Selection of Saccharomyces cerevisiae with peculiar oenological properties from naturally fermented Negroamaro must

Spontaneous grape must fermentation, induced by the indigenous micro flora, is believed to be associated with a specific vineyard and to give a distinctive style and quality to that wine. The alcohol-tolerant Saccharomyces cerevisiae strains invariably dominate the latter stage of natural wine fermentation. The S. cerevisiae population and other specific yeasts present in the vineyard niche habitats are considered autochthonous and their involvement in natural fermentation allows the production of wines with particular features in each microclimatic area. The present study was aimed to the individuation of autochthonous yeast strains useful in the improvement of oenological production of Salento, which is a very important wine-producing area of Southern Italy. Grapes were sampled from the most representative areas of Salento region (Brindisi, Salice Salentino, Copertino, Melissano) for "Negroamaro" production and separately subjected to natural fermentation in an experimental scale. The identification of micro biota present during the last step of wine fermentation (>1 °Bé) of Negroamaro grapes, was carried out to select autochthonous yeast strains for industrial wine production. Aliquots of must samples(0.1 ml each) of several dilutions (from 10-1 to 10-6) were spread on YPD agar medium supplemented with ampicillin (100 mg/l) for 3-5 days at 26 °C. As preliminary screening, averages of 1500 colonies for each of the four fermentations were assayed for H2S production on BIGGY agar. The colonies appearing white or light brown (about the 10% of initial 1500 individual clones) were selected and recognized as Saccharomyces cerevisiae by a microbiological screening based on the implementation of the taxonomical keys for identification of yeasts belonging to Saccharomyces genus. All the yeasts analysed were identified as S.cerevisiae. The criterion for S. cerevisiae strain differentiation was provided by the PCR analysis of polymorphism of amplified ? interspersed element sequences, which has been demonstrated to share a similar discriminative power of the mtDNA restriction patterns. Unexpectedly, the obtained results indicated that in all the four analysed Negro Amaro fermentations the S. cerevisiae population was composed by a high number of different strains, thus allowing the identification of a limited number (6) of dominating strains associated with 81 secondary strains. The Negroamaro grape variety was used for microvinification studies with all these yeasts isolates.. The grape must was clarified centrifugation, inoculated with a final concentration of 106 cell ml-1 of pure yeast culture. Transparency and foam production of the fermented products were directly evaluated whilst HPLC analysis was employed to assay glucose, fructose, ethanol, glycerol and acetic acid contents. The results of these technological analyses for the selection of autochthonous starters for Negroamaro wine production will be discussed.