Sulfhydryl−Disulfide Interchange-Induced Interparticle Protein Polymerization in Whey Protein-Stabilized Emulsions and Its Relation to Emulsion Stability

Polymerization of proteins via sulfhydryl−disulfide interchange occurs at the oil−water interface. However, it is not known if this polymerization reaction takes place only within the protein film of an emulsion particle or also occurs between the protein films of emulsion particles. To elucidate this, emulsions made with pure β-lactoglobulin and pure α-lactalbumin were mixed at 1:1 ratio and the time-dependent intermolecular sulfhydryl−disulfide interchange between the protein films of β-lactoglobulin-stabilized and α-lactalbumin-stabilized emulsion droplets were studied. In pure protein emulsions, polymerization of β-lactoglobulin occurred whereas it did not occur in α-lactalbumin. However, disulfide cross-linked β-lactoglobulin−α-lactalbumin polymers formed in the cream phase but not in the serum phase of the emulsion-mix emulsions of pure β-lactoglobulin and pure α-lactalbumin emulsions. The extent of polymerization increased with storage time, indicating the occurrence of interparticle polymerization in this emulsion system. The interparticle polymerization promoted aggregation of emulsion particles and decreased the kinetic stability of the emulsion.