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International audience; Aims: The ability of lactic acid bacteria (LAB) to metabolize certain phenolic precursors to vanillin was investigated. Methods ans results : Gas chromatography-mass spectrometry (GC-MS) or HPLC was used to evaluate the biosynthesis of vanillin from simple phenolic precursors. LAB were not able to form vanillin from eugenol, isoeugenol or vanillic acid. However Oenococcus oeni or Lactobacillus sp. could convert ferulic acid to vanillin, but in low yield. Only Lactobacillus sp. or Pediococcus sp. strains were able to produce significant quantities of 4-vinylguaiacol from ferulic acid. Moreover, LAB reduced vanillin to the corresponding vanillyl alcohol. Conclusions: The transformation of phenolic compounds tested by LAB could not explain the concentrations of vanillin observed during LAB growth in contact with wood. Significance and impact of the study : Important details of the role of LAB in the conversion of phenolic compounds to vanillin have been elucidated. These findings contribute to the understanding of malolactic fermentation in the production of aroma compounds.

National audience; 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.

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Studies were conducted to assess establishment of malolactic starters based on Leuconostoc oenos (Oenococcus oeni) in wines. 6 commercial freeze dried starters were used in trials conducted in 5 wineries. Studies on identification of the starters by genomic DNA fingerprinting using NotI, ApaI and SfiI RFLP patterns indicated that 3 of the starters were the same strain; this was confirmed by Southern hybridization of EcoRI-restricted DNA with phi MC 10 prophage DNA as probe. The wines were inoculated immediately after racking; some had already started malolactic fermentation. Samples with added starters showed faster malic acid decomposition than those made without starters. Pulse field gel electrophoresis patterns of bacteria isolated at the end of malolactic fermentation indicated that in 15 of 21 trials, the pattern of the starter appeared either alone or superimposed on that of other bacteria. This indicated that, even when an indigenous microflora was abundant and active, added starters could survive and predominate. The number of trials conducted did not permit determination of whether any of the starters studied showed superior ability to establish in wines. Further studies are required to determine whether use of malolactic starters influences the sensory properties of wines., L'expérimentation porte sur six préparations industrielles de levains malolactiques. La première partie du travail a consisté à établir le profil de restriction des souches utilisées. Parmi les six levains on retrouve le même profil pour trois. L'étude conduite avec la sonde d e bactériophage confirme que ces trois levains ont probablement été préparés avec la même souche. Dans la plupart des lots inoculés, les profils caractéristiques des levains sont retrouvés prédominants en fin de fermentation malolactique. Malgré la présence d’une population élevée et active de bactéries indigènes, les levains semblent avoir participé efficacement à la fermentation. Il reste à établir l’incidence de chacun des levains sur les caractères des vins obtenus.

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