3.4.2. Scattering Coefficient–SSC–Soluble Sugars Contents Relations

The values of r between μ*<sup>a</sup>* and either SSC or contents of soluble sugars, when compared with the respective r between μ- *<sup>s</sup>* and the same variables, exhibited different patterns; μ- *s* decreased gradually with the increase in wavelength (Figure 6c,d) which may be related to the decreased values of μ- *s* over the whole investigated wavelength range. It was also observed that soluble solid and soluble sugars were positively correlated with scattering properties. At 25 ◦C, the average *r* values of SSC, TSS fructose, glucose and sucrose with μ- *<sup>s</sup>* were 0.855, 0.811, 0.771, 0.695 and 0.884, respectively. At 0 ◦C, the average values of *r* were 0.808, 0.742, 0.704, 0.659 and 0.887, respectively. Moreover, SSC, TSS, fructose and sucrose have higher correlation with μ- *s* than glucose. Among the three types of soluble sugars, μ- *<sup>s</sup>* was most related to sucrose (*r* = 0.789–0.939), followed by fructose (*r* = 0.599–0.816) and glucose (*r* = 0.544–0.738). The same result was found in the correlation of SSC and μ*<sup>a</sup>* with the three soluble sugars. The findings proved that the sucrose contributed the most to the strong relationship between optical properties and SSC.

According to the results of Sections 3.4.1 and 3.4.2, the correlation between soluble sugars and μ*<sup>a</sup>* was stronger than that between soluble sugars and μ- *<sup>s</sup>* in the whole visible-medium wave near infrared region. Besides, with the increase in wavelength, the correlation between soluble sugars and μ*<sup>a</sup>* increased, while the correlation between soluble sugars and μ- *s* decreased. It could be inferred that μ*<sup>a</sup>* was more associated with chemical components, while μ- *s* was less correlated with them [7,30,36,46]. Furthermore, previous study presented that scattering property was influenced by microstructure. Rowe et al. [26] measured the optical spectra of apples over the range of 400–1050 nm using IS and obtained an average *r* of–0.68 between the μ- *<sup>s</sup>* and firmness. Liu et al. showed that μ- *s* of kiwifruit had stronger correlation with firmness (*r* = 0.74–0.87) than with SSC (*r* = 0.49–0.81) and moisture content (*r* = 0.59–0.80) [7]. These studies indicated that the absorption and scattering properties had potential to determine the internal quality of apple.
