**Li Fang 1, Kangli Wei 1, Li Feng 1, Kang Tu 1, Jing Peng 1, Jiahong Wang <sup>2</sup> and Leiqing Pan 1,\***


Received: 10 November 2020; Accepted: 15 December 2020; Published: 17 December 2020

**Abstract:** Soluble solid content (SSC) is regarded as the most significant internal quality associated with the taste and maturity in fruits. Evaluating the relationship between the optical properties and soluble sugars facilitates exploration of the mechanism of optical techniques in SSC assessment. In this research, absorption coefficient (μ*a*) and reduced scattering coefficient (μ- *<sup>s</sup>*) of Fuji apple during storage were obtained using automatic integrating sphere (AIS) at 905–1650 nm. Relationships between μ*a*, μ- *<sup>s</sup>* and SSC, and soluble sugars contents, were investigated. The result showed that SSC, the content of total soluble sugars (TSS), fructose, glucose and sucrose were all decreasing after storage, and the same trend appeared in the change of μ*<sup>a</sup>* and μ- *<sup>s</sup>*. In the whole wavelength range, both μ*<sup>a</sup>* and μ- *<sup>s</sup>* were positively related to SSC and soluble sugars contents. The correlations between μ*<sup>a</sup>* and SSC, and soluble sugars contents, showed increasing tendencies with increasing wavelengths, while for μ- *<sup>s</sup>*, correlations had the opposite trend. The strongest correlations between μ*<sup>a</sup>* and SSC, and soluble sugars contents, were observed in the correlation of μ*<sup>a</sup>* with sucrose, with an *r* of 0.934. Furthermore, a partial least square (PLS) model for sucrose based on μ*<sup>a</sup>* was built with the coefficient of determination of prediction (*Rp* 2) and the root mean square error of prediction (RMSEP) of 0.851 and 1.047, respectively. The overall results demonstrate that optical properties at the range of 905–1650 nm could be used to evaluate SSC of apples and this may due to the strong correlation between sucrose content and μ*a*.

**Keywords:** apple flesh; absorption; scattering; soluble sugars; 905–1650 nm
