3.4.1. Absorption Coefficient–SSC–Soluble Sugars Contents Relations

The *r* values between the absorption coefficient of apple flesh at 905–1600 nm and SSC, TSS content and soluble sugars contents at 25 and 0 ◦C are shown in Figure 6a,b, respectively. The μ*<sup>a</sup>* was positively associated with SSC, TSS content and soluble sugars contents in the whole range, exhibiting high *r* values of 0.873–0.960 at 25 ◦C and 0.863–0.963 at 0 ◦C. A previous study reported a similar result, specifically that SSC is positively correlated with μ*<sup>a</sup>* at 1198 nm [35]. At 905–1600 nm, the *r* profile between μ*<sup>a</sup>* and the SSC, TSS content and soluble sugars contents had similar patterns, with almost increasing tendencies with increasing wavelengths. Therefore, from Figure 6a,b, we could observe that SSC and soluble sugars presented stronger correlation in the medium-wave near infrared region than in the short-wave near infrared region. Compared to the report of Wei et al. [36], the average *r* values μ*<sup>a</sup>* and the SSC and TSS content and soluble sugars contents at 905–1050 nm were different. This may be due to different samples used in the two studies and different rates of accumulation or decomposition of soluble sugars. However, due to increasing response to the content information of chemical components, the correlation between absorption properties and soluble sugars increases with the increasing of the wavelength in the whole visible-medium wave near infrared region in general [45].

Overall, SSC (*r* = 0.933) and sucrose (*r* = 0.934) had the highest average correlation coefficients with μ*a*, followed by TSS (*r* = 0.928), and finally fructose (*r* = 0.915) and glucose (*r* = 0.899) at 25 ◦C. The phenomena may be connected to the strong correlation between SSC and sucrose content. At 0 ◦C, *r* values of μ*<sup>a</sup>* and the SSC, TSS content and soluble sugars contents ranged from 0.890 to 0.930. The result also showed that μ*<sup>a</sup>* peaked at 980, 1169 and 1485 nm, and the correlation at the three peaks increased successively, with *r* ranges of 0.881–0.922, 0.894–0.938 and 0.913–0.961, which was in good agreement with the successively stronger spectral signal. To further investigate the correlations with SSC and sucrose content, Table 3 plotted the direct linear correlations of μ*<sup>a</sup>* at 980, 1169 and 1485 nm with SSC and sucrose content (25 ◦C). The coefficients of determination (*R*2) of the equations were between 0.848 and 0.921. Results from the equations indicate that SSC and sucrose content gave good linear correlations with μ*a*, with an *R*<sup>2</sup> of 0.915 and 0.921, respectively, which confirms the results shown in Figure 6a.

**Figure 6.** Correlations coefficient of μ*a* and μ- *s* with content of soluble solid (SSC), total soluble sugars (TSS) and three types of soluble sugars of apples at 25 ◦C (**a**,**c**) and 0 ◦C (**b**,**d**).


