Colorimetric hydrogel indicators for real-time food spoilage monitoring: From fundamental principles, via monitoring mechanism to emerging applications.

Global food spoilage necessitates real-time monitoring. Colorimetric hydrogel indicators offer cost-effective, visually intuitive smart packaging solutions. This review surveys advances in field over 2016-2026, beginning with basic principles of hydrogel matrices, ranging from simple single-network to mechanically enhanced double-network systems. It then explores various monitoring mechanisms, explaining how colorimetric agents, such as halochromic dyes, plasmonic nanoparticles, enzyme-based systems, redox-active metal ions, and biomarker-specific system, translate chemical changes into visible color signals. We evaluate incorporation strategies, contrasting physical doping with covalent grafting. Applications are illustrated across diverse food groups, including detecting TVB-N in meat and seafood, pH shifts in dairy, and volatile organic compounds in fruits and vegetables. Furthermore, critical challenges regarding sensitivity, response-time, stability, and cost are addressed. Finally, the review highlights emerging trends such as multi-analyte sensors, advanced nanomaterials, and Internet of Things integration. These innovations are poised to transform food safety monitoring and global supply chain management.