Isolation, Identification, Stress-Tolerance and Gas-Producing Characteristics of Yeasts from Sayram Yogurt

Sayram yogurt, a traditional fermented dairy product from Aksu Prefecture, Xinjiang, derives its characteristic flavor and functional attributes largely from the indigenous yeast community present in the raw materials. In this study, six dominant yeast isolates (Y4, Y6, Y10, Y18, Y35, Y44) were recovered and purified from locally produced, handmade Sayram yogurt. ITS rDNA sequencing identified five isolates as Kluyveromyces marxianus and one as Pichia fermentans. We then systematically assessed key physiological stress responses relevant to dairy fermentation and gastrointestinal survival, including acid–base tolerance, bile salt tolerance, resistance to high sugar and hyperosmotic conditions, ethanol tolerance, temperature adaptability, and tolerance to simulated gastrointestinal fluids. The tested yeast strains demonstrated robust growth across a broad range of conditions: pH 2.5–6.5, bile salts 0.25–2%, glucose 300–900 g/L, ethanol 10–20%, and culture temperatures 25–37 °C. After treatment with artificially simulated gastric and intestinal fluids, the bacterial cell turbidity (OD600) remained at a relatively high level. Gas-production profiles varied markedly among strains; notably, Y4 and Y6 initiated gas production earlier and produced larger gas volumes than the others. These results define the physiological tolerances and functional strengths of yeast isolates from Sayram yogurt and provide experimental evidence and strain resources to support the exploitation of microbial diversity in Xinjiang traditional fermented milk, the development of functional fermented foods, and the selection of candidate probiotic strains.