Modification of walnut shell lignin nanoparticles through deep eutectic solvent for application in active food packaging films

The depletion and polluting nature of fossil fuels, coupled with the threat to human health posed by microplastics generated from plastic packaging, have underscored the significance of renewable resources in addressing environmental degradation. Among these, biomass has emerged as a prominent contender. The complex molecular structure of lignocellulose and limitations of conventional pretreatments hinder its efficient utilization. To address this, a ternary deep eutectic solvent (DES) was developed to extract bioactive lignin nanoparticles (LNPs) from walnut shells. Employing a continuous processing platform, 56.84 % of the lignin was converted into LNPs via DES and anti-solvent precipitation. Ethylene glycol in the DES preserved lignin's side-chain functionality, yielding LNPs with a ζ-potential of -28.29 mV, a particle size of 203.48 nm, and superior antioxidant activity compared to LNPs prepared by traditional DES. Electrostatic and non-covalent interactions between LNPs and ε-polylysine (ε-PL) within a polyvinyl alcohol (PVA) matrix produced a composite film with a tensile strength of 33.32 MPa, 90 % UV-blocking efficiency, and 60 % microbial growth inhibition. When applied to fresh walnut packaging, the film's gradual release of LNPs suppressed mold proliferation via phenolic hydroxyl groups, reduced oxidative degradation of nutrients, and extended shelf life. This study introduces a scalable, circular approach to repurposing agricultural waste into biodegradable active packaging, aligning lignin valorization with sustainable food preservation strategies.