Study of the microbial ecosystem present on the barrels surface used during the winemaking | Etude de l'écosystème microbien présent à la surface des barriques utilisées lors de la vinification

The microbial community on the surface of wood barrels used in winemaking was determined just before their first use, during first contact with wine, and some months after wine contact during successive racking at different steps of wine elaboration. Five microbial populations (total yeast, non-Saccharomyces yeast, lactic acid bacteria, acetic acid bacteria, and anaerobic and anaerobic tolerant Gram negative bacteria) were identified using 5 different selective culture media. Among each population, species and strains were identified using molecular tools such as the polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis, PCR-restriction fragment length polymorphism and pulsed field gel electrophoresis. At the laboratory scale, the risks of contamination were evaluated by wine inoculation trials and their impact on wine microbial population and chemical properties (volatile phenols and acetic acid) was determined. Before the first use and during the first wine contact, the microbial community on the barrel wood was mainly composed of basidiomycetes (Cryptococcus, Bulleromyces and Rhodotorula) and enterobacteria (Serratia sp. and Shigella sp.). After wine contact, these microorganisms became less and less detectable, which could be explained by their low ethanol resistance. However, some species could resist ethanol and remain detectable some weeks after the first barrelling. The ability of these species to degrade some wood components could affect the wine microbial consortium. In addition, since the first racking, the majority of microbial species found on the wood barrel surface was composed of major wine yeast (Saccharomyces cerevisiae) and bacterial (Oenococcus oeni) species, as well as spoilage organisms such as Brettanomyces bruxellensis, Pediococcus parvulus and Gluconobacter oxydans. Laboratory trials of contamination suggested that these residual populations were unable to cause wine alteration such as volatile phenol and acetic acid production. The microorganisms present on the wood surface could not modify the microbial steady state of wine containing some populations regulated by oenological practices (sulfating, racking) and also by microbial interactions between each species. When the microbial population was low as in the case after filtration or heat treatment, the contamination was more possible. In any cases, washing of barrels at each racking is strongly recommended to progressively eliminate the microorganisms that are adhered to the wood and to obtain an efficient microbial stability of the wine.