From macroscopic phase separation to microscopic interactions: effects of pH, mass ratio, and ionic strength on phase behavior and thermodynamic mechanism of whey protein isolate - fucoidan complexes.

This study investigated the effects of pH, mass ratio, and ionic strength on the phase behavior and thermodynamic interactions of whey protein isolate (WPI)-fucoidan (FUC) complexes. The mass ratio was identified as the primary factor governing phase behavior, with a stable homogeneous system observed at a 1:1 (w/w) ratio across pH 2-7. This work quantitatively demonstrated the dual role of ionic strength in regulating WPI/FUC complexation: low NaCl promoted complex formation via enhanced electrostatic interactions, whereas high salt induced phase modulation through combined electrostatic screening and excluded volume effects. Thermodynamic analysis revealed a pH-dependent transition in interaction mechanisms. At pH 4.0 and 5.5, complexation was spontaneous and enthalpy-driven, dominated by electrostatic interactions, while at pH 2.5 the driving force shifted to entropy-driven hydrophobic interactions. These findings provide new mechanistic insights into salt- and pH-regulated phase behavior in protein-polysaccharide systems.