The aim of the master's thesis was to isolate wild yeasts of the genus Saccharomyces, related to the species S. uvarum and S. eubayanus, and Hanseniaspora from the natural environment. We sampled tree bark and fruits from fruit trees in remote parts of the Koroška and Gorenjska regions, as well as from various tree species of the pre-alpine world. We sampled bark from olive trees in Slovenian Istria and grapes from vine-growing areas throughout Slovenia. In the first part of the thesis, yeasts were isolated on selective media. DNA was isolated from the pure cultures and the yeasts were identified using sequences of the internal transcribed region (ITS) or the D1/D2 regions of the ribosomal gene. In total, we isolated 196 yeast strains, which were accomodated to 31 different species: 18 isolates to the genus Saccharomyces and 58 to the genus Hanseniaspora. The species of the Saccharomyces genus, S. cerevisiae, S. jurei, and S. paradoxus, were most frequently isolated from tree bark and from fruits of forest stands. Isolates of the genus Hanseniaspora (H. uvarum and H. vineae) were most frequently isolated from fruit of tree and shrub species and from grapes in vineyards. Yeasts of both genera were most frequently isolated using a yeast nitrogen base (YNB) selective medium with the addition of ethanol and glucose. The second part of the thesis included the chemical-physical analysis of fermenting grape juice from three Slovenian wine-growing regions, from which mainly yeasts of the genus Hanseniaspora were isolated. We found out that these species ferment grape juice slowly and do not complete fermentation, as the fermented juice contains large amounts of residual sugar. These species also produce large amounts of volatile acids.
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