## 3 Results and Discussion  
### Fluorescence Characteristics of Stainless Steel and Fruits Samples  
Generally, 2B-finished stainless-steel has a smooth surface and slight reflection similar to a mirror, while #4-finished stainless steel has a fine-polishing grit line with uniform appearance and directionality on its brushed surface [34]. Figure 3 shows the fluorescence spectra of the two types of stainless steel. The 2B spectrum has a characteristic broad fluorescence peak in the 600-740 nm range, while the #4 spectrum is relatively flat at all wavelengths across the 450-730 nm range. Figure 3: Average spectra of the stainless-steel surfaces. Figure 2: Flowchart for the development of the optimal residual-organic detection algorithm. Figure 4 shows, for each fruit, the average spectra for each concentration of juice residue on the surfaces of 2B-finished SSS. All the samples show a broad fluorescence peak in the 600-740 nm region similar to that of the 2B-finished stainless steel spectrum in Figure 3.  
The honeybee residue spectra show a tendency for fluorescence peak intensities in the 450-600 nm range to decrease with increasing dilution (decreasing juice concentration); the most diluted samples (1:100) show nearly the same fluorescence properties as 2B stainless steel in this region. In the 600-730 nm range, the fluorescence peaks of stainless steel are the highest, and the 1:5, 1:20, 1:50, and 1:100 dilutions also exhibit fluorescence properties similar to those of the 2B stainless steel. Similarly, the orange residue spectra show a decrease in fluorescence peak intensity with the increasing dilutions for the 450-600 nm range. Again, in the 600-730 nm region, the peak values of 2B stainless steel are the highest, and there are almost no differences in the fluorescence peaks for all the dilution concentrations. The apple residue spectra also show a decrease in the fluorescence peak intensity with the increasing dilutions for the 450-600 nm range; the values for the 1:5 and 1:10 samples increase, whereas the remaining samples show fluorescence properties similar to those of stainless steel. In the 600-730 nm region, the fluorescence peak of stainless steel is the highest, and there are differences between the fluorescence peaks exhibited by the original juice and the 1:5 dilution concentration. However, there are almost no differences among the fluorescence peaks at the other dilution concentrations. The watermelon residue spectra also show a decrease in fluorescence peak intensities in the 450-600 nm range with increasing dilution; the values for the 1:5, 1:10, and 1:20 samples increase, whereas the remaining samples exhibit fluorescence properties similar to those of stainless-steel. In the 600-730 nm range, the peak value of stainless steel is the highest, and there are almost no differences among the fluorescence peaks at all the dilution concentrations. On the 2B finished surface, all the fruits used in the experiment show significant differences in the spectra for the different concentrations in the 450-600 nm range. In the 600-730 nm range, the peak value of stainless-steel is the highest for all four fruits, and it is difficult to distinguish based on the dilution concentration. The 450-600 nm and 600-730 nm ranges are known to be related to carotenoids and chlorophyll characteristics, respectively [35; 36; 37]. Figure 5 shows, for each fruit, the average spectra for each concentration of juice residue on the surfaces of #4-finished SSS. For all the fruits, the original juice exhibits the highest fluorescence intensities and the #4 surface has much less influence on the juice residue spectra in Figure 5 compared to the influence of the 2B surface in the juice residue spectra in Figure 4. In the 450-600 nm range, all the samples have fluorescence properties similar to those of #4-finished SSS.