Structure–Property Relationships in Gamma-Irradiated Films Based on Gelatin/Modified Cassava Starch and Gelatin/Gluten Blends

This study investigates the effect of gamma irradiation on the physicochemical, mechanical, and antimicrobial properties of biodegradable composite films based on modified cassava starch/gelatin (GS) and wheat gluten/gelatin (GG), including their active formulations with 2% potassium sorbate (GS2S and GG2S, respectively). Films were produced by casting and irradiated at 2–32 kGy. Irradiation modulated the structure–property relationships of starch–protein matrices in a dose-dependent manner. In GG systems, tensile strength increased while elongation decreased, indicating enhanced intermolecular interactions. At higher doses (16–32 kGy), excessive rigidity was observed, whereas lower doses (2–8 kGy) provided a more favorable balance between strength and flexibility. Water vapor permeability decreased in selected formulations, while solubility results indicated the coexistence of crosslinking and chain scission mechanisms. Among the tested conditions, 2 kGy was identified as the optimal dose. Despite the incorporation of 2% potassium sorbate, antimicrobial performance remained limited, with modest inhibition against Wallemia sebi and negligible effects on Aspergillus chevalieri and Aspergillus montevidensis. HPLC analysis demonstrated a significant reduction in sorbate release after irradiation (up to ~80%), indicating that restricted mass transfer, rather than intrinsic antimicrobial inefficiency, governs the observed behavior. The main novelty of this work lies in demonstrating that irradiation-induced structural modifications improve mechanical and barrier properties but simultaneously hinder active compound release. This decoupling between structural performance and functional activity highlights a critical limitation in irradiated active films. Overall, gamma irradiation at 2 kGy is effective for tuning material properties; however, controlling release kinetics is essential to achieve functional antimicrobial performance in biodegradable active packaging systems.