## 3 Results and Discussion  
### SK-NIRS Spectra and Correlated Chemical Concentrations  
Figure 1 described the qualitative analysis results of four main fatty acids compositions in seed oils as the maturation stages increased. It also showed the crude protein percent concentration of seed at five different maturation stages. The measured concentrations of crude protein had a slight increase between the MS2 (32.36%) and MS3 (34.57%) phase, and after that stayed relatively steady around 33.9%. It showed that two saturated fatty acids (palmitic acid and stearic acid) concentrations remained nearly constant as the maturation stage of the cucumber seeds progressed. However, the concentrations of two unsaturated fatty acids showed obvious fluctuations as the maturation stage increased. Specifically, the maximum concentration of oleic acid occurred in MS1; it then decreased steeply in MS2, remained steady in MS3 and MS4, and finally increased greatly in MS5. In contrast, the concentration of linoleic acid, which is the dominant constituent of oil in cucumber seeds, was completely opposite to that of oleic acid. This strong negative relationship between oleic acid and linoleic acid in cucumber seeds agreed with that in the study of [PERSON] et al. [34]. In terms of the average spectra in Figure 2b, the most distinctive maturational stages were found in MS1 and MS5. Considering that cucumber seeds are very rich in oil (nearly 40%), the obvious differences in the concentrations of oleic and linoleic acids  
may account for the differences in absorption of incident light in the near-infrared region, especially in MS1 and MS5. As shown in Figure 2, the main absorption peaks of cucumber seeds were observed at approximately 1210 nm, 1471 nm, 1726 nm, and 1929 nm in the average SK-NIRS spectra of five maturational stages. According to the interpretations of [PERSON] and [PERSON] [35], the overlapping peak with a maximum of approximately 1210 nm results from the second overtone arising from the C-H stretching mode; the absorption band at 1471 nm is characteristic of the first overtone N-H stretching; the absorption peak around 1726 nm is due to the first overtone of C-H stretching; the absorption band centered near 1929 nm results from a combination of O-H stretching and H-O-H bending. These absorption properties of cucumber seeds may be explained by the high protein and oil content of the seeds [17] and the differences in the specific composition of the seeds as the maturation stage of the seeds increases. In addition, there was an apparent negative correlation between the height of the absorption peaks and the maturation stage of the seeds. It would make sense based on the fact that the cucumber seeds get thicker and thicker as their maturation stage increases. Thus, there is more incident NIR light reflected back to the detector, and the NIR absorption intensity decreases as the maturation stages increase. As far as we know, little information is available about how the maturation stage affects the absorption of incident light in the near-infrared region. The NIR spectra contain information from all the chemical constituents of the sample, and direct interpretation of the absorbance values obtained from complex mixtures such as those present in intact kernels is difficult. Figure 1: Concentrations of four fatty acid compositions in cucumber seed oil at five maturation stages. The raw spectral features can be enhanced using the second Savitzky-Golay derivative (second polynomial with a filter width of 15), which has a trough corresponding to each peak in the raw spectra. The second-derivative SK-NIRS spectra of all regions and one special interval are plotted in Figure 3.  
As the peak value at a wavelength in the raw spectra goes up much higher, the trough value at the corresponding wavelength in the second derivative spectra moves downward much deeper. Particularly, the position of the minima correlates with the position of the peaks in the raw spectra.