Additive-free pectin-rich macromolecular films from citrus peel: Process-controlled composition, colloidal structure, and functional properties.

Citrus peel waste is a significant by-product of citrus processing with potential for recovering pectin-rich materials. This study developed additive-free, pectin-rich bioplastic films from citrus peel using integrated hydrothermal treatment (HT) and microfiltration (MF). A 23 full factorial design was employed to investigate the effects of temperature (100-120 ℃), reaction time (5-25 min), and pore size (0.7-2.7 µm). Increasing hydrothermal severity enhanced the release of pectin, carbohydrates, and proteins, while β-elimination altered the soluble pectic fraction, reducing galacturonic acid content and increasing the apparent degree of esterification. MF independently redistributed the dispersed colloidal phase, with smaller pore sizes producing more densely packed particle networks. Factorial analysis showed that temperature and reaction time primarily governed tensile strength (up to 3.07 MPa), whereas the temperature × pore-size interaction-controlled water vapor permeability. Multiple factor analysis confirmed that HT and MF functioned as partially independent process variables, controlling composition and colloidal structure, respectively. Multi-response optimization identified 114 ℃, 25 min, and 0.70 µm as the most balanced condition (desirability = 0.59), satisfying feasibility constraints for thickness and flexibility. The integrated HT-MF process provides an additive-free, process-controlled route for valorizing citrus peel waste into functional, pectin-rich packaging films.