Impact of oxidized sucrose as a bio-based crosslinker on thermoformable films made from lupin protein isolate (Lupinus angustifolius L.)

This study investigates the effect of oxidized sucrose (OS) on selected functional properties of cast films from lupin protein isolate (LPI). LPI with a protein content larger than 0.9 g g⁻¹ was obtained by alkaline extraction and isoelectric precipitation from bitter narrow-leaved lupins (Lupinus angustifolius L.). OS was synthesized by vicinal diol cleavage using sodium periodate and was successfully characterized by Fourier-transform infrared-, ¹H-nuclear magnetic resonance spectroscopy, and aldehyde titration. Films were produced from heated (85°C, 30 min), alkaline (pH 10), aqueous solutions of LPI (0.1 mL⁻¹), glycerol (300 mg g⁻¹ of LPI) and OS (25, 50, 75, and 100 mg g⁻¹ of LPI). The effect of covalent crosslinking between OS and protein chains was studied by investigating mechanical properties, moisture content, total soluble matter, water vapor permeability, and protein solubility for all films. LPI films produced with the addition of OS showed increased mechanical performance as the tensile strength was increased from 3.5 up to 9.3 MPa and elongation at break values could be raised from 118% to 176%. Taken together with the facts that these films are thermoformable and show improved wet strength compared with control films, make them promising materials for sustainable packaging and short-term applications in agriculture and forestry.