Colorimetric detection of milk spoilage at low temperatures: a novel PDA/ZnO@NC membrane for pH-based freshness monitoring

Food waste is a huge problem worldwide, and spoilage during storage is a major contributor. Food freshness monitoring systems can help reduce food waste by providing consumers with information to more accurately predict the shelf life of their food. In the case of milk, pH can be used as a freshness indicator. As milk spoils, microbial activity converts lactose to lactic acid, which produces hydrogen ions (H⁺) and consequently lowers the pH of the milk. In this paper, we propose a paper sensor that can detect the pH of milk in a refrigerated state (− 4 ℃) by color change. Polydiacetylene (PDA)/zinc oxide (ZnO) nanocomposite, which changes its color from blue to red as the pH decreases, was used as the color changing material. Specifically, the nanocomposite was applied to a nitrocellulose (NC) membrane, which is approved as a food packaging material for monitoring freshness during transport or on the shelf. The freshness monitoring function of the sensor was verified by applying artificially spoiled milk samples to the PDA/ZnO@NC membrane. In particular, it was demonstrated that the PDA/ZnO@NC membrane was shown to detect spoiled milk samples with pH 4.5 in 10 min at low temperature. Furthermore, the color change of the sensor upon detection of spoilage remained stable even after long-term storage. In conclusion, the PDA/ZnO@NC membrane presented in this study can be widely applied to the freshness monitoring of various food products stored at low temperatures whose spoilage is evaluated by pH.