Allyl isothiocyanate (AITC) is a small electrophilic molecule which can be found in cabbage after degradation of glucosinolates. The covalent attachment of AITC to whey protein isolate (WPI) was previously reported to increase their hydrophobicity and structural flexibility at acidic pH values. It is thus hypothesized, that the o/w interface adsorption behaviour and interfacial structure will be altered. To further understand the effect of the AITC-modification on the emulsifying capacity, adsorption kinetic and interfacial properties of unmodified and modified WPI were investigated at the o/w interface. The WPI-modification resulted in a significantly increased surface adsorption kinetic and a lower equilibrium interfacial tension at acidic pH values. The ratio of α-lactalbumin (ALA) and β-lactoglobulin (BLG) at the oil droplet surface differed between unmodified and modified WPI (modBLG > ALA+modALA). Several layers of loosely attached proteins were evident on the oil droplet surface in all modified WPI emulsions. The hyperfine coupling (aN) of the EPR spin probe TB residing at the oil droplet surface reflected an increased hydrophobicity of the modified proteins. A lower order parameter (S) in the lipid phase of the modified WPI emulsions gave evidence of an altered alignment of the modified proteins at the interface, probably sticking into the oil phase. In conclusion, the present results indicate that the increased flexibility and hydrophobicity of the modified whey proteins, especially of modified BLG, surpass the surface activity of unmodified ALA in acidic pH values.
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