Optical properties of ‘Gala’ (Malus pumila) apple pulp and their relationship with internal quality

To understand the relationship between the optical properties (OPs) and internal qualities of apple over a near-infrared range, 'Gala' apple was used as sample to obtain the absorption coefficient (μa) and reduced scattering coefficient (μs') in 950–1650 nm, as well as the soluble solids content (SSC), water content, firmness of apple pulp. Meanwhile, the microstructure of pulp tissue was observed. The relationship between OPs and internal quality was analyzed, and the prediction models for internal quality using the spectra of OPs were established by partial least squares regression (PLSR). The results showed that μa had three obvious peaks at 980, 1192, and 1425 nm. μs' decreased slowly first then increased later with the turning point at 1425 nm. During the storage, the SSC increased first then decreased, while the water content and firmness decreased generally. The μa presented a positive and high correlation with SSC with the correlation coefficient (R) of 0.72–0.94, but had negative and poor correlations with water content and firmness (|R|<0.46). μs' was negatively correlated with SSC in the investigated wavelength range, but it was negatively correlated with water content and firmness below 1387 nm and 1347 nm, respectively, and positively above 1387 nm and 1347 nm. The established PLSR models based on μa had the best prediction performance for SSC and water content, while the built PLSR model using μs' was the best for determining firmness. This study provides a theoretical basis for nondestructive detection of internal qualities of 'Gala' apple using near-infrared spectroscopy.