Tuning mechanical, barrier and water resistance properties of pectin films through almond shell-derived nanocellulose reinforcement and Ca2+ crosslinking

Pectin films were produced from citrus pectin and almond-shell nanocellulose (NC) to assess the effects of NC content (0-6 wt%) and calcium (Ca2+) crosslinking. Films were cast without Ca2+, with Ca2+ in the film-forming solution, or with both Ca2+ in the film-forming solution and additional post-casting Ca2+ bath. NC and Ca2+ during casting caused limited stiffening; however, the bath greatly increased tensile strength and Young’s Modulus, with the strongest effect at 4% NC (up to 102 and 156% higher than the control without NC or crosslinking). Increasing NC concentration improved oxygen barrier properties, reducing oxygen permeability by 48% versus the control (reaching 6.71 ± 0.10·10-21 m3·m/m2·s·Pa for films containing 6 wt% NC), while Ca2+ had no significant effect on oxygen but reduced water vapour permeability by up to 41%. Non-bathed films were fully soluble in water, whereas bath-treated films showed higher water resistance, further enhanced by increasing NC content, reaching solubilities up to 26% lower than the double-crosslinked film without NC (88% solubility). NC also increased thermal stability, raising the main degradation onset by up to 24% relative to the control (158 °C). Overall, sequential Ca2+ crosslinking and residue-derived NC yield pectin films with tunable mechanics, water and thermal resistance and high oxygen barrier, making them very interesting as biodegradable food packaging.

本研究以柑橘果胶与杏仁壳纳米纤维素（nanocellulose, NC）为原料制备果胶基复合膜，旨在考察纳米纤维素添加量（0~6 wt%）与钙离子（calcium, Ca²⁺）交联作用对膜性能的影响。实验共设置三类成膜体系：未添加Ca²⁺的空白对照组膜、成膜液中引入Ca²⁺的交联膜，以及成膜液中添加Ca²⁺且在铸膜完成后经Ca²⁺后处理浴浸渍的双重交联膜。成膜过程中引入的NC与Ca²⁺仅能带来有限的力学强化效果；而后处理浴浸渍则可显著提升膜的拉伸强度与杨氏模量（Young’s Modulus），当NC添加量为4 wt%时强化效果最优，此时膜的拉伸强度与杨氏模量较无NC且未交联的空白对照组分别提升最高可达102%与156%。提升NC添加量可显著改善膜的氧气阻隔性能，相较于空白对照组，膜的氧气透过率最高可降低48%；当NC添加量为6 wt%时，膜的氧气透过率可达6.71±0.10×10⁻²¹ m³·m/(m²·s·Pa)。而Ca²⁺对膜的氧气阻隔性能无显著影响，但可将水蒸气透过率最高降低41%。未经后处理浴浸渍的膜可完全溶于水，经后处理浴浸渍的膜则具备更优的耐水性能，且耐水性随NC添加量提升进一步增强：相较于未添加NC的双重交联膜（溶解度为88%），经后处理的膜溶解度最高可降低26%。NC还可提升膜的热稳定性，相较于空白对照组（初始热降解温度为158 ℃），膜的主要降解起始温度最高可提升24%。综上，通过分步钙离子交联工艺结合杏仁壳衍生纳米纤维素制备的果胶基膜，可实现力学性能、耐水与耐热性能的可调性，并具备优异的氧气阻隔性能，在可生物降解食品包装领域具备良好的应用前景。