Smart eco-friendly packaging for spoilage detection of fresh chicken, salmon, and shrimp products

At the consumer level, labels for in-situ monitoring by the naked eye are useful tools as indicators of food freshness. Smart labels are often pH-sensitive and reactive to volatile bases. Expanding their response range to various food spoilage VOCs can improve performance and applications. In this study, we aimed to develop an innovative, easy-to-assemble, and multi-reactive colorimetric label based on environmentally friendly and cost-effective ingredients. The label was produced by layer-by-layer deposition of PVA, pectin, and copper (II) gluconate within preformed commercial polystyrene trays for food packaging. The copper salt, commonly used in dietary supplements, serves as the sensing compound, while the pectin host matrix acts as a carrier, forming a hydrogel that allows interaction with volatile analytes. The label provides a colorimetric response in the presence of various N- and S-based VOCs produced during the spoilage of protein foods (salmon, shrimp, and chicken) at storage conditions of 4 °C and 25 °C. To investigate the mechanism, we collected data on the interaction of copper (II) gluconate with ammonia, trimethylamine, hydrogen sulfide, and thiols, and attributed the colorimetric response to various coordination, redox, and precipitation reactions. A representative model of the salt response was obtained through theoretical calculations. The labels used were analyzed by scanning electron microscopy and gas chromatography, attempting to correlate the N- and S-based VOC profile with the colorimetric response. Ease of production, eco-sustainability, and low-temperature efficacy combine with the convenience of monitoring food preservation via color management software on a common smartphone.