Enhancing the aqueous solubility of curcumin at acidic condition through the complexation with whey protein nanofibrils

In the present study, whey protein nanofibrils (WPN) were used as carriers to improve the aqueous solubility of curcumin at acidic conditions (pH 3.2) for expanding its applications in functional beverages and drinks. Nanofibrils with nanometric diameter (less than 15 nm) and micrometric length were produced by 5 h heating (85 °C) of whey protein isolate (WPI) solution at pH 2.0. WPN showed a higher surface hydrophobicity compared to the native parental proteins which improved its ability to form a soluble complex with curcumin. The water solubility of curcumin was increased by about 1200-folds through the complexation with WPN, whereas the complexation with WPI increased its solubility by approximately 180-folds. Fluorescence measurements and Fourier transform infra-red spectroscopy indicated that the hydrogen bonding and hydrophobic interactions were mainly contributed to the formation of curcumin-WPN complexes. Circular dichroism spectroscopy revealed that the secondary structures of WPN were not significantly affected by binding to curcumin. The structural phase of nanocomplexes also was studied using X-ray diffraction analysis. Complexation of whey protein nanofibrils with curcumin improved their apparent viscosity and surface activity which makes them appropriate candidates to design new functional food emulsions and beverages. The results of antioxidant activity measurements (DPPH radical scavenging activity and reducing power) also showed that the antioxidant capacity of curcumin was drastically improved through the complexation with WPN. Generally, this study suggests that the whey protein nanofibril could be employed as a material to enhance the food applications of curcumin as a water-insoluble bioactive compound.