Rheology of whey protein–dextran conjugate films at the air/water interface

The film properties of whey protein isolate (WPI) and WPI–dextran Maillard conjugates with different levels of dextran attachment were examined using surface shear and dilatational rheology. The changes in film properties were assessed in relation to the changes in protein unfolding induced by various levels of attachment of dextrans of different molecular weight. Attachment of ∼1 dextran per mole of WPI (low conjugation) did not change the surface modulus or the surface dilatational modulus. At the low level of conjugation there was little effect on protein structure, as measured by the negligible effect of dextran attachment on tryptophan fluorescence emission and circular dichroism spectra. Increasing the number of dextrans attached to ∼5 dextrans per mole WPI (moderate conjugation), which caused substantial protein unfolding, markedly decreased both the surface and dilatational moduli. The results suggested that the loss of film strength was a result of protein unfolding. The consequences of this are discussed in terms of the origin of film strength and in relation to the functional behaviour of these interfacial films.