## 3 Results and Discussion
### Optimal Wavelengths Selection for Chilling Injury Detection of Green Jujubes  
In order to accelerate the processing speed of hypercube images, and ultimately, realize on-line chilling injury detection, the important wavelengths that could retain most information of the original data with fewer feature number by using feature selection method needs to determine. Figure 4 presents the selection probability of each wavelength calculated by random frog at scanning speed of 8 mm/s and 20 mm/s, with different spectral resolutions, respectively. For the data with spectral resolution of 1.25 nm (Figure 4a), a similar pattern of selection probability of each wavelengths over the range of 430-1023 nm could be observed between the two scanning speeds, implying that the  
Figure 3: Mean spectra of normal (n = 120), slightly (n = 120) and severely (n = 120) chilling samples at different scanning speeds and spectral resolutions. (**a,b**) were 8 mm/s and 20 mm/s with spectral resolution of 1.25 nm, respectively. (**c,d**) were 8 mm/s and 20 mm/s with spectral resolution of 5.03 nm, respectively. contribution of each variable was not significantly influenced by the scanning speed during the chilling injury detection of green jujubes. The selection probabilities of wavelengths in the range of 430-700 nm and 931-1023 nm were relatively lower than those over 701-930 nm. It could also be found that the wavelengths over 701-930 nm from hyperspectral images with spectral resolution of 5.03 nm were also considered as important spectral regions for two speeds (Figure 4b). Figure S1 also shows the selection probability of each wavelength at the scanning speeds of 8 mm/s and 20 mm/s with spectral resolutions of 2.51 nm and 10.08 nm, respectively, which was similar to those of 1.25 nm and 5.03 nm. The selected wavelengths with higher selection probability over 701-930 nm, linking to light scattering information within the pulp tissue might be watery and soft tissues from cell structure breakdown, which might present potential for discriminating chilled jujubes from normal ones. To determine the optimal feature number for jujube chilling detection, the first ten wavelengths selected by random frog for different scanning speeds and spectral resolutions were used for further analysis (Table 1). Although the selection probability of each wavelength at two scanning speeds was slightly different, the first ten selected wavelengths were quite similar under the same spectral resolution. When compared with different spectral resolutions, the important wavelengths for data with spectral resolution of 5.03 nm distributed dispersedly than those of 1.25 nm on account of that some adjacent wavelengths (i.e., 723 nm, 724 nm, and 726 nm) were considered as important wavelengths for hyperspectral images with 1.25 nm spectral resolution. In addition, most of the selected wavelengths for data with spectral resolution of 5.03 nm were over 800 nm, while it presented a reverse pattern to that of 1.25 nm. Table S1 also presents the first ten selected wavelengths at 8 mm/s and 20 mm/s, with spectral resolutions of 2.51 nm and 10.08 nm, respectively. For 2.51 nm, most selected wavelengths were over 800 nm, which was similar to those of 5.03 nm, but opposite to those of 10.08 nm (Table S1). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Spectral Resolution (nm)** & **Speed (mm/s)** & **Wavelengths (nm)** \\ \hline
1.25 & 8 & 726, 724, 889, 840, 888, 764, 709, 723, 728, 841 \\  & 20 & 888, 708, 723, 726, 839, 727, 841, 709, 724, 743 \\ \hline
5.03 & 8 & 839, 875, 870, 865, 736, 844, 849, 890, 756, 880 \\  & 20 & 844, 880, 870, 849, 865, 736, 885, 875, 731, 751 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The first ten selected wavelengths selected by random frog at different scanning speeds and spectral resolutions for chilling injury identification of green jujube.