*3.2. Spectral Analysis*

#### 3.2.1. Absorption Coefficient

The results obtained from the mean μ*<sup>a</sup>* spectra of apple samples storage at 25 and 0 ◦C are set out in Figure 3a,b. The graphs show that there has been a similar spectral trend, peaking at 980, 1169 and 1485 nm, which corresponds to the vibration absorption of O-H, C-H and C-C [41]. In addition, the absorption at 1485 nm was significantly stronger than that at 980 and 1169 nm, with values (1.52–1.88 mm−1) being 10 times higher than that of the others. In addition, the μ*<sup>a</sup>* value peaked at 1485 nm when the apple was stored at 25 ◦C after 10 days. There was no significant difference between day 0 and day 20, and μ*<sup>a</sup>* decreased significantly on the 30th, 40th and 50th days. At 0 ◦C, μ*<sup>a</sup>* at 1485 nm showed a similar trend with that at 25 ◦C, with the largest value on the 30th day during storage.

The μ*<sup>a</sup>* values of apple flesh stored at 25 and 0 ◦C were within 0.04 and 1.88, and within 0.05 and 1.67 mm<sup>−</sup>1, respectively. Generally, the μ*<sup>a</sup>* values in this study were comparable to those reported in other research. In the study by Ma et al. [35], the μ*<sup>a</sup>* values of apple samples were 0.00–0.50 mm−1, measured by HIS at 1000–1800 nm. The research of Saeys et al. reported the μ*<sup>a</sup>* values of Royal Gala (0.00–2.20 mm<sup>−</sup>1) based on IS at 350–2200 nm [33]. An important issue that emerged from the data is the dramatic difference in absorption values of apple flesh measured by different techniques. The μ*<sup>a</sup>* obtained using IS is much higher than that with HIS. This may be due to the different sample tissue state, different varieties of apples, different wavelength ranges as well as different measured method in the absorption property [42].

**Figure 3.** Average optical properties of Fuji apple flesh for six storage times: absorption (μ*a*) and scattering (μ- *s*) at 25 ◦C (**a**,**c**); absorption (μ*a*) and scattering (μ- *s*) at 0 ◦C (**b**,**d**). d, day.
