Lignin-reinforced film of curcumin-conjugated chitosan as photodynamically active antibacterial food packaging.

While curcumin-containing chitosan films have been widely investigated as active food packaging, their application requires formulating curcumin into nanoparticles/inclusion-complex to overcome its poor aqueous solubility and stability. To address this limitation, the present work investigated the feasibility of using curcumin-conjugated chitosan as film former. The curcumin-chitosan conjugate was synthesized by a simple and safe one-step free-radical grafting method. Lignin was added to enhance the mechanical, optical, and water-barrier properties of curcumin-chitosan conjugate films. The effects of lignin content (0-20 wt%) on the film's surface morphology, UV resistance, mechanical properties, water sensitivity, and thermal stability were investigated. Subsequently, the photodynamic antibacterial activity of the lignin-reinforced curcumin-chitosan conjugate films was evaluated against Staphylococcus aureus and Escherichia coli. Its potential application as food packaging was evaluated using fresh shrimps as the model food. The results showed that the curcumin-conjugated chitosan film shared similar characteristics as chitosan-only films, indicating the minimal impact of curcumin conjugation on chitosan's film formation. The addition of lignin at 8 wt% significantly enhanced the film's UV resistance (<2% transmittance), thermal stability, tensile strength (≈40 MPa), while lowered its water solubility and water vapor permeability by ≈12% and 23%, respectively, attributed to lignin-chitosan hydrogen bonding that resulted in denser crosslinked polymeric structures. The film exhibited high antibacterial activities against E. coli and S. aureus under light irradiation (<5% survival rates) due to the photosensitizing activity of curcumin. The film was superior to non-conjugated curcumin-chitosan film in suppressing bacterial contamination of shrimps during storage (≈2.2 log CFU/mL improvement in bacterial eradication).