Gelatin Hydrogel pH Sensors Incorporating Anthocyanins for Intelligent Food Packaging: Towards Enhanced Food Spoilage...

Protein-based hydrogels composed of gelatin, whey and glycerol were functionalized with red cabbage extract (RCE) to develop natural colorimetric pH sensors for intelligent food packaging. Structural analysis by X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed amorphous, hierarchically organized networks where RCE molecules interact with protein chains. The resulting microstructure, consisting of compact surface domains and a porous internal network, may regulate the diffusion of volatile amines into the hydrogel matrix, enabling gradual and stable pH-dependent color transitions. The resulting biocomposite hydrogel exhibited a stable and time-resolved optical response to meat spoilage, correlating structural relaxation with colorimetric sensitivity. Color difference values (ΔE₀₀) calculated based on recorded images indicated strong chromatic changes in the presence of spoilage-related volatiles. Under refrigeration, ΔE₀₀ remained below five, suggesting negligible color shifts. At room temperature, ΔE₀₀ exceeded 20 after 48 h, confirming significant anthocyanin transformation linked to increased alkalinity (pH 7.2-7.5). A positive correlation between ΔE₀₀ and pH was observed, highlighting the hydrogel's high sensitivity to environmental changes. These findings confirm the potential of RCE-loaded hydrogels as eco-friendly, visual freshness indicators suitable for intelligent packaging applications. The hydrogel films demonstrated a distinct color transition within the pH range of 5.75-7.5, corresponding to the freshness variation interval of chicken meat.