Abstract
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 (ΔE(00)) calculated based on recorded images indicated strong chromatic changes in the presence of spoilage-related volatiles. Under refrigeration, ΔE(00) remained below five, suggesting negligible color shifts. At room temperature, ΔE(00) exceeded 20 after 48 h, confirming significant anthocyanin transformation linked to increased alkalinity (pH 7.2-7.5). A positive correlation between ΔE(00) 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.