Structure-function relationships and biodegradation behavior of protein-based films derived from Passiflora edulis seed flour: valorization of an agro-industrial by-product for sustainable food packaging

• Protein films from Passiflora edulis seed flour valorise agro-industrial waste. • Spectroscopic and thermal analyses linked protein conformation to film performance. • Higher flour content formed protein networks, improving strength and stability. • Homogeneous gel-like microstructure correlated with elastic and tensile behaviour. • Complete biodegradation in nine days supports sustainable food packaging use. Biodegradable protein-based films were developed from flour obtained from non-edible Passiflora edulis seeds as a strategy for the chemical valorization of an underutilised agro-industrial by-product. The films were formulated using two concentrations of flour (2.5 and 3.0%) and two levels of glycerol (50-60%) as a plasticising agent. Their physicochemical, thermal, mechanical, functional, barrier and microstructural properties were characterized. The films exhibited a compact and continuous matrix with a dark appearance and elastic behavior. Micrographs revealed a gel type with a uniform pattern of spherical granules distributed homogeneously on the surface, with no visible pores or cracks. The formulation of 3.0% flour and 50% glycerol showed greater surface hydrophobicity and a higher melting point, suggesting a denser and more cohesive structure. Spectroscopic (FTIR), thermal (DSC, TGA) and structural (XRD) analyses demonstrated significant modifications in the secondary conformation of the proteins, which were directly associated with improved mechanical performance, including higher tensile strength and elongation at break. Water vapor permeability increased (2 × 10 -9 - 1.1 × 10 -8 g/Pa ms) with higher glycerol and flour content, reflecting the balance between matrix flexibility and barrier efficiency. All formulations showed complete biodegradability within nine days, confirming their enviromental compatibility.