# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Sensors
#### 4.1.1 Optical Sensors  
Optical sensors play an important role in determining the freshness of vegetables, due to their capability of sensing the optical variations resulting from changes in chlorophyll content. These sensors can also sense pathogens' growth using fluorescence or hyperspectral imaging to indicate deterioration and spoilage. Optical sensors provide accurate results and are highly sensitive. Optical sensor technologies are considered promising in detecting the deterioration of packaged green leaves such as bagged lettuce, spinach and arugula. Hyperspectral imaging relies on the intensity and spectral characteristics of light reflected from the leaves. On the other hand, chlorophyll (Chl) fluorescence imaging relies on the reflection as a result of exciting the chlorophyll using light [57]. However, the current cost of such systems is still a drawback for their commercial use. At present, chlorophyll fluorescence imaging use fluorometers to measure the amount of emitted light after excitations of Chl with light at certain wavelengths. Illuminating the leaves will give a photon of energy, \(\hbar v_{EX}\), to Chl pigments. This will lead to creation of excited unstable state (S\({}_{1}\)'), that has a very short half-life in the order of nanoseconds [64]. Therefore, through the relaxation process, light will be emitted as shown in Jablonski diagram in Figure 5a. The wavelength of the emitted light depends on the energy difference in the two states. Chl pigments are classified into Chl-a and Chl-b. Chl-a was found to be a good fluorescent material, hence fluorescence is measured based on the excitation of Chl-a. Chl-a has two absorption peaks, one at 430 nm (blue light) and the other at 665 nm (red light) with higher absorption at the blue light [65; 66]. For emission, Chl-a emits in the red-light region at 680 nm (higher) and 750 nm [65; 66]. This means that for better resolution, a fluorometer will excite at 430 nm and measure fluorescence at 680 nm. However, due to the decomposition of Chl-a at 430 nm into by-products such as pheophytin, 460 nm light is used for excitation of Chl-a [65; 66]. The connection between the fluorescence intensity and Chl is based on Beer-Lamberts law of absorption [65; 66]. Although the quantum efficiency of Chl is low and in the range of 2% to 10% of the absorbed light [67], it is still a very useful and powerful technique for plant physiologists in studying Chl fluorescence effect in vegetation [68; 69; 70]. There are many techniques used to make fluorometers, such as the ones based on the pulse amplitude modulation (PAM) [71], or on pump and probe signals [72; 73]. These techniques rely on the wide-field detection and have a limited spatial resolution in the orders of millimeter [74]. These methods depend on the transition of Chl for adaptation from dark to light environment. An example of such system is as shown in Figure 5b. Fluorescence imaging systems consists of light source for excitation of the Chl in the sample, hence the re-emission will be captured by the camera after which it  
Figure 5: (a) Jablonski diagram illustrating the excitation and emission process. (b) Schematic of general Chl fluorometer system. is processed in a computer. Light emitting diodes (LEDs) are widely used in fluorometers because of their ability to control the intensity of light and its duration. While for the detecting camera, normally the commercial version is enough for this purpose without packaging. However, for Chl fluorescence imaging of leafy greens inside plastic bags, the decayed tissue with no fluorescence will not be distinguished from the background which also does not fluoresce [58]. Therefore, cameras with higher sensitivity are required for such systems or the use of hyperspectral images can be an alternative for more accuracy [58; 59]. Hyperspectral imaging, which is also known as imaging spectrometer, obtains a spectrum for each pixel in the image, it provides the electromagnetic spectral information for the use in the identification of surfaces.