Tuning mechanical, barrier and water resistance 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.