Screening of Fermented Strains in Handmade Yogurt in Aksu Region of Xinjiang and Its Fermentation Performance

The Lactobacillus strains with excellent fermentation performance play a crucial role in the preparation of starter cultures, which has a significant impact on the quality of fermented dairy products. Therefore, to select the Lactobacillus strains with high fermentation performance is essential for the preparation of starter cultures. In this study, four Lactobacillus strains with high stability of milk-clotting after the 10^th passage were derived from three samples of Aksu handmade yogurt (a sort of Xinjiang specialty). By means of 16S rRNA sequencing and morphophysiological identification, q8-1 and q28-2 were identified as Lactobacillus casei, R22-2 was identified as Lactobacillus paracasei, and q4-1 was identified as Pediococcus acidilactici. Meanwhile, some other bacteria with the stability of milk-clotting that had been previously selected in the lab including K5-2 (Pediococcus pentosaceus), 7-1 (Lactobacillus bulgaricus) and A-2 (Streptococcus thermophilus) were juxtaposed with these four Lactobacillus strains for performance assessment. In this assessment, a measurement was performed to determine their single-strain growth ability, acid production performance, post-acidification performance, viable cell count (VCC) during storage in diary products, sensory analysis results, texture properties and aroma-producing ability. Results showed that, the Lactobacillus bulgaricus 7-1 had the best fermentation performance with coagulation time and fermentation end times of 8 and 9.5 hours, respectively. It had a high acid yield but a weak capacity for post-acidification. After 1 day of post-ripening, the acidity was 97.6 °T. After 20 days of storage at 4 ℃, the acidity did not vary by more than 20 °T, and the number of viable bacteria remained above 1×10⁶ CFU/mL. Good texture characterized the fermented milk. The firmness, consistency, cohesiveness and index of viscosity were (60.26±0.11) g, (1209.37±0.62) g·s, (−11.81±0.04) g, (−5.37±0.04) g·s, respectively. In the fermented milk, the concentrations of acetaldehyde and diacetyl were 14.48 and 1.23 mg/L, respectively. Good fermentation performance was demonstrated by Streptococcus thermophilus A-2, and Pediococcus pentosaceus K5-2, which also met the fundamental parameters of starter culture. Results showed that Lactobacillus bulgaricus 7-1, Streptococcus thermophilus A-2, and Pediococcus pentosaceus K5-2 had good yoghurt fermentation abilities and could be employed as yoghurt starter cultures.