# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Sensors
#### 4.1.1 Optical Sensors
These devices measure the reflectance of the light from a surface as a series of narrow and contiguous wavelengths bands. The fact that the pixel is shown as a spectrum gives much more information about the leaves and their freshness. The schematic of such a system is very similar to that used in fluorescence imaging, shown in Figure 5b. The broad-spectrum light source, with typical spectral range of 380 nm to 1012 nm, will cause elastic scattering in the leaves. Then, the reflectance is captured by the spectrometer in the form of simultaneous images in a high number of bands [75]. Each resulting pixel from the contiguous reflectance spectrum will give three-dimensional hyperspectral cubic data, with two of them in the spatial domain and one in the spectral domain for further processing. For leafy greens monitoring, the use of reflectance at 671 nm was found better in differentiating between the white parts of leafy greens as in lettuce from the white background [75]. The main difference between spectral imaging and fluorescence imaging is that measurements with fluorometers rely on the visible spectrum fluorescence re-emission of photons. While in hyper-spectral imaging, the reflectance is measured from the whole electromagnetic spectrum, providing more accurate results with higher resolution and a wider wavelength range to evaluate surfaces of interest [75; 76]. Decay sensors were developed to evaluate the reflectance and fluorescence spectra of leafy greens through Chl fluorescence imaging and hyperspectral imaging. In [58], both techniques were used to detect freshness of plastic bagged lettuce. One of the lettuce decay indices was developed based on the use of modified hyperspectral indices developed in [77]. The modified relation of reflectance was chosen based on the reflectance difference spectrum between fresh and decayed tissues. They provided a logarithmic ratio between the reflectance at different wavelengths as a performance index called LEDI4. The other lettuce decay index was used to measure Chl fluorescence known as LEDICF, which is defined as the ratio between the minimum Chl fluorescence to the maximum fluorescence. The minimum level of fluorescence is measured usually after switching the measuring light, because the exciting beam is not sufficient to induce electron transport [78]. The maximum fluorescence is then measured after applying a saturated light pulse [78]. The best performance was observed with the use of hyper-spectral imaging in terms of sensitivity, specificity, and early decay detection for different color groups of lettuce. In [58], for lettuce, the white color image indicated fresh tissue, red for decayed tissue, and blue was for the background. However, the accuracy of such a sensor was low when it came to white stacks of lettuce and was misidentified as decayed, giving a false negative output of the sample. Thus, it was concluded that the hyperspectral technique for freshness detection is better suited to evaluate green leaves such as baby spinach or butterhead lettuce. The other imaging technique that was tested in [58], used a fluorescence index which gave a better performance and higher sensitivity in identifying the white parts, but it misclassified as decayed the red fresh tissues, found in red oak lettuce. Here, the white color indicated decayed tissue, and the blue color, fresh tissue. Evaluations of these parameters (LEDI4 and LEDICF) showed positive correlations between each other and the visual rating scale, as illustrated in Figure 6. The results of this experiment were successful in classifying fresh from decayed tissue of plastic bagged lettuce packaged with modified atmosphere (MAP). It is harder to generalize whether this technique can work for all packaging types, but it works on different types of leafy green vegetables. Furthermore, this approach needs adjustment with the cut-off values for the indices if more specific classification is needed such as: fresh, intermediate, or decayed.