Isolation and Identification of Lactic Acid Bacteria with High Fermentation Performance from Naturally Fermented Yak Yogurt in Tibetan Pastoral Areas of Gansu

A total of 195 strains of lactic acid bacteria(LAB) were isolated from 23 naturally fermented yak yogurt samples collected from different Tibetan pastoral areas of Gansu province. Totally 12 LAB strains showing better fermentation performance were obtained through preliminary selection based on genetic stability, coagulation time, titratable acidity, water retention and sensory evaluation. Finally, 5 strains of LAB with excellent fermentation performance were selected by assessing acidification ability, post-acidification capacity, viability and viscosity during cold storage, and aroma-producing capacity. The coagulation time and titratable acidity of the 5 strains were 4 to 6 h and 75.90 to 83.87 °T, respectively. During cold storage, the value of titratable acidity increased by 13.72 to 21.16 °T. The viable bacteria counts were 107 to 1011 CFU/mL and the viscosity values were 3.01 to 8.21 Pa·s. The contents of acetaldehyde and diacetyl were 11.00 to 42.68 μg/mL and 2.91 to 13.18 μg/mL, respectively. The results of 16S rDNA molecular identification showed that Q3 and Q4 were classified as Lactococcus lactis subsp. lactis and Streptococcus thermophilus, respectively while G3, G4 and G5 belonged to Lactobacillus delbrueckii subsp. bulgaricus. These 5 trains LAB is useful for industrial production of fermented dairy products.