# Intelligent Packaging for Real-Time Monitoring of Food-Quality: Current and Future Developments
## 3 Methods for Real-Time Food Monitoring
### Specific Chemicals and pH Changes  
During food spoilage, several chemical modifications may occur in the packaging environment related to specific microorganism growth, especially for meat and fish products [55]. Thereby, as described below, different types of colorimetric assays exploiting chemical indicators have been vastly used to monitor food quality owing to their accuracy, simplicity, low cost, rapidity, and reliability [99; 100; 101; 102; 103]. As an example, polyaniline-based films embedded within the packaging lead to visible color changes in the presence of biogenic amines [104]. This sensor exploits the unique capability of polyaniline to change conductivity and color as a consequence of pH variation. The colorimetric assay has also been demonstrated by exploiting 16 different chemosensitive compounds that are able to detect several chemical species including trimethylamine, dimethylamine, cadaverine, and putrescine [105]. Remarkably, since food spoilage is diversified depending on both the sample type and world location, such a sensor acts as a universal system displaying great versatility in detecting a wide range of compound varieties [106; 107; 108]. Alternatively, electrochemical sensors, which are based on the redox reaction of a certain species on an electrode surface generating an electrical signal, have been broadly explored for the detection of volatile amine compounds during food deterioration, especially in the case of animal-derived products (i.e., meat and fish) [48; 109; 110; 111; 112]. In this regard, a wireless sensor was recently fabricated by using a hydrogel-coated pH-electrode sensor sensible to the environmental amount of volatile amine with a detection limit of 0.001 mg/L [113]. These sensors have the main advantage of providing a rapid and highly sensitive response to food spoilage, while also providing information as regards the concentration of the analyte species. On the other hand, due to their greater sophistication with respect to colorimetric sensors, it is noteworthy that electrochemical sensors are much harder to implement in packaging systems. Additionally, several meat spoilage sensors have been developed by means of pH-responsive materials directly incorporated within the packages [114; 115]. Again, the simple optical determination of vapor diffusion in thin polymeric films has been demonstrated as being a useful tool for food monitoring applications [23], as shown in Figure 2. The detection system exploits the optical interference of a commercial cling-wrap film (Figure 2a, red line). Upon exposure to different vapor phase analytes, the wrap increases its thickness, and its interference pattern is modified (Figure 2a, black line). Recording the dynamics of these variations (Figure 2b), it is possible to assess the diffusion coefficient of the diffusing species (Figure 2c, top panel) and relate the spectral shift of the interference pattern to the van der Waals volume of the analyte (Figure 2c, bottom panel). Yet, photonic crystals have also been shown to be able to act as indicators of food spoilage, owing to their capability to provide an easily discernible optical response to nitrogen-derived compounds [116; 117; 118].