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Understanding mechanisms of the plasmin-induced dissociation of the casein micelle
Bovine plasmin is a highly heat-resistant enzyme that is naturally present in milk. Plasmin can survive severe heat treatments such as UHT and may act on casein during the storage of milk products and lead to proteolysis, gelation, and bitterness.
We explored the plasmin-induced dissociation of the casein micelle to achieve a better understanding of gelation and sedimentation mechanisms in different milk products. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and reverse phase high performance liquid chromatography (RP-HPLC) were used to monitor the extent of hydrolysis of casein. The particle size, turbidity, mineral level in the serum phase, and sedimentation were also analyzed and were correlated with different extents of hydrolysis.
The particle size and turbidity decreased during the initial hydrolysis, whereas increased dramatically towards the end of hydrolysis. This indicated that casein micelle dissociation occurred during early stages and that aggregation of hydrolyzed peptides occurred towards the final stages of hydrolysis. The total calcium and phosphorus level in the serum phase increased linearly with an increase in the extent of hydrolysis, suggesting the release of peptides containing colloidal calcium phosphate from the casein micelle. The SDS-PAGE and RP-HPLC results indicated that hydrophilic peptides, e.g. proteose peptones, were the first to dissociate from the casein micelle on plasmin-induced hydrolysis; hydrophobic peptides, e.g. γ-caseins, dissociated slowly and with dissociation patterns that were identical to those of κ-casein, suggesting that, even after breakage of the anchor points, the release of κ-casein from the micelle was too slow to cause gelation. These results provide new insights into the dissociation pattern of the casein micelle and how this relates to plasmin-induced sedimentation or gelation of UHT milk systems.
Keywords: Plasmin, UHT milk, Gelation