## 3 Results and Discussion
### Optical Properties of Gala Apples that had Irreversible Damage during Storage in Refrigerator  
Consumers generally expect that apples remain fresh for a long time when kept under atmosphere-controlled storage conditions, such as in the refrigerator [24]. It is difficult to detect bruises in apples in the early stage because the skin often obscures the appearance of the underlying damage. In our study, all six apples looked intact when we purchased them on day 1; however, one of the apples had visible damage on day 7. We kept the injured apple, and analyzed the optical properties of the normal and injured apples separately. We collected five frequency-domain reflectance at 5 mm SDS from a certain spot of each apple. For the injured apple, we concentrated on the spot of visible damage. Figures 7 and 8 show the optical properties of the normal and injured apples on different days at wavelengths of 660 and 980 nm, respectively. The average \(\mu_{a}\) and \(\mu_{s}\)' of normal apples and injured apple are listed in Tables 1 and 2. We observed that the \(\mu_{a}\) and \(\mu_{s}\)' of the injured apple significantly differed from those of the normal apples. The \(\mu_{a}\) of the normal apples at a wavelength of 660 nm decreased with time due to chlorophyll breakdown, but the \(\mu_{a}\) of the injured apple at 660 nm increased. This is caused by the extent of browning and the phenolic composition of the apples. The composition of the apple was modified after injury, and the polyphenols in the apples were oxidized to their corresponding quinones by polyphenol oxidase; these quinones are then polymerized with other quinones and amines to form brown pigments [25; 26]. Thus, the increase in the \(\mu_{a}\) of the injured  
Figure 6: (**a**) Absorption and (**b**) reduced scattering coefficients of an apple recovered at a wavelength of 980 nm at source–detection separation (SDS) of 1 mm (black squares) and 5 mm (red circles) 1, 7, and 14 days after purchase. apple, as shown in Figure 7a, indicates the accumulation of brown pigments after injury. The trend of decrease in the \(\mu_{a}\) of the normal apples at 660 nm is 12% when they were refrigerated, compared to 24% when stored in an air-conditioned room; the \(\mu_{s}\)' did not vary significantly (\(p>0.05\)) in 14 days. Thus, we speculate that the storage of fruits in suitably cold temperature sustains the freshness longer. This finding agrees with the phenomenon that low temperature storage has the benefit of protecting nonappearance qualities: texture, nutrition, aroma, and flavor [27]. In Figure 7, a slight difference was observed in the optical properties at 660 nm between the injured and normal apples on day 1. However, the difference was not visually perceptible on day 1. The optical properties of the injured and normal apples have significant variation trends. These findings suggest that our frequency-domain DRS system has the potential to detect injured apples in the early stage. Figure 8a shows that the \(\mu_{a}\) of normal apples at 980 nm decreased slightly, but that of the injured apple increased rapidly. We suspect that the vacuolation and cytoplasmic collapse of the damaged cells caused the cell wall degradation, and caused the cytoplasm to spread out, thereby causing the increase in the \(\mu_{a}\) of the injured apple at 980 nm. Figures 7b and 8b show that the \(\mu_{s}\)' of the normal apples were almost invariant; however, they were significantly decreased in the injured apple in the first 7 days. We suspect that the vacuolation and cytoplasmic collapse of the damaged cells caused the \(\mu_{s}\)' to decrease rapidly at 660 and 980 nm. From day 7 to day 14, the \(\mu_{s}\)' of the injured apple increased. It is assumed that the extensive freezing damages caused by the change in cell structure led to the increase in the \(\mu_{s}\)' [28].