Pectin-microfibrillated cellulose composite modified by calcium diffusion: Analysis of calcic microstructures,...

Pectin and microfibrillated cellulose (MFC) form a composite with a modifiable microstructure during calcium (Ca²⁺) diffusion. This study aimed to analyze the microstructural changes, chemical composition, and structural connectivity in the pectin-microfibrillated cellulose (PMFC) composite caused by Ca²⁺ diffusion. It also examined how the modified structure influences thermal treatment and compression forces. PMFC was formulated with MFC suspension and pectin to simulate plant cell wall components, pretreated with a calcium hydroxide solution, and exposed to thermal treatment. SEM analysis revealed that pectin and MFC formed large, fibrillated, amorphous structures. When calcium pretreatment was applied to PMFC, it changed into a uniform firm structure with modified three-dimensional networks consisting of shorter microstructures with more skeletons and branches (Ca²⁺-branched arrangement) compared to calcium-free structures. EDX analysis showed calcium peaks at different PMFC thicknesses, with calcium content increasing in the PMFC until equilibrium, reaching a calcium diffusion coefficient of 3.25 × 10^-10 m²/s. The Ca²⁺-branched structures remained stable during thermal treatment, preventing deformation and collapse, and a moisture diffusion coefficient of 2.80 × 10^-9 m²/s. Furthermore, the Ca²⁺-branched structures provided hardness, elasticity, and resistance to stress-deformation during compression. Ca²⁺ modifies three-dimensional networks even after the physicochemical interaction between pectin and MFC has occurred.