Characterization of a traditional ripened cheese, Kurdish Kope cheese: Proteolysis, microstructural, and rheological characteristics

This study explored the primary and secondary proteolysis reactions in Kope cheese made from raw sheep milk, revealing significant biochemical and structural changes throughout the ripening process. The ripening index and proteolysis depth increased significantly (p < 0.05) during the ripening period. Electrophoresis results indicated that by the end of ripening, 44.7% of αs1-casein and 70.2% of β-casein residues remained intact, reflecting progressive protein breakdown. The low-molecular-weight protein-to-casein ratio rose from 0.12 to 0.40, signifying intensified proteolysis over time. Analysis of the ethanol-soluble fraction's peptide profile showed two prominent peaks between 40 and 50 min and a single major peak between 20 and 25 min, demonstrating the formation of specific peptides during maturation. Free amino acid analysis revealed higher concentrations of glutamic acid, leucine, and lysine in Kope cheese, signifying a rich profile of flavor-enhancing compounds. Principal component analysis classified the cheeses into three distinct ripening categories: (1) Young-ripened cheese (37-67 days) with low proteolysis intensity; (2) Mild-ripened cheese aged 67-127 days, characterized by the lowest mean storage modulus (G') value (576 KPa); (3) Old-ripened cheese (127-187 days) displaying significant proteolysis intensity and a comparatively low G' value. Scanning Electron Microscopy (SEM) imaging revealed dramatic structural changes during ripening. Initially, the cheese had a compact and rough texture, which evolved into an open, crumbly structure by the end of the ripening period. These changes corresponded with the biochemical processes of proteolysis and the resulting textural softening.