Chemical physics of whey protein isolate in the presence of mucin: From macromolecular interactions to functionality

Oral processing, textural perception and functionality of colloidal foods are strongly influenced by the interactions between the salivary mucins and the food proteins. This work studies the physico-chemical aspects of mixtures of a typical food protein, whey protein isolate (WPI) and mucin. Phase separations result from aggregation between the two components at pH 7 and at pH 3. ζ-potential and fluorimetry data show that electrostatics contribute to entropically-driven interactions at pH 3, while at pH 7, two different non-electrostatic interactions, an entropically-driven and an enthalpically-driven one lead to aggregation and phase separation. Substitution of WPI with increasing mucin concentrations at pH 7 results in a marked increase of the shear viscosity in comparison with pH 3. Mucin enhances the extensional viscosity in a similar fashion, e.g. the incorporation of mucin into a WPI system at 6:4 ratio increases the extensional viscosity ≥ 3-fold (0.27˗0.85 Pa s) and ≥2-fold (0.38˗0.89 Pa s) at pH 3 and pH 7, respectively. These results indicate a notable increase of the extensional over shear viscosity ratio (Trouton's ratio). The above highlight the effect of the molecular-level interactions between food and salivary macromolecules on phase behavior and flow during oral processing.