Abstract:A yeast strain with proteolytic activity was screened from traditional fermented milk from Xinjiang and its proteolytic activity and in vitro free radical scavenging activity determined before combination with the commercial yogurt starter (Lactobacillus delbrueckii subsp. bulgaricus. CICC6047 and Streptococcus thermophilus CICC6038) for study of the starter activity, proteolysis, texture change, acidification ability, and in vitro free radical scavenging activity during fermentation and cold storage. The results showed the highest proteolytic activity for strain 0807-39, which had a significantly higher leucine content than other strains at 0.34 mg/mL (P<0.05), with free radical scavenging rates of 68.13% and 74.24% obtained for ABTS and DPPH, respectively. Strain 0807-39, which was not associated with any harmful biogenic amines, was found sensitive to antibiotic resistance and did not show hemolysis, indicating that the strain is safe. Identification by 26S rDNA sequencing indicated the strain as K. marxianus, and it was found to promote the number of live lactic acid bacteria and increase the hydrolysis of milk protein during fermentation. During cold storage, K. marxianus DPUL-F15 significantly promoted the degradation of α-casein and β-casein in fermented milk (P<0.05), which decreased by 0.24 mg/mL and 1.23 mg/mL respectively. The content of free amino acids in the fermented milk increased by 0.1 mg/mL. The post-acidification of fermented milk was accelerated, with acidity reaching 120.56 T° after 21 days in storage, and the highest scavenging activities of 68.87% and 71.1% were observed for ABTS and DPPH free radicals, respectively, after 14 days in cold storage. The strain was also found to influence the texture characteristics. The results indicate that K. marxianus DPUL-F15 has the potential to be used as an auxiliary starter that can improve proteolysis in fermented milk.
