ABSTRACT Bio-based films were developed from soy protein isolate (SPI) reinforced with upcycled carrot pulp (UCP) obtained from carrot-processing side streams by continuous subcritical-water hydrolysis. This study aimed to valorise food-processing residues within a circular bioeconomy framework while establishing structure–processing–property relationships for SPI-based composite films produced by thermomechanical processing. Two bio-based crosslinking strategies were evaluated: citric acid (CA) and gallic acid (GA), with alkaline pretreatment used to modify SPI conformation and reactivity. Films were prepared by internal mixing at 140 °C and 60 rpm followed by compression moulding at 150 °C, yielding flexible films with an average thickness of approximately 0.35 mm. Crosslinking chemistry strongly influenced the tolerance of SPI matrices to UCP incorporation over the 0–10 wt% range. CA-crosslinked SPI films maintained relatively stable tensile strength, 3.5–4.2 MPa, and Young’s modulus, 55–70 MPa, across all UCP loadings, consistent with the formation of a more robust covalently stabilised network . In contrast, GA-crosslinked films based on alkaline-pretreated SPI showed higher initial flexibility, with elongation at break values of approximately 50–75%, but were more sensitive to UCP loading, suggesting greater sensitivity of non-covalent protein–polyphenol–polysaccharide interactions to structural perturbation by the complex polysaccharide-rich filler. UCP showed a median particle diameter close to 62 μm, comparable to SPI, and contained hydroxyl-rich hemicellulosic and pectic fractions favourable for matrix–filler interaction. Optical transmittance, X-ray radiography and refractive-index similarity in aqueous dispersions were consistent with good macroscopic compatibility at suitable filler contents. Overall, rational crosslinker selection enabled effective incorporation of hydrothermally derived carrot side-stream fractions into SPI films, providing promising candidate materials for potential sustainable flexible packaging applications based on food-waste valorisation, reduced-solvent processing and circular bioeconomy principles.
Fortunatti-Montoya et al. (Mon,) studied this question.