Structural elucidation of citric acid cross-linked pectin and its impact on the properties of nanocellulose-reinforced packaging films

To advance sustainable packaging development, this study investigates modifying pectin-based films with citric acid cross-linking and nanocellulose reinforcement to enhance their functional properties. Cross-linked citrus pectin (CLCP) films were obtained from film-forming solutions containing different amounts of pectin (2.5-3.5 g 100 mL^-1) and citric acid (1-2 g 100 mL^-1), with glycerol (1-3 mL 100 mL^-1), and nanocellulose (0-2 g 100 mL^-1) added as a plasticizer and reinforcing agent, respectively. CLCP films were comprehensively characterized concerning structural, mechanical, and barrier properties. FTIR confirmed successful cross-linking, whereas the structural modifications occurred in the pectin network after cross-linking were elucidated for the first time using advanced 2D-NMR. Principal component analysis (PCA) revealed that nanocellulose concentration was the most significant factor in improving mechanical properties, with strong positive correlations with tensile strength (r = 0.586) and Young's modulus (r = 0.639). Conversely, glycerol concentration showed the strongest negative correlation (r = -0.746 and -0.741, respectively), confirming its plasticizing effect. Barrier properties were also significantly influenced: higher citric acid concentration and processing temperature improved the oxygen barrier. In contrast, higher pectin and nanocellulose concentrations decreased the oxygen transmission rate (r = -0.636 and r = -0.733, respectively). This systematic analysis provides a comprehensive design map for tuning film properties, demonstrating that co-modifying pectin with citric acid and nanocellulose is a highly effective strategy for developing advanced, sustainable packaging materials with tailored performance.