THE CONTRIBUTION OF LACTOCOCCAL STARTER PROTEINASES TO PROTEOLYSIS INCHEDDAR CHEESE

The contribution of the lactococcal proteinase to proteolysis and flav or development in Cheddar cheese was investigated using the starter st rains Lactococcus lactis ssp. lactis UC317, its proteinase-negative de rivative FH041, and variants of UC317 modified in proteinase productio n, location, and specificity. Lactococcus lactis ssp. lactis FH041 was transformed by electroporation with plasmids pCI3601, pCI3602, or pNZ 521. Plasmids pCI3601 and pCI3602 harbor the cloned proteinase genes o f L. lactis ssp. lactis UC317 on a high copy number vector and, as suc h, encode an increased concentration of cell wall-associated and secre ted enzymes, respectively. Plasmid pNZ521 contains the cloned proteina se genes from Lactococcus lactis ssp. cremoris SK11. Assessment of pro teolysis and flavor development in Cheddar cheese made with these stra ins revealed that starter proteinases are required for the accumulatio n of small peptides and free amino acids in Cheddar cheese. Proteolysi s was not enhanced by an approximately three-fold increase in concentr ation of the lactococcal proteinase. The strain in which the proteinas e remained attached to the cell wall appeared to contribute more to pr oteolysis than the strain that secreted the enzyme. Water-soluble pept ides unique to Lactococcus lactis ssp. cremoris SK11 and L lactis ssp. lactis UC317 were detected by PAGE and HPLC, respectively. Sensory ev aluation showed that the flavors of all cheeses made with proteinase-p ositive starters were similar, but cheeses made with proteinase-negati ve starters lacked flavor.