Design and characterization of soluble biopolymer complexes produced by electrostatic self-assembly of a whey protein isolate and sodium alginate

The aim of this study was to design and characterize soluble biopolymer complexes formed by self-assembly of an anionic polysaccharide (sodium alginate, SA) and a whey protein isolate (WPI). First, conditions for producing stable protein aggregates were studied by analysing the effect of concentration (2, 4, 6% w/w) and heating temperature (55, 70, 85 °C) on WPI structural characteristics, using spectroscopic techniques UV–Vis and fluorescence. As heat treatment of WPI at the highest temperature induced the formation of soluble protein aggregates with a greater exposure of hydrophobic patches on their surface, this condition was selected to form protein particles. Secondly, from aqueous solutions of 6% w/w heat-treated WPI at 85 °C, WPI/SA systems were obtained at different ratios (2:1, 4:1 and 6:1) and transmittance measurements as a function of pH (6.0–3.0) were made. Dynamic behaviour of biopolymers in WPI/SA systems was discussed in terms of transition pHs, pHc and pHϕ, corresponding to the formation of WPI-SA soluble complexes and the beginning of associative phase separation (coacervation), respectively. Finally, with these pH values, phase diagrams were constructed for each system. From these diagrams, pH 4.0 was selected to perform deposition of SA onto WPI aggregates surface, as at this pH value formation of WPI-SA soluble complexes was observed for all WPI:SA ratios. Resultant self-assembled biopolymer complexes were then characterized by the same set of spectroscopic techniques. Higher WPI:SA ratios produced soluble self-assembled biopolymer particles where hydrophobic patches of protein aggregates would be more occluded ensuring the protection of potential lipophilic bioactive agents attached inside.