Preparation, Characterisation, and Fresh-Keeping Application of Pullulan/Konjac Glucomannan/Low-Acyl Gellan Gum Ternary Edible Composite Films for Extending the Shelf Life of Plums

The escalating environmental burden of conventional plastic food packaging has intensified the search for sustainable, biodegradable alternatives. However, single-component polysaccharide edible films often exhibit suboptimal mechanical and barrier properties, which limits their practical applicability. In this study, pullulan (PUL), konjac glucomannan (KGM) and low-acyl gellan gum (LGG) were blended as film-forming substrates with glycerol as a plasticiser. Single-factor experiments combined with Box–Behnken response surface methodology (RSM) were employed to optimise the formulation of ternary edible composite films. The effects of PUL, KGM and LGG concentrations on mechanical properties and water vapour permeability (WVP) were systematically investigated. The optimised film was comprehensively characterised for microstructure, physical barrier performance, antioxidant activity, soil disintegration, bacteriostatic capacity and plum preservation efficiency. The optimal formulation comprised 1.43 g/100 mL PUL, 0.24 g/100 mL KGM and 0.48 g/100 mL LGG. Under these conditions, the composite film exhibited a transparent and smooth appearance, with a continuous and dense internal microstructure. The comprehensive performance score (K) reached 0.805 ± 0.013, with a WVP of 2.75 ± 0.17 g·μm/(m2·h·kPa), light transmittance of 89.93% ± 1.26%, tensile strength (TS) of 3.05 ± 0.14 N/cm2, elongation at break (EB) of 64.57% ± 0.58% and DPPH radical scavenging activity of 34.12% ± 0.96%. Storage trials on two plum varieties demonstrated that the composite coating effectively mitigated fruit weight loss, retarded peel browning and pulp softening, and maintained superior sensory quality throughout storage. Overall, the PUL/KGM/LGG ternary edible composite film exhibited favourable mechanical strength, promising water vapour barrier properties and moderate antioxidant activity, suggesting its potential as a useful reference for the further development and industrial adoption of biodegradable edible films in fruit and vegetable preservation.