The yeast flora associated with the surface of grapes during ripening was studied with regard to different sectors of the grape skin and the position in the bunch by means of traditional as well as more vigorous preisolation and precounting treatments. The yeast number per square centimeter of skin increases with ripening and is highest in the area immediately surrounding the stem. The cluster sector closer to the peduncle seems to constitute a favorable substrate for yeasts, hosting a resident flora about 10 and 100 times higher than the central and lower parts of the bunch, respectively.Kloeckera apiculata was the normal resident species of grapes regardless of the sector or the ripening period, and constitutes the fermenting flora of mature grapes. The ecological implications of the results of this survey are discussed.

Strains of the genera Saccharomyces, Saccharomycodes, Schizosaccharomyces, Hanseniaspora, Kluyveromyces, Pichia, Kloeckera, and Torulopsis were examined for killer, sensitive, neutral, and killer-sensitive characteristics against a killer strain (NCYC738) and a sensitive strain (NYCC1006) of Saccharomyces cerevisiae. Thirty-one Saccharomyces cerevisiae strains out of 782 screened were able to kill the sensitive test strain; 707 appeared to be sensitive and 44 neutral. A high frequency of killer phenotypes (71.4%) was found in Hansenula anomala var. anomala strains. No killer strains were present in the remaining 17 species considered.

The yeast flora associated with the surface of various natural materials have been studied on the assumption that individual cells and microcolonies are strongly retained by gummy and mucous secretions. Preisolation treatments based on vigorous shaking, percolation with an excess of water, and the sonication of samples allowed the recovery of a much higher number of species than did traditional isolation procedures. Results confirmed that a sequence of aggressive and disruptive actions on samples, together with an enrichment culture, is necessary to obtain an exhaustive picture of the actual yeast flora.

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