Occurrence, origin and fate of pyroglutamyl-γ3-casein in cheese

The proteolysis of β-casein (CN) by action of plasmin characterizes the ripening of several cheeses, and one of the newly formed peptides, pyroglutamyl-g3-CN (pg3-CN), originates from the cyclization of the N-terminal Glu of g3-CN (β-CN f108-209). This peptide revealed in both Grana Padano PDO and Parmigiano-Reggiano PDO cheeses 2 months after manufacture progressively increased in content during ripening. A preliminary survey revealed pg3-CN in both hard and extra-hard cheese varieties, whereas in semi-hard cheeses it was present only in mature samples. To understand the mechanisms of pg3-CN formation/degradation, in vitro trials (real cheese, model cheeses and buffered g3-CN solutions) were performed. Both the effect of cheese pH and the capacity of selected microorganisms (L. helveticus, E. faecalis and E. faecium) to cyclize the N-terminal Glu residue of g3-CN were studied. The results suggested that while pg3-CN formed spontaneously during cheese ripening, mainly at the acid pH characteristic of hard and extra-hard cheeses, the role of cyclase enzymes was negligible. Differences in pg3-CN degradation were observed, due to the proteolytic activity of bacteria involved in the cheese ripening.