# Using Fluorescence Spectroscopy to Detect Rot in Fruit and Vegetable Crops
## 3 Results and Discussion  
### Experiments with a 405 nm Continuous Laser  
Figure 5 shows the spectrum of fluorescence of the surface (left) and pulp (right) of a healthy and rotten apple. Two peaks can be identified on the spectra: in the green (520-530 nm) and in the red (680 nm) regions. Peaks in the blue-green fluorescence region (440-540 nm) are mainly associated with cinnamic and coumarin acids, some alkaloids and flavonoids [19]. The latter are usually located in the cell wall [19] and perform mainly antioxidant functions and protect against fungal and viral diseases [20]. The peak in the red area is well described in many studies; it is the peak of chlorophyll. Previously, for green leaves, it was shown that the ratio of the intensities of the peaks 680/740 depends on the concentration of chlorophyll [21]. The existence of a peak at 680 nm and the absence of a peak at an excitation of 405 nm at 740 nm confirms the presence of a small amount of chlorophyll in apple samples. Figure 4: Apples and potatoes used in experiments with pulsed fluorescence spectroscopy. Healthy (**top**) and rotten (**bottom**) samples. In Figure 5, the spectral differences between healthy and rott-affected fruits are visible. In samples affected by rotting, the peak of the maximum of chlorophyll is shifted from 680 nm to 675.5 nm, indicating a decrease in chlorophyll concentration. The second difference in the surface is the \"plateau\" between peaks in green (520 nm) and red (680 nm) regions. Healthy samples do not show a significant decrease in intensity between peaks. This plateau in the yellow-orange region (550-650 nm) is most likely due to the presence of riboflavin, carotenoids, and flavonoids [19]. The differences are even more pronounced for the apple pulp (Figure 5b). The fluorescence amplitude of the apple pulp affected by rot is reliably much smaller than the healthy specimens in the whole investigated region. Healthy and rotten potato surface is less different in fluorescence spectra in contrast to apple. The peak of chlorophyll (680 nm) for healthy potatoes is less pronounced than for the rotten samples, and the peaks in the green region (530 nm) are the same (Figure 6). However, there are reliable differences in the pulp. The spectrum of the rotten sample goes beyond the sensitivity of the spectrometer in the green area (Figure 6b). This can be explained by the assumption that rotten potato pulp absorbs laser radiation much more efficiently, which leads to increased fluorescence. The efficiency of fluorescence quenching is rather high for the healthy plant, so any damage of the plant material should result in a change of its chemical composition; thus, this will decrease the quenching efficiency and hence the induced fluorescence intensity should be increased. Additionally, bacteria and fungi in rotten potato had high fluorescence, so with the increase of such organisms' concentration, the fluorescence will be higher. An increase in fluorescence occurs when the thickness of healthy potato pulp changes (Figure 6b, top right). The amplitude of the fluorescence spectrum increases 2.5-3 times with an increase in the slice thickness from 0.5 to 2 mm. Intensity of fluorescence is not linearly proportional to the sample thickness. Figure 7 shows the kinetics of the decrease in fluorescence over time. The observed effect is similar, with the well-known Kautsky effect observed for the chlorophyll peak in green leaves [22]. However, there are differences; the Kautsky effect occurs in leaves previously placed in the dark, and times of growth of the fluorescence are \(\sim\)300-500 ms, and decay times are \(\sim\)1-10 s, although in some cases the fluorescence decay takes \(\sim\)1-10 min [21,23].