Enhanced colorimetric freshness indicator based on grape skin anthocyanin‑metal complex in zein/pullulan/PVA nanofiber membrane

Anthocyanins are widely explored for developing freshness indicators. However, their applications are often limited by low color sensitivity and inability to accurately identify the sub-fresh stage of foods. This study developed a novel smart packaging indicator using a Cu²⁺-complexed grape skin anthocyanin (GSA). The complexation significantly enhanced colorimetric accuracy within the critical pH 6–8 range, enabling distinct transitions from pink (pH = 6) to light purple (pH = 7) and deep purple (pH = 8). The indicator was fabricated by incorporating the Cu²⁺-GSA complex and tea polyphenol into an electrospun zein/pullulan-polyvinyl alcohol nanofiber membrane. When monitoring shrimp freshness at 4°C, the indicator's color shift from purple to green accurately corresponded to the sub-fresh (TVB-N > 16.74 mg/100 g) and spoiled (TVB-N > 30 mg/100 g) stages, with RGB values falling below 73 and 55, respectively. The integrated tea polyphenol extended the shelf life of the shrimp by approximately one day. The indicator also effectively monitored poultry and beef spoilage, with an RGB threshold < 85 reliably signaling spoilage (TVB-N > 15 mg/100 g). The platform establishes a strong correlation between visual color parameters and key freshness indices, offering a reliable, real-time solution for monitoring protein-rich food quality.