Isolation and clonal pre-selection of enological Saccharomyces

The aim of the present study was to perform a fast pre-selection from a great number of wine yeasts using a simple phenotypic-based methodology that allows many different strains to be simultaneously tested. A total of 150 elliptic yeasts, isolated from must and wine from black grapes of a distinctive Italian variety, were studied. Yeasts were identified to genus level by assessing their ability to ferment glucose and their production of spores on acetate agar. The Saccharomyces strains were seeded on BiGGY agar to determine their H(2)S production, on calcium carbonate agar to test their acetic acid production, and on grape-skin agar and on grape-seed agar to assess their interaction with phenolic compounds. The Saccharomyces strains were also examined for fermentative vigor after 2 d or 7 d both with and without the addition of 100 mg L(-1) of SO(2) in must at 20 degrees brix and pH 3.20. At the end of fermentation, the wines produced by the 18 best yeasts were analyzed and the strains were studied for additional biochemical and technological characteristics. The resistance of the strains to simultaneous acid-stress and osmotic-stress was studied carrying out in duplicate winemaking tests in must at 30 degrees brix and pH 2.60. A remarkable heterogeneity among the 150 autochthonous yeasts studied was demonstrated. The phenotypical biodiversity is particularly interesting for several technological characteristics useful in winemaking, such as fermentation vigor, acetic acid production and malic acid content of the wines. The vast majority of the elliptic wine yeasts isolated did not show suitable characteristics, so only 18 strains, 12% of the total, remained for the final tests. Many of the strains that had passed the preliminary screenings revealed some defects when they were studied for fermentation performance, both in standard winemaking and under stressors. Two strains exhibited particularly interesting performances: one strain for winemaking of normal musts and the other for winemaking of musts from dried grapes or under stressful conditions.