A nature pH indicator with high colorimetric response sensitivity for pork freshness monitoring

Smart packaging with colorimetric pH indicators is vital for food quality monitoring. However, synthetic pH-sensitive dyes pose safety hazards, and conventional methods to fabricate pH-sensitive films lack real-time detection. In this study, blueberry anthocyanin (Ant) and petunia dye (PD) were chosen among six natural dyes due to their enhanced pH responsiveness. They were utilized as chromogenic agents, while petunia dye composite film (PDCF) and anthocyanin composite film (AntCF) were synthesized using electrostatic spinning that incorporated natural dye, chitosan (CS), trehalose (TRE), and polyvinyl alcohol (PVA). Finally, a "sandwich" structure pH indicator was created by overlaying two layers of PET on the top and bottom of the composite film and perforating one side. The pH indicator based on PD composite film (PDCF-s) demonstrated superior effectiveness and stability compared to the Ant composite film (AntCF-s) in monitoring pork freshness. The indicators exhibited excellent sensitivity to low concentrations of ammonia vapor, completing the reaction within 2 min. As the pork spoiled, the color of PDCF-s changed visibly from pink to purple, with an ΔE value of 40. Moreover, the ΔE variation remained stable across different conditions. This study innovatively designed "sandwich" pH indicators, providing a promising natural safe dye for smart packaging, and promoting the use of pH-responsive indicators.