kappacasein

Kappa-casein, also known as κ-casein, is one of the main protein components of milk casein micelles. In bovine milk it is encoded by the CSN3 gene. The mature protein is a glycoprotein of about 169 amino acids, with a molecular weight roughly in the range of 19 to 23 kDa. Its structure features a heavily glycosylated, hydrophilic C-terminal region and a more charged N-terminal portion that associates with calcium phosphate on the micelle surface, helping to stabilize the micellar structure.

Function and significance: κ-casein stabilizes milk micelles and contributes to the emulsion stability and viscosity of milk. By forming a protective surface layer, it prevents premature aggregation of micelles, maintaining milk in a stable colloidal suspension.

Role in cheese making: κ-casein is the primary substrate for the milk-clotting enzyme chymosin (rennet). Cleavage occurs at a specific peptide bond between Phe105 and Met106, releasing para-kappa-casein and glycomacropeptide (GMP). Removal of the surface-stabilizing portion destabilizes the micelle surface, promoting aggregation and curd formation, which is essential for traditional cheese production.

Genetic variation: κ-casein is polymorphic, with several alleles identified in cattle. The A and B alleles are among the best studied. Different allelic forms can influence milk coagulation properties, gel strength, and cheese yield, and are of interest in breeding programs aiming to optimize cheesemaking traits. Glycosylation patterns can also vary, potentially affecting micelle stability and processing behavior.

Occurrence and relevance: κ-casein is present across the milk of many mammalian species, with concentration and structure varying by species, breed, diet, and lactation stage. In dairy production, κ-casein status informs processing strategies and can influence cheese quality and yield.