Dual-network low-methoxyl amidated pectin–protein films: mechanism, optimization, and application to fresh foods

Abstract

This study reports an edible-film platform that valorizes Hàm Yên orange peel low-methoxyl amidated pectin (LM/LMA) and bee brood protein (BBP, Apis mellifera) through dual structuring: complex coacervation between pectin(−) and BBP(+) at pH 4.7, and Ca²⁺ “egg-box” crosslinking of homogalacturonan junctions. BBP was obtained via defatting → mild alkaline extraction → isoelectric precipitation; LM/LMA pectin was prepared from local citrus peel and plasticized with glycerol. A design–make–measure loop combined turbidity/ζ-potential/DLS maps to locate the coacervation window, and then tuned casting and post-CaCl₂ treatment. Under the optimized recipe (LMA 2.5% w/v; BBP 0.5% w/v; glycerol 25% on polymer; CaCl₂ 1.5% w/v; pH 4.7), the hybrid film achieved a balanced property set: tensile strength (TS) ≈ 51 MPa, elongation at break (EAB) ≈ 20.5%, storage elastic tissue E′ (DMA E′) ≈ 1.18 GPa, water vapor permeability (WVP) ≈ 4.6 g mm per m² per day per kPa, oxygen transmission rate (OTR) ≈ 60 cm³ per m² per day, glass transition temperature (T_g) ≈ 54 °C, T₆₀₀ ≈ 80%, haze ≈ 14%, contact angle ≈ 71°. FTIR resolved amide–COO⁻ electrostatic pairing and Ca–pectinate signatures; SEM revealed a compact, defect-poor cross-section; TGA/DSC showed multi-step dehydration/plasticizer loss and polysaccharide degradation typical of Ca²⁺-reinforced pectin networks. Safety profiling met food-contact expectations (Pb/Cd/As/Hg at trace levels; total plate count (TPC) and yeast–mold <10 CFU g⁻¹; E. coli, Salmonella, S. aureus not detected) and the film disintegrated to ∼18% mass remaining at day 60 under lab composting. In pilot applications, coatings on beef (4 °C) and tomato (12 °C) slowed drip/weight loss, lipid oxidation (TBARS), texture softening, pH drift, microbial growth, and decay incidence versus pectin-only and control, indicating effective moisture/oxygen moderation and surface conditioning by the pectin–protein–Ca²⁺ network. The results demonstrate a circular, locally sourced, protein–polysaccharide film that reconciles mechanical robustness, gas/water-vapor resistance, optics, safety, and biodegradability, with clear translational potential for fresh-food preservation.