Effect of sodium chloride on formation and structure of whey protein isolate/hyaluronic acid complex and its ability to loading curcumin

Effects of ionic strength (0–150 mM NaCl) on formation and properties of whey protein isolate (WPI) and hyaluronic acid (HA) complexes (WPI/HA = 4:1, total concentration of 0.1%, w/w) were investigated. Subsequently, coacervates with corresponding NaCl were applied as delivery vehicle for curcumin. Turbidity analysis showed that NaCl enhanced WPI and HA complexation at 5–40 mM. Adding NaCl (0–150 mM) shifted pHφ1 and pHφ2 towards lower and higher pH values, respectively. Zeta potential measurement showed that surface charge of complexes decreased with increasing NaCl concentration. Circular dichroism and fluorescence spectra indicated that α-helix content and fluorescence intensity of WPI involved in complexation changed with increase of varied NaCl concentration. Isothermal Titration Calorimetry data and Confocal Laser Microscope analysis showed that screening effect of NaCl hindered electrostatic interaction, whereas hydrophobic force may promote complexation at NaCl concentration of 0–30 mM. Fluorescence quenching data revealed that presence of salt at low (5–40 mM) and high (50–150 mM) levels increased and decreased binding ability of WPI/HA with curcumin, respectively. Presence of salt at 5–40 mM improved particle size, encapsulation efficiency and loading capacity compared with control. Data demonstrated that critical NaCl concentration was around 40 mM and this finding could facilitate applications of WPI/HA complex as delivery vehicle of bioactive substances.