Ultrasound-assisted construction of pea protein isolate-folic acid covalent complex as self-assembled nanocarrier: Enhancing the stability, precise release property, and bioaccessibility of curcumin

Curcumin (Cur) is a hydrophobic phenolic compound with superior biological activity, but the limited water solubility, chemical instability, and poor bioaccessibility of curcumin restrict its application. This study developed pea protein isolate (PPI)-folic acid (FA) covalent complexes as nanocarriers for the encapsulation, protection, and precise delivery of curcumin. The results of SDS-PAGE, XPS, and FTIR demonstrated that covalent complexation between PPI and FA was formed through an ultrasound-assisted free radical grafting method. Notably, ultrasonication for 15 min (PPI-FA-US15) achieved a higher FA loading capacity (44.78 ± 0.37 µg/mg) than the traditional free radical grafting for 24 h (36.45 ± 0.43 µg/mg). The ultrasonic treatment and covalent complexation of FA greatly improved the solubility of PPI (from 61.34 ± 0.67% to 97.02 ± 0.88%). Curcumin was efficiently encapsulated within the core of PPI-FA-US15 conjugates to form PPI-FA15-Cur nanocomplexes by hydrogen bonding and hydrophobic interaction. The complexation of FA improved the binding affinity of PPI for curcumin. PPI-FA15-Cur demonstrated a 1.97-fold superior loading capacity for curcumin compared to PPI-Cur. Meanwhile, PPI-FA15-Cur displayed a spherical morphology, and maintained a stable particle size distribution during 77-d storage. The heating stability, photochemical stability and antioxidant property of curcumin were significantly improved. Furthermore,in vitrodigestion studies demonstrated that PPI-FA15-Cur showed a sustained release of curcumin. The bioaccessibility of curcumin within PPI-FA15-Cur was about 4.75-fold and 1.11-fold higher than that of free curcumin and PPI-Cur, respectively. This study provides a promising approach for developing plant protein-based carriers that enable precise delivery of curcumin in functional foods.