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Cross-linking of milk proteins can reduce its susceptibility to plasmin-induced hydrolysis
Plasmin-induced proteolysis is a major problem in milk and dairy beverages and must be controlled, as it leads to flavor (bitterness) and texture (gelation, sedimentation) defects. Plasmin cleaves proteins at the carboxyl site of lysine−X and arginine−X bonds with a preference for the lysine−X bond. We therefore hypothesized that cross-linking of milk proteins and modification of lysine residues through transglutamination and the Maillard reaction would reduce the susceptibility of milk proteins to the action of plasmin. Lysine residues on the β-casein backbone were cross-linked with glutamyl residues to different extents by transglutamination and lysine−sugar−lysine cross-linking was achieved through the Maillard reaction. The modified systems were then hydrolyzed by plasmin. The extents of cross-linking and the hydrolysis reaction were monitored by quantifying the formation of the different hydrolyzed products, e.g. γ-casein and proteose peptones, using sodium dodecyl sulfate polyacrylamide gel electrophoresis and reverse phase high performance liquid chromatography.
Cross-linking of lysine residues with glutamyl residues by transglutamination and lysine−sugar−lysine cross-linking through the Maillard reaction clearly affected plasmin-induced hydrolysis negatively and reduced the susceptibility of b-casein to plasmin-induced hydrolysis. This could have been due to (1) the modification of lysine making it unrecognizable to the substrate-binding pocket of plasmin and (2) the cross-linking preventing the release of hydrolyzed peptides. In terms of controlling plasmin-induced hydrolysis, it appeared that effect (1) played a more major role in Maillard reaction cross-linking and that effect (2) played a more major role in transglutamination.
It can be concluded from this study that the cross-linking of proteins may be a useful tool for controlling the plasmin-induced hydrolysis of milk proteins and therefore for minimizing the texture- and flavor-related defects that are caused by the release of hydrolyzed peptides.
Keywords: Plasmin, Transglutamination, Maillard reaction