**4. Conclusions**

The optical properties of Fuji apple during storage in two different storage conditions (25 ◦C, 50 days; 0 ◦C, 150 days) were evaluated by the AIS system at 905–1600 nm, and the exact relationships of absorption and scattering properties with SSC and soluble sugars contents were explored. The changes of μ*a*, μ- *<sup>s</sup>*, SSC and soluble sugars contents showed a similar pattern at the two experimented temperatures during storage. Three prominent absorption peaks at 980, 1169 and 1485 nm were noticed for the spectra of μ*a*. In addition, μ*<sup>a</sup>* and μ- *<sup>s</sup>* were positively associated with SSC and soluble sugars contents (*r* = 0.659–0.936), and their values are greatly relied on the wavelength. Thus, it could be easily understood that the decreases in SSC and soluble sugar content were accompanied by declines in μ*<sup>a</sup>* and μ- *<sup>s</sup>*. Additionally, the μ*<sup>a</sup>* had stronger correlation with these soluble components than with μ- *s*. Another important conclusion was that both SSC and μ*<sup>a</sup>* were more strongly correlated with sucrose content, with *r* values of 0.933, 0.934 (at 25 ◦C) and 0.930, 0.927 (at 0 ◦C). Moreover, partial least square regression (PLS) models for SSC and soluble sugars contents were developed, with the best performance of *Rp* <sup>2</sup> = 0.851 and *RMSEP* = 1.047, which appeared in the model of sucrose based on μ*a*. Furthermore, due to better measuring bonds of C-H, O-H, C-C at the longer wavelength range, the prediction model at 905–1600 nm performed better than the models developed in the 400–1050 nm range in the report of Wei et al. [36]. Thus, these results made the mechanism for detecting SSC of apples based on optical techniques clear. The strong association between the absorption properties and the sucrose content provided potential and theoretical support in assessing SSC based on optical techniques.

However, this study was carried out on a relatively limited number of fruit and only on apples from one cultivar. Further work needs to involve the investigation of exact relationships between absorption and scattering properties and soluble sugars contents of other varieties of apple flesh, as well as the correlation of optical properties with other components which contribute to quality, such as to firmness.

**Author Contributions:** Conceptualization, K.T. and L.P.; methodology and data curation, K.W. and L.F. (Li Fang); formal analysis, J.P., L.F. (Li Feng) and J.W.; investigation, K.W.; writing—original draft preparation, L.F. (Li Fang); writing—review and editing, L.P. and K.T.; visualization, L.F. (Li Feng) and J.W.; supervision, K.T. and J.P.; project administration, K.T. and L.P.; funding acquisition, L.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Chinese National Foundation of Nature and Science (31671926), Fundamental Research Funds for the Central Universities (KYLH202003) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

**Conflicts of Interest:** The authors declare no conflict of interest.
