Fabrication of Polygonatum cyrtonema Hua Flavonoid-AgNPs and its PLA Composite Active Film for Extending the Shelf Life of Frozen Litchi

Abstract

This study presents an eco-friendly strategy to valorize Polygonatum cryptonym Hua flavonoids (PC) from the residues of polysaccharide extraction by synthesizing silver nanoparticles (PC-AgNPs) with enhanced bioactivity. Optimized synthesis (pH 7.5, 10 mM Ag⁺, 55 °C, 30 min) yielded monodisperse, spherical PC-AgNPs (avg. 16.2 nm) with face-centered cubic crystal structure. Nanoparticles showed remarkable scavenging ability for DPPH free radical (SC₅₀ = 2.61 µg/mL) and ABTS free radical (SC₅₀ = 1.65 µg/mL) compared to the native PC samples. The PC-AgNPs were incorporated into polylactic acid (PLA) films (1% w/w), achieving superior mechanical performance (tensile strength: 54.6 MPa; elongation at break: 6.0%) while demonstrating broad-spectrum antimicrobial activity against E. coli, S. aureus, and A. niger. Mechanistic studies revealed that the nanocomposite film disrupted bacterial membrane integrity in E. coli. Applied to litchi preservation at -18°C, the 1%NPs/PLA coating effectively maintained mitochondrial enzyme activities (Succinate dehydrogenase, Cytochrome c oxidase, H⁺-ATPase, Ca²⁺-ATPase) at more than 80% of fresh fruit levels for a certain storage life, significantly delaying senescence compared to controls. These findings establish PC-AgNPs/PLA as a dual-functional active packaging material that synergistically combines antioxidant reinforcement, antimicrobial protection, and physiological regulation for postharvest fruit preservation.