Challenges in lignin integration within biopolymer matrices: Toward stable and effective lignin nanoparticles as additives for sustainable food packaging

Lignin nanoparticles (LNPs), synthesized from Kraft lignin (KL) using a green, cost-effective and scalable hybrid process, were incorporated into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) to develop sustainable food packaging materials. Electrospun fibers with varying LNPs content were annealed into continuous films (PHBV/LNPs). The morphological, thermal, optical, antioxidant, antimicrobial, barrier, and mechanical properties of the films were evaluated and compared with those of PHBV/KL films. Results showed that PHBV/LNPs films exhibited superior properties compared to PHBV/KL ones. Indeed, LNPs ensured better dispersion within the polymer matrix than neat KL, resulting in enhanced thermal stability, UV-Vis blocking properties and reliable antioxidant performance (% DPPH inhibition of about 90 % even after 20 days of film production). PHBV/LNPs films reduced bacterial counts against S. aureus by approximately 10 times (R∼1) compared to PHBV/KL ones. Moreover, the addition of LNPs did not alter the mechanical properties of pure PHBV, maintaining its suitability for food packaging, while not significantly affecting water barrier properties, particularly for oxygen permeability a reduction up to 10 times lower than neat PHBV was observed, thus outperforming conventional petroleum-based plastics. Therefore, the uniform size distribution of the LNPs facilitated their dispersion within the polymer matrix without the need for additional chemical functionalisation, ultimately resulting in a stable and effective material suitable for active food packaging applications.