Antibacterial starch-cellulose-based films with pH-responsive colorimetric properties for real-time food freshness monitoring

Starch/cellulose-based materials represent one of the most promising classes of biodegradable biomass substrates. However, their susceptibility to microbial contamination and limited function limit their applications in modern intelligent food packaging. In this study, cinnamaldehyde (CA) was employed as an antimicrobial agent, while the food-safe synthetic dyes neutral red (NR) and bromothymol blue (BTB) served as pH-responsive indicators. Using a one-pot strategy, starch granules were simultaneously gelatinized and encapsulated CA, while co-blending with bacterial cellulose nanofibers (CNFs) to yield a uniform film-forming solution. Upon incorporation of NR and BTB into the film solution, the resulting CA/HAS-CNFs-(NR&BTB) film exhibited excellent mechanical performance (tensile strength: 10.20 MPa; elongation at break: 14.76 %) owing to hydrogen bond reorganization between CA/HAS inclusion complexes and CNFs. FTIR and CLSM analyses confirmed the successful encapsulation of CA and uniform dispersion within the matrix. The film demonstrated distinct and reversible color transitions in response to pH variation and effectively tracked shrimp freshness, with a visible change from orange to yellow-green over time during storage. The R, G and B values of the film can be directly obtained through the mobile phone app, and then the GCS value can be calculated to obtain the freshness status of the food. Biodegradation studies under simulated microplastic-contaminated environments revealed that the composite film had favorable environmental compatibility. This work provides a sustainable and intelligent packaging solution with significant potential for real-time food freshness monitoring.