Viscoelastic properties of oil-water interfaces covered by bovine beta-casein tryptic peptides

A combination of proteolysis and dilational rheology has been used to study the behavior of films of beta-casein (beta-CN) and of peptides spread at the oil-water interface. Identification of the peptides produced by trypsin hydrolysis of beta-CN in emulsion at 37 degrees C provided information on the structure of beta-CN adsorbed at the oil-water interface. Good interface properties were observed for beta-CN or its peptides, probably because of the amphipathic nature of beta-CN or a synergistic effect between hydrophilic and hydrophobic peptides. Remarkable surface activity was found for the amphipathic peptide beta-CN (f114-169). Rheological studies had shown that interface films made with peptide fractions or with beta-CN were elastic rather than viscous. Film made with the purified peptide beta-CN (f114-169) was merely elastic at the triolein-water interface. A decrease of the viscoelastic modulus was observed for aging beta-CN film but not for aging peptide films; The beta-CN decrease was related to the flexibility of its structure. When the interface is increased by the dilation of an aqueous droplet plunged into oil, beta-CN may expose new polypeptide trains to cover the increased interface, unlike peptides with simpler structures.