Preparation of pH-colorimetric films based on soy protein isolate/ZnO nanoparticles and grape-skin red for monitoring pork freshness

The ability to monitor food quality without opening packages has received growing attention, but traditional methods for real-time monitoring may not accurately reflect the freshness of food. Therefore, in this study, pH-colorimetric films based on soy protein isolate (SPI)/grape-skin red (GSR)/ZnO nanoparticles (NPs) were developed for real-time monitoring of pork quality during storage. The mechanical properties, water resistance, antioxidant activity, temperature/light stability, and pH sensitivity of the prepared films were determined. The values for tensile strength, elongation at break, water vapor permeability, water absorption, water solubility, and antioxidant activity were 2.6–4.7 MPa, 87.9%–118.8%, 3.4–4.6 × 10−12 g cm/cm2·s·Pa, 8.9%–20.0%, 22.2%–27.2%, and 32.2%–65.1%, respectively. Furthermore, Scanning Electron Microscopy analysis showed that GSR could promote ZnO NPs fusion within the SPI to obtain homogeneous and smooth films while Differential Scanning Calorimetry, X-ray diffraction and Fourier-Transform Infrared Spectroscopy analyses revealed that intra- and intermolecular interactions between the different film components enhanced crystallinity and thermal stability, with hydrogen bonds and electrostatic interactions being the main forces involved. It was also found that the pH-colorimetric films had excellent pH sensitivity and as such, they could effectively monitor food freshness in real-time, at cool temperatures, under light-proof conditions and using only the bare eye. These findings indicated that pH-colorimetric films can be a viable option for evaluating pork freshness.