Effects of Phenolic-Rich Extracts from <i>Castanea sativa</i> Mill. Wood Processing Byproducts on the Development of Compostable Polylactic Acid-Based Materials with Antioxidant and Antibacterial Properties

In a circular economy framework, valorizing agro-industrial waste into high-performance active materials is crucial for sustainable packaging and biomedical innovative applications. This study reported the design, development, and characterization of multifunctional polylactic acid (PLA)-derived films obtained by extrusion, incorporating PLA with an extract from <i>Castanea sativa</i> Mill. wood (CSW) by-products. The phenolic profile of the extract was identified through HPLC-DAD-MS analysis, highlighting the presence of gallic acid, ellagic acid, and hydrolyzable tannins as vescalagin and castalagin. TGA analysis evidenced thermal stability till 250 °C according to the extrusion process. Interestingly, CSW extract exhibited <i>in vitro</i> antimicrobial properties against Gram-positive relevant bacterial strains (MBC from 0.5% to 1.0% w/v). Finally, active films (PLA_1CSW, PLA_3CSW, and PLA_5CSW) were prepared by extruding PLA with CSW extract at 1, 3, and 5%, w/w, respectively. Mechanical characterization of these films showed that tensile strength and modulus remained comparable to neat PLA, while elongation at break decreased with increasing CSW content. Antioxidant assays showed dose-dependent DPPH radical scavenging activity, reaching a value of 96% for PLA_3CSW. PLA_5CSW revealed strong antimicrobial performance against <i>Staphylococcus aureus</i> and <i>Bacillus cereus</i>. For all formulations, overall migration into food simulant remained below EU limits (<10 mg/dm²) and, according to ISO 20200:2016, disintegration under controlled composting conditions reached >90% within 14 days. These findings revealed the potential of CSW extract in the production of sustainable, compostable, and bioactive PLA-based films, according to a sustainable transition following circular economy principles in the food and biomedical sectors.