Composites of Thermoplastic Starch and Lignin-Rich Agricultural Waste for the Packaging of Fatty Foods

Abstract

Walnut shell (WS) has been demonstrated to be a promising and sustainable reinforcement in polymer composites. However, the evaluation of the overall properties provided by this lignin-rich food residue to biopolymer composites, for their use in the food packaging market, has not been evaluated yet. In this work, composite films of thermoplastic starch (TPS) containing either WS or hydrolyzed WS (HWS) and plasticized with a polyglycerol were prepared by spray drying, followed by extrusion and compression molding. TPS composites with HWS exhibited excellent UV light-blocking properties and sustained antioxidant activity for 2 weeks, ideal for preserving fatty foods. Furthermore, the compatibility of WS with TPS has been improved after the hydrolysis of WS with acetic acid. This treatment reduced the dimensions of the WS particles and exposed their hydroxyl groups, improving their compatibility with TPS, and consequently enhancing the mechanical and water vapor barrier properties. Overall, the composites containing 5 wt % of HWS exhibited the best compromise between the mechanical properties and antioxidant activity. Moreover, these composites complied with the safety requirements of the EU legislation regarding the migration of substances from the packaging to the food, in order to be used as food contact materials.