cerevisiae L15, for its lactone production capacity. In a second part, a study of the metabolic pathways involved in the production of these compounds was carried out. The role of phenylalanine and tryptophan in the synthesis of certain benzene derivatives, including benzaldehyde and benzyl alcohol, by Starm. bacillaris was established. Moreover, it was shown that leucine is not the direct precursor of terpenes synthesis in this strain, likely synthesised from acetyl-CoA via a metabolic intermediate of the sterol pathway, isopentenyl pyrophosphate. The last part of this work was to evaluate the impact of a sequential Starm. bacillaris / S. cerevisiae inoculation on the fermentation process and the quality of the products. This study was carried out on a pilot scale in fermenters equipped with an on-line monitoring system of the majority of aromas. The obtained dynamic data were completed with chemical analysis of volatile compounds composition and sensory analysis (triangular tests and descriptive tests) of the wines. Interestingly, the wines produced using sequential or pure S. cerevisiae fermentations were discriminated both by chemical and sensory analyses. This project provided new knowledge on the metabolic capacities of wine yeasts to produce unconventional aroma compounds that provide a diversification of the offer and a real typicity of the wines. This knowledge has also made it possible to define a strategy of directed evolution to improve Starm. bacillaris 3800 for the production of benzaldehyde, which has been initiated.