Phenolic acid-grafted high-amylose maize starch as a multifunctional emulsifier in agarose-based emulsion gels for thymol loading and release: Application in active coatings for strawberry preservation

Three benzoic-type phenolic acids were covalently grafted onto HAMS via esterification. • Bioactivities followed a hydroxyl-dependent ranking: Ga-HAMS > Da-HAMS > Ph-HAMS. • LF-NMR/MRI showed uniform water distribution in modified emulsion gels. • Ga-HAMS active coating significantly extends strawberry shelf life at 25 °C. Three benzoic-type phenolic acids with progressively increasing hydroxyl substitution ( p -hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid) were covalently grafted onto native high-amylose maize starch (Na-HAMS) via EDC·HCl/DMAP-mediated esterification, yielding Ph-HAMS, Da-HAMS, and Ga-HAMS at a grafting ratio of approximately 12%. Successful esterification at the C-6 primary hydroxyl groups was confirmed by FTIR, UV-Vis, XPS, and NMR analyses. Phenolic acid grafting significantly enhanced surface hydrophobicity and conferred marked antioxidant and antimicrobial activities, both following a consistent structure-dependent ranking of Ga-HAMS > Da-HAMS > Ph-HAMS, reflecting differences in phenolic hydroxyl number and ortho-cooperative configuration. Thymol-loaded emulsion gels were subsequently constructed using each modified starch as an emulsifier combined with an agarose network. Compared with Na-HAMS, modified starch-based emulsion gels exhibited substantially improved emulsifying activity, reduced droplet size, and enhanced thymol loading efficiency (∼30% to 92%), along with superior physical stability. Rheological analysis confirmed elasticity-dominated viscoelastic behavior ( G ′ > G ″) stable across 20-90 °C, while LF-NMR and MRI revealed more homogeneous internal water distribution within the modified gel matrices. Based on a comprehensive performance evaluation, Ga-HAMS emulsion gel was applied as an active coating for strawberry preservation at 25 °C. Compared with control and Na-HAMS groups, Ga-HAMS coating effectively suppressed microbial growth, reduced decay and weight loss, and better maintained firmness, color, and soluble solids. LF-NMR and MRI further confirmed the preservation of cellular water compartmentalization. This work systematically elucidates the structure-activity relationships of benzoic-type phenolic acid-grafted HAMS and provides a theoretical basis for polysaccharide-based active emulsion gels in postharvest fruit preservation. Download: Download high-res image (362KB) Download: Download full-size image