Influence of Defined Shear Rates on Structural Changes and Functional Properties of Highly Concentrated Whey Protein Isolate-Citrus Pectin Blends at Elevated Temperatures

Abstract

The functional properties of whey proteins can be improved by conjugation with citrus pectin. Although protein-polysaccharide conjugates can be performed using extrusion processing, little is known about the influence of the extrusion conditions (e.g., temperature, shear stress, time) on the reactions taking place. As during extrusion processing, thermal and mechanical stresses are coupled to each other, their influence on the reactions taking place cannot be investigated separately. This study aims to get a deeper understanding of the influence of defined shear rates on structural changes and functional properties of highly concentrated whey protein-citrus pectin blends treated at elevated temperatures by using a closed-cavity rheometer (CCR). The CCR provides the opportunity to examine the impact of thermal and mechanical stresses in highly concentrated systems independently. The analyses of structural changes showed that the formation of disulfide bonds was accelerated with increasing shear. Temperature treatments at 120 °C and 140 °C resulted in the formation of non-disulfide covalent cross-links (e.g., Maillard reaction products and isopeptides), while shear inhibited their formation at treatment conditions up to 140 °C and 2 min. The samples treated at 140 °C and 2 min (with and without the application of shear) exhibited improved emulsifying capacities which is attributed to changes in their interfacial properties. This might be due to high concentrations of fluorescent compounds indicating the formation of Maillard reaction products (e.g., conjugates).