Changes in Proteolysis in Fermented Milk Produced by Streptococcus thermophilus in Co-Culture with Lactobacillus plantarum or Bifidobacterium animalis subsp. lactis During Refrigerated Storage

Proteolysis in fermented milk, a complex and dynamic process, depends on the starter cultures used. This study aimed to evaluate the influence of Lactobacillus plantarum or Bifidobacterium animalis subsp. lactis, or both, co-fermented with Streptococcus thermophilus, on the changes in the proteolysis profile of fermented milk during 21-day storage at 4 °C, including the pH value, proteolytic degree, protease activity, aminopeptidase activity, free amino acid content, and electrophoresis performance. The results showed that the treatments with co-cultures exhibited a higher amount of free amino groups and neutral protease activity at an extracellular level, whereas lower pH values and aminopeptidase activities towards the six substrates at an intracellular level than the ones with a single-strain of S. thermophilus over the refrigerated storage were observed. In co-fermentation with S. thermophilus, B. animalis subsp. lactis did not significantly affect the concentrations of most free amino acids, while contributions of L. plantarum were found. Electrophoresis indicated that the mixed starters, especially the co-cultures containing L. plantarum, showed a stronger degradation for caseins than the pure S. thermophilus culture. These findings suggest that culture combinations may influence the proteolysis characteristics of the fermented products, and probiotic cultures must be carefully chosen for fermented production.