PHAGE-INDUCED LYSIS OF LACTOCOCCUS-LACTIS DURING SAINT-PAULIN CHEESE RIPENING AND ITS IMPACT ON PROTEOLYSIS

The autolysis of starter bacteria during cheese ripening results in th e release of intracellular enzymes, especially peptidases, into the cu rd and in consequence is expected to accelerate and enhance the develo pment of cheese flavour. In the course of a series of cheese assays wi th the non-autolytic Lactococcus lactis NCD0763 strain, the presence o f a homologous bacteriophage was encountered. The phage was present at different levels which allowed us to evaluate the impact of the exten t of starter bacteria lysis on Saint-Paulin-type cheese proteolysis du ring ripening. We estimated bacterial lysis by viable cell enumeration and electron microscopy observations over a 2-month ripening period. On the same cheeses, we have estimated proteolysis by measuring solubl e nitrogen (SN) and phosphotungstic acid-soluble nitrogen (PTA-N) and by quantifying free amino acids. For phage levels up to 10(10) pfu/g o f cheese, the pH and dry matter values were in the standard range, whe reas for a higher phage level incorrect acidification was observed. Th e extent of starter lysis increased with the phage level and it was al so correlated with the rate of formation of small peptides and free am ino acids. After 2 months of ripening, the PTA-N value was three-fold higher and the total free amino acid content was seven-fold higher in the cheese with a phage level of 10(10) pfu/g than in the control chee ses. These results confirm that the early lysis of starter bacteria al lows the release of intracellular peptidases which are active in chees e, and that the extent of starter lysis can be correlated with the pro teolysis rate.