A biodegradable film based on sodium alginate–gelatin involving a mulberry anthocyanin extract for fish oil packaging

Driven by the 'dual carbon' strategy and the imperative of a circular economy, food packaging is undergoing a ternary upgrade. In this study, mulberry anthocyanin extract (MAE) was upcycled into a natural, dual-function active principle that concurrently delivers antioxidant capacity and ultraviolet (UV) barrier activity. It was incorporated with an edible sodium alginate–gelatin (SG) based film to engineer a next-generation biodegradable antioxidant film. The dose-dependent impacts of MAE on the film's mechanical signature, radical-scavenging efficacy, optical performance, and practical utility in fish oil packaging were systematically examined. MAE obtained from a residue of mulberry juice fermentation had 296.86 mg/g total polyphenols and 146.50 mg/g anthocyanins, primarily cyanidin-3-glucoside and cyanidin-3-rutinoside. When 0.2 mg/mL MAE (SG-2MAE) was added, the tensile strength of SG films increased from 48.33 to 50.98 MPa. When 0.4 mg/mL MAE was incorporated, the antioxidant activity of SG-4MAE film against 2,2'-biazido-bis-3-ethyl-benzothiazoline-6-sulfonic acid (ABTS) increased by 158%, whereas transmittance (600 nm) decreased by 33.7%, indicating enhanced UV blocking. When applied for packaging fish oil, the SG-MAE film delayed oxidation better than the unpackaged sample after 4 weeks' storage by directly blocking oxygen, releasing antioxidant constituents, and adsorbing residual oxygen. In summary, SG-MAE films showed excellent comprehensive properties and high antioxidant properties, and could be used as active packaging materials for fish oil.