The highly stable indicator film incorporating roselle anthocyanin co-pigmented with oxalic acid: Preparation, characterization and freshness monitoring application

A novel stable PVA/HPMC/roselle anthocyanin (RAE) indicator film co-pigmented with oxalic acid (OA) was prepared, its properties, application effects and stability enhancement mechanism were investigated correspondingly. The structural characterization revealed that more stable network was formed due to the co-pigmentation facilitated generation of molecular interactions. Meanwhile, the co-pigmentation improved film mechanical and hydrophobic properties compared to both PVA/HPMC/RAE newly prepared (PHRN) or stored (PHRS) film, expressing as higher tensile strength values (12.25% and 14.44% higher than PHRN and PHRS), lower water solubility (7.22% and 10.09% lower than PHRN and PHRS) and water vapor permeability values (33.20% and 21.05% lower than PHRN and PHRS) of PVA/HPMC/RAE/OA newly prepared (PHON) or stored (PHOS) film. Compared with the PHRS film, the PHOS film still presented more distinguishable color variations when being applied to monitor shrimp freshness, owing to the stabilization behaviors of co-pigmentation in anthocyanin conformation. Hence, the co-pigmentation was an effective strategy to enhance film stability, physical and pH-responsive properties after long term storage, leading to better film monitoring effects when applied in real-time freshness monitoring.