L. Vassal

Involvement of a Prophage in the Lysis of Lactococcus lactis subsp. cremoris AM2 during Cheese Ripening

Lactococcus lactis subsp. cremoris AM2 strain was previously shown to lyse early and extensively during cheese ripening. This strain is lysogenic and contains a prophage named ΦAM2. Lysis of strain AM2 and its prophage-cured derivative AM2-C in Saint-Paulin pressed-type cheese was monitored by the following parameters: cell viability, morphological changes of bacteria observed by electron microscopy and release of intracytoplasmic peptidases. Proteolysis was quantified by measuring soluble nitrogen (SN), phosphotungstic acid soluble nitrogen (PTA-N) and free amino acids. By contrast to the wild type strain AM2 which lyses early and extensively, its prophage-cured derivative AM2-C lyses only slowly and to a limited extent in cheese. These results indicate that the prophage ΦAM2 is involved in the lytic behaviour of L. lactis AM2 during cheese ripening. In addition, the comparison of two isogenic strains with similar enzymatic potential but different ability to lyse demonstrates that starter strain lysis results in a higher free amino acids production rate and a decrease of bitter taste.

Lactococcal Lysis and Curd Proteolysis: Two Predictable Events Important for the Development of Cheese Flavour

We have previously estimated the lytic ability of many lactococcal strains as well as their proteinase and global peptidase activities using buffer and pseudo-curd tests. In the present study, we have selected five lactococcal starters having different lytic and proteolytic properties to manufacture semi-hard cheeses. The lytic behaviour of the five strains in cheese correlated with their estimated lytic ability. Cell lysis positively influenced the ripening of cheese: proteolysis was more advanced in cheeses made with the three lytic strains. However, high cell lysis was not sufficient to obtain cheeses with good flavour scores and in particular non-bitter cheeses. The cheeses made with the strain Lactococcus lactis subsp. cremoris RD251, whose cells lysed highly during ripening but whose global peptidase potential was low, were bitter as were the cheeses made with the low lytic strain Lactococcus lactis subsp. lactis RD232 whose cells possessed a high proteolytic potential.