Abstract
The sensory profile of fermented beverages is one of their most important properties. This sensory profile relies on a unique and complex blend of volatiles molecules that impact our olfactive system. While aroma compounds provide a positive note, some are extremely detrimental to the global profile. Hydrogen sulfide (H2S), the main volatile sulfur compound produced by Saccharomyces cerevisiae during alcoholic fermentation, is one of the most negative compounds given its high impact and low sensory threshold. Under specific conditions such nitrogen nutrient limitation, it can be overproduced, which leads to poor aromatic quality. In order to investigate these phenomena, we first developed a new sensitive method for the evaluation of total H₂S production during alcoholic fermentation using a metal trap and a fluorescent probe. This method, enabled us to evaluate the combined impact of assimilable nitrogen, sulfur dioxide and strain on sulfide production by yeast during wine alcoholic fermentation using a full factorial experimental design. All factors were significant, but yeast strains showed the highest impact, followed by nitrogen and sulfite content. We then applied this method to evaluate the variability in H2S production of strains isolated in different ecological environment (wine fermentation, flor velum and oak bark). Surprisingly, wine strains displayed a significantly higher production than flor and oak strain, that became even sharper in presence of SO2. The analysis of copper resistance of these strains varied similarly to H2S production. The analysis of the number of copies of the CUP1 gene present in the genome of these strains was significantly correlated to the total production of sulfide in absence of SO2. This enables us to propose that the amplification of the CUP1 gene of some strains may participate in an excessive H2S production caused by the stimulation of the activity of the SAP pathway for the biosynthesis of cysteine required for the synthesis of Cup1 protein. In a third phase, we tried to decipher the genetic bases of the differences between strains in their ability to produce H2S in the presence of SO₂, using a cross between a wine and a flor strain. We applied a Bulk Segregant Analysis (BSA) to a 96 strain progeny derived from these two strains having two different abilities to produce H₂S. The comparison of the allelic frequencies along the genome of pools of segregants producing a small or a high quantity of total H₂S during alcoholic fermentation pointed out to two regions involved in the variations of H₂S production. The variations in four genes activating different mechanisms were found to explain this trait. This data represents a new insight into the regulation of H₂S production during wine fermentation and show the interplay of sulfur assimilation pathway and cell metabolism.