Heat-sealable chitosan- DES composite film: performance optimization, multi-mechanism synergistic effect, and extension of bread shelf life

Chitosan (CS), an excellent antibacterial biomacromolecule, cannot be directly heat-sealed, which severely limits its application as packaging material. In this study, heat sealable thermoformed CS films were prepared and their feasibility as independent packaging bags was explored, with deep eutectic solvents (DES) composed of citric acid (CA) and choline chloride (ChCl) as plasticizers. Orthogonal analysis was used to evaluate the effects of temperature, pressure, time and the mass ratio of CS to DES on the properties of thermoplastic CS films. Further, the CS-DES films were heat-sealed directly into pouches and used to preserve white bread. The results showed that the mass ratio of CS to DES has the most significant effect on the heat-sealing performance. When the mass ratio reached 6/4, the best heat-sealing strength was 3.77 N/15 mm. Multiple interactions occurred between DES and CS, including hydrogen bonding between choline chloride, CA and chitosan, as well as esterification and amidation reactions between CA and chitosan. The esterification and amidation reactions between CA and CS in the excessive DES may be the reason for the heat-sealing ability of CS-DES films. The freshness preservation results for white bread slices indicated that the CS-DES-7 film significantly inhibits microbial growth during storage, maintaining microbial counts within acceptable limits for 37 days. However, the film's moisture barrier properties require further enhancement to achieve superior freshness retention. Although chitosan has attracted widespread research interest due to its excellent properties, it cannot be heat sealed or made into bags, which limits its practical application in food. This study overcomes the limitation of chitosan's inability to heat seal, building a bridge from theoretical research to practical use of chitosan as packaging material.