## 3 Results  
### Spectral Characteristics of Prunes with Different Damage Levels  
As shown in Figure 3, the mean reflectance spectra of prunes under different cumulative damage levels exhibited distinctive patterns, particularly in the near-infrared region (700-1050 nm). Intact samples consistently showed the highest spectral intensity, especially beyond 850 nm. A progressive decline in reflectance was observed with increasing bruise severity, most notably around 818 nm and 980 nm. The absorption near 818 nm is primarily associated with the second overtone of O-H stretching in water molecules, reflecting changes in tissue hydration status and intracellular water mobility [39; 40]. Meanwhile, the 980 nm region corresponds to a strong combination band of O-H stretching and bending vibrations, also indicative of the water content and the structural integrity of parenchyma cells [41]. These variations suggest that cumulative mechanical stress may lead to progressive disruption of cell membranes and water redistribution, resulting in reduced absorption and scattering in these bands. Such observations are consistent with prior studies on fruit bruising and water-related spectral features in horticultural products [20; 42].  
These spectral changes are physiologically meaningful because water distribution and retention are closely related to tissue viability. In bruised fruit tissues, mechanical stress compromises cell membrane integrity, resulting in intracellular water leakage and intercellular redistribution. This leads to a measurable reduction in water-associated absorbance features at 818 nm and 980 nm. Additionally, the degradation of parenchyma cell walls due to repeated low-energy impacts may accelerate water loss and enzymatic activity, further altering the spectral responses [43]. Thus, the observed changes in these NIR bands provide a biochemical basis for differentiating bruise severity in prunes using spectroscopic techniques.  
In addition to the NIR trends described above, some atypical reflectance behaviors were observed in the visible region. Interestingly, severely bruised samples exhibited reflectance values lower than those of moderately bruised ones beyond 850 nm, which may be attributed to more extensive tissue degradation and dehydration. However, in the visible region (especially below 650 nm), severely damaged samples sometimes displayed slightly higher reflectance than moderately damaged or even intact ones. This could be due to reduced light absorption by and increased scattering from loosened or broken skin and flesh tissue, where cell walls have lost their buffering capacity [44]. Such spectral reversal near 650 nm has also been reported in other fruit bruise studies and is considered a potential indicator of tissue structure saturation.  
These spectral characteristics suggest that Vis-NIR reflectance signals in the 400-1050 nm range effectively capture bruise-related physiological changes in prunes. In particular, the 600-900 nm range demonstrated both a sufficient signal-to-noise ratio and strong biological relevance, providing a robust basis for subsequent feature selection and modeling  
Figure 3: Average reflectance spectra of intact and mildly, moderately, and severely damaged prunes in the 400–1050 nm range.  
strategies. The selected spectral range also aligns well with the optimal response range of the spectrometer used, ensuring data quality and reproducibility [20].