Functionality of whey proteins covalently modified by allyl isothiocyanate. Part 1 physicochemical and antibacterial properties of native and modified whey proteins at pH 2 to 7

Abstract

Whey protein isolate (WPI) (∼75% β-lactoglobulin (β-LG)) is frequently used in foods as a natural emulsifying agent. However, at an acidic pH value, its emulsification capacity is greatly reduced. The covalent attachment of natural electrophilic hydrophobic molecules to WPI proteins is a promising method for changing the physicochemical properties of WPI in favor of a higher functionality at acidic pH values. In the present study, different concentrations of allyl isothiocyanate (AITC) were covalently bound to WPI and the related changes in physicochemical properties (charge, aggregation, surface hydrophobicity and secondary structure) were monitored over a wide pH range (pH 2 to 7). In addition, the antibacterial activity against different strains of S. aureus and E. coli before and after AITC modification was assessed. The results showed that both whey proteins β-LG and α-lactalbumin (ALA) were modified by AITC. This modification remained stable during pH adjustment. Unbound AITC was successfully removed by the lyophilization process, which reduced the strong odor of the volatile AITC. A shift of the isoelectric point towards acidic conditions was observed. The hydrophobicity of the modified WPI increased significantly and the protein's secondary and tertiary structure was altered. In addition, a more loose protein folding was observed. These effects were less pronounced at pH 6 and 7. The relatively mild antibacterial effect of native WPI was not significantly influenced by the addition of AITC.