Mechanical, thermal, and antioxidant properties of composite films prepared from durum wheat starch and lignin

Lignin conveys several important and useful characteristics to starch films depending on the lignin MW, extraction procedure, and lignin botanical source. In this study, lignin was obtained from durum wheat straw through the organosolv process, followed by fractionation according to solubility in alcohol. The alcohol-soluble lignin (ASL) was then used to prepare starch-ASL films. The mechanical properties, thermal stability, water solubility, color, and antioxidant activity of the resulting films were evaluated, and the starch-ASL films were also analyzed by means of SEM and attenuated total reflection infrared spectroscopy (ATR-IR). A procedure, to incorporate ASL, was developed and used to produce homogenous surfaces in starch-ASL films; agglomerations or undissolved particles of lignin were not observed in the starch matrix at microscopic level. ASL promoted significant decrease of the tensile strength (TS) and elastic modulus (EM), but an increase of elongation at break (EB) in films. Thermal analysis of the starch-ASL films showed high resistance to thermal degradation, due to the incorporation of ASL. In addition, the starch-ASL films exhibited antioxidant activity (2,2-diphenyl-1-picrylhydrazyl [DPPH]), which increased with increasing ASL content.