Phenolic acid-grafted starch nanoparticles as Pickering emulsion stabilizers: Linking the molecular structure of phenolic acids to emulsion stability.

Abstract

Phenolic acid-grafted starch nanoparticles (PA-SNPs) were fabricated via laccase-mediated oxidation using ferulic acid (FA), caffeic acid (CA), gallic acid (GA), and tannic acid (TA) to develop multifunctional Pickering stabilizers with enhanced interfacial and antioxidant properties. Phenolic acids (PAs) grafting significantly altered the physicochemical characteristics of SNPs, including particle size, ζ-potential, hydrophobicity, and antioxidant activity, in a structure-dependent manner. The methoxy-substituted aromatic structure of FA primarily enhanced particle wettability, while the higher density of phenolic hydroxyl groups in CA and GA and the multi-galloyl structure of TA increased phenolic binding and radical-scavenging activity, thereby reducing physical destabilization and enhancing oxidative protection. Among the modified particles, TA-SNPs exhibited the highest phenolic binding content and radical-scavenging activity due to the polyphenolic structure of TA, which provides abundant phenolic hydroxyl groups and multiple binding sites. Compared with unmodified SNPs, PA-SNPs exhibited higher emulsifying activity and stability indices, as well as lower interfacial tension. Pickering emulsions stabilized by GA-SNPs and TA-SNPs (GA-SNPs-em and TA-SNPs-em, respectively) exhibited smaller droplet sizes. Confocal laser scanning microscopy (CLSM) revealed more continuous particle-associated fluorescence at the oil-water interfaces in GA-SNPs-em and TA-SNPs-em, consistent with greater apparent interfacial accumulation of the modified particles. Accordingly, emulsions stabilized by PA-SNPs exhibited improved physical and storage stability. Furthermore, PAs grafting effectively retarded lipid oxidation, with TA-SNPs-em and GA-SNPs-em reducing the final peroxide value by 70%-75% compared with SNPs-em. These results establish a relationship among PAs structure, PA-SNPs properties, interfacial assembly, and emulsion stability, supporting the view that tailoring the molecular structure of grafted PAs is an effective strategy for designing antioxidant emulsions with enhanced physical and oxidative stability.