Visual Freshness Indicators for Fresh Produce Packaging: Response Mechanisms, Produce-Specific Applications, Current Challenges, and Future Perspectives

Fruits and vegetables remain metabolically active after harvest, so fixed-date labels inadequately represent quality changes during storage. Visual freshness indicators (VFIs) convert quality-related signals into visible color or pattern changes, enabling nondestructive, package-level monitoring. This review classifies VFIs by responsive signals – pH, CO2, ethylene, moisture, microbial metabolites, and temperature – and compares sensing mechanisms, indicator materials, and produce-specific applications. A central limitation is signal ambiguity: ethylene and respiratory CO2 changes in climacteric fruits may reflect normal ripening and senescence rather than microbial spoilage. Poorly calibrated VFIs can therefore trigger false spoilage warnings and premature rejection of marketable produce, or provide false reassurance when deterioration is missed. Additional barriers include indicator-component migration and food-contact compliance, environmental interference in visual recognition, insufficient cross-variety and real-world validation, and lack of standardized performance evaluation. We identify priorities for distinguishing ripening-stage assessment from spoilage detection, evaluating migration and protective-barrier strategies, improving AI-assisted label-reading robustness, and establishing a standardized evaluation framework covering response performance, environmental stability, reproducibility, safety, and industrial feasibility. These measures are essential for translating laboratory-scale indicators into reliable decision-support tools for fresh produce supply chains.