Comprehensive Peptide Profiling Reveals the Influence of Streptococcus Thermophilus , Lactobacillus Delbrueckii Subsp. bulgaricus , and Endogenous Milk Proteases on the Hydrolysis of Milk Proteins During Early‐Stage Lactic Acid Fermentation

The present study analyzed the impact of Streptococcus thermophilus (ST), Lactobacillus delbrueckii subsp. bulgaricus (LB), and endogenous milk enzymes on protein hydrolysis during early‐stage lactic acid fermentation of milk. Untargeted peptide profiling of milk fermented with single or mixed cultures of ST and LB and unfermented controls was applied covering peptides between 4 and 45 amino acids. Thus, more than 3000 peptides were identified. LB in single and mixed culture led to the highest proteolytic activity (up to 50% higher than controls), the highest number of peptides [1270 unique peptides vs. 124 (ST) and 439 (controls)], and the highest number of bioactive peptides [109 vs. 37 (ST) and 79 (controls)]. Shorter peptides (4–15 amino acids) were predominantly hydrolyzed by ST and generated by LB. Distinct cleavage sites that were detected in all samples could be attributed to milk enzymes like plasmin. Proteolysis by bacterial enzymes showed low specificity.