Chitosan (CS) is widely utilized as a natural preservative for fruits and vegetables; however, its practical application is constrained by inferior mechanical properties and low water resistance. In this study, citric acid (CA) was incorporated into the chitosan matrix to leverage its multifunctional carboxyl groups and inherent antibacterial properties, aiming to enhance both the mechanical and antibacterial performance of the resulting composite films. The structure of the CS/CA composite films was characterized using Fourier transform infrared spectroscopy (FT-IR), 1H nuclear magnetic resonance (1H NMR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The physicochemical properties, antimicrobial activity, and potential for blueberry preservation of CS/CA films with varying CA content were systematically evaluated. Results indicated that the incorporation of CA induced cross-linking of CS chains via hydrogen bonding and amidation, leading to significant improvements in optical transparency, thermal stability, and hydrophilicity. The mechanical properties of the composite films were remarkably enhanced, with the maximum tensile strength increasing by up to fourfold compared to that of pure CS films. Composite films, particularly CS/CA4–CS/CA6, exhibited strong antibacterial effects against S. aureus and E. coli. Preservation tests on blueberries demonstrated the superior efficacy of the CS/CA composite films, especially CS/CA7, which showed optimal performance with only 5.6 % weight loss, maintained firmness at 228.3 g, the highest anthocyanin retention, and a 46.75 % reduction in decay rate compared to the control. These findings indicate that CS/CA composite films are promising environmentally friendly materials for active food packaging applications.
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