To explain the limited nutritional value of milk cultured with
proteinase-positive (Prt+) strains of Lactococcus lactis for the subsequent growth of
dairy lactococci, we investigated further the time courses of modifications in the free
amino acid and peptide contents of cultured milk. When growing in milk for up to
24 h, Prt+ strains of Lc. lactis progressively accumulated amino acids and casein-derived peptides. The growth of proteinase-negative (Prt−) wild-type strains and
peptide transport mutants of Lc. lactis in cultured milk showed that casein-derived
peptides could sustain growth up to 5×108 cfu/ml, depending on the extent of casein
degradation during the preliminary growth of Prt+ strains and the Prt− strains. Of
the casein-derived oligopeptides, <25% were transported into the cell and used for
Lc. lactis growth. However, they played a prominent role, contributing 90% to
growth. In contrast, di- and tripeptides did not contribute to growth, suggesting that
either few were released from caseins or they did not supply essential amino acids.