La microbiologie des vins issus de raisins botrytisés au cours de l'élevage. Caractérisation des souches de Saccharomyces cerevisiae responsables de refermentation

Botrytis-affected wines microbiology during maturation. Characterization of Saccharomyces cerevisiae strains responsible for refermentation

• Auteur / Autrice : Benoit Divol
• Direction : Pierre Strehaiano
• Type : Thèse de doctorat
• Discipline(s) : Génie des procédés et de l'environnement
• Date : Soutenance en 2004
• Etablissement(s) : Toulouse, INPT

Mots clés

Mots clés contrôlés

Dioxyde de soufre

Aspect économique

Saccharomyces cerevisiae

Fermentation

Vin de liqueur

Résumé

La fermentation alcoolique des vins liquoreux issus de raisin botrytisés est arrêtee brutalement par ajout massif de dioxyde de soufre. Certaines sousches de levures survivent et parfois provoquent une refermentation. Ces levures survivent dans un état comparable à celui qualifié de viable non cultivable chez les bactéries. Seules certaines espèces fermentaires et oxydatives survivent. L'utilisation potentielle du diméthyldicarbonate en complément du sioxyde de soufre a été étudiée. Les souches de saccharomyces cerevisiae de refermentation isolées exhibent des singularités de séquence de leur ADNr, les apparentant aux souches de voile de certains vins spéciaux. Ces souches surexpriment constitutivement le gène ssu1 et synthétisent rapidement une forte concentration d'éthanal en réponse à la présence de dioxyde de soufre. La refermentation est le résultat d'une adaptation génétique et d'une sélection, fruit d'une multitude de paramètres microbiologiques physico-chimiques et humains.

The alcoholic fermentation of Botrytis-affected wines is stopped by addition of sulphur dioxide. Some fermenting yeast species can survive during maturation and grow in spite of high ethanol, sugars and sulphur dioxide concentrations. An undesirable new fermentation, named ''refermentation'', can sometimes occur. In this study, it was proved that some yeast species were able to survive in a viable but non-culturable-like state. This state explains the apparent sterility of wines during maturation. Within Saccharomyces cerevisiae species, an intraspecific selection was spontaneously operated. After some weeks, only one strain could often survive. An ecological study was realized. Some highly fermentative and oxidative species could survive. In spite of slower metabolism, they synthesized acetaldehyde during maturation. The exit from the VBNC state and the high sulphur dioxide binding power were the keys of refermentations. The use of dimethyldicarbonate to stop alcoholic fermentation was studied. The most efficient action was observed for the mixed sulphur dioxide and DMDC addition. Some Saccharomyces cerevisiae strains responsible for refermentations were isolated. These strains exhibited rDNA sequence singularities, showing that they were close to flor strains, responsible for velum formation in some special wines. Moreover, those strains constitutively over-expressed SSU1 gene and could rapidly synthesize high concentrations of acetaldehyde in response to sulphur dioxide addition. High sulphur dioxide concentrations had probably selected the most resistant strains. Refermentation is the result of genetic adaptation and selection, under the influence of microbiological, physical, chemical and human parameters.