Fermentation characteristics of Lactobacillus delbrueckii subsp. bulgaricus T50 and Streptococcus thermophilus S10 complex starter: Enhancing fermentation performance, metabolic interaction, and storage stability

Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are commonly used together as a fermented milk starter. This study aimed to find the optimal ratio of two strains showing favorable fermentation characteristics, Lactobacillus delbrueckii subsp. bulgaricus T50 (T50) and Streptococcus thermophilus S10 (S10), in fermentation and explore their metabolic interaction. The optimal strain ratio was determined to be 1: 1000. The milk fermented with the optimal ratio had higher elasticity and viscosity, reached the gel point quickly, and had a strong gel structure compared to the commercial starters. T50+S10 exhibited favorable fermentation characteristics and storage stability, most similar to YF-L922, suggesting its potential to be developed as a commercial starter for fermented milk. Metabolomics analysis revealed that the T50+S10 combination enhanced the utilization of succinic acid, L-tyrosine, and L-tryptophan, promoting their mutualistic growth. T50 likely increased the availability of amino acid derivatives, small peptides, and calcium pantothenate to support S10 growth. These findings confirm the impact of L. bulgaricus and S. thermophilus ratio on fermented milk quality and provide insights into their metabolic-level interaction during fermentation. This study holds significance for the development and application of starter cultures in the dairy industry. • Optimal starter ratio: 1:1000 of L. bulgaricus T50 and S. thermophilus S10. • Shortens gelation time, enhanced gel structure, and storage stability. • Enhances succinate, tyrosine, and tryptophan metabolism. • Provides amino acid derivatives and small peptides. • Facilitates pantothenate synthesis.