Improvement of mechanical and bioactive properties of chitosan films plasticized with novel thymol-based deep eutectic solvents

The limited mechanical and barrier properties of chitosan (CS) have restricted its widespread adoption in food packaging. To overcome this challenge, a straightforward approach was presented to prepare multifunctional bioactive CS packaging films using novel deep eutectic solvents (DES). DESs were initially prepared from menthol (ME), octanoic acid (OA), and thymol (THY). The successful of preparation of three DESs (OA:THY, ME:THY and OA:ME:THY) was confirmed by FTIR and 1 H NMR analysis. Then, the effects of the composition and addition level (0.5, 1.0 and 1.5 %) of DESs on structures, functional properties of CS films were studied. Rheological analysis, FTIR, XRD, and SEM results demonstrated excellent biocompatibility and effective intermolecular interactions between DESs and CS matrix. The thermal stability of CS-DES composite films decreased with the increase of DES. The incorporation of DES significantly enhanced the mechanical strength and hydrophobicity of films. The CS-ME:OA:THY-0.5 composite films exhibited the highest tensile strength (69.34 MPa), while the CS-ME:OA:THY-1.5 composite films showed the highest hydrophobicity (water contact angle = 107.95°). With the increase of DESs, the transmittance of films decreased especially for CS-ME:OA:THY-1.5 composite films with strong ultraviolent barrier property. Additionally, CS-DES composite films showed excellent antibacterial activity (96% of inhibition rate over 24 h), antioxidant activity, and bio-safety. Finally, the CS-ME:OA:THY composite films could effectively extend shelf life of grape by 3 days. The tested DESs represent promising plasticizers for improving the defects of CS-based films. This work paves the way for the practical application of CS-DES composite films in food related fields.