Novel Co-MOF-doped gelatin/agar intelligent film for beef freshness visual tracking based on the structural change of ZIF-67 under ammonia etching effect.

It is an important task to construct intelligent packaging for meat freshness monitoring with good color stability and indication function. Herein, cobalt-based metal-organic framework nanomaterials (Co-MOF, ZIF-67) with antimicrobial and ammonia-sensitive properties were successfully synthesized and added into gelatin/agar (GA) matrix to develop highly stable intelligent films (GA/ZIF67). The incorporation of ZIF-67 nanoparticles enhanced the hydrophobicity (water contact angle >90°) and UV-blocking properties (close to 0.3 % at T280) of the films and endowed the films with excellent antimicrobial activity. The GA/ZIF67 films exhibited outstanding ammonia-sensitive functionality and color stability. More crucially, the ammonia-sensitive color-changing mechanism of ZIF-67 may be attributed to the slow etching effect of its crystal facets upon exposure to ammonia. With the gradual spoilage of beef, the color of the GA/ZIF67 film changes from blue-violet to gray-yellow/orange-red. In addition, the RGB value of the film collected by a mobile device App was used to quickly determine the freshness of beef, reducing individual variability in color perception. Overall, this work offers novel insights into exploring Co-MOF as a new type of indicator for developing a rapid and real-time beef freshness monitoring system, which has great potential for application in the intelligent packaging field.