Abstract
•TRS absorption coefficients are more related to pigment and water of the fruit flesh.•SRS absorption coefficients are strongly affected by pigments in the fruit skin.•Fruit softening was accompanied by increasing values of reduced scattering coefficient.•TRS and SRS capture valuable information on the changes in flesh texture.
Bulk optical properties, in terms of absorption (μa) and reduced scattering coefficients (μ′s), can be used for the non-destructive monitoring of fruit quality during ripening. In this study, the performance of time-resolved (TRS) and spatially-resolved (SRS) spectroscopy were compared by analyzing ‘Braeburn’ apples over a 21 d period of ripening. Nine batches of 20 apples each were measured on the blush side by TRS (540−1064 nm) and SRS (550−1000 nm). Every fruit was analyzed for skin color, texture characteristics, relative internal space volume (RISV), total solid soluble and titratable acidity contents. TRS absorption spectra showed two maxima, the highest at 980 nm (water) and the second at 670 nm (chlorophyll), while in SRS spectra the main peak was measured at 550 nm (anthocyanins) followed by that at 670 nm. The values of μa580SRS and of μa670SRS were much higher than those measured at the same wavelengths by TRS suggesting that TRS and SRS actually explore the apple tissue (skin and/or flesh) in a different way. The values of μa980TRS were higher than those of μa980SRS, probably due to the fact that water content was lower in the skin (mostly probed by SRS) than in the flesh (mostly probed by TRS). No significant correlations were found between μa580SRS and μa580TRS and between μa980SRS and μa980TRS but a low positive relationship was observed between μa670TRS and μa670SRS. On the contrary, high correlations were found between μa670SRS and the spectral index IAD (index of absorbance difference) related to chlorophyll in the skin and between μa580SRS and the spectral index ARI (anthocyanin reflectance index), related to anthocyanin content in the peel, suggesting that μa580SRS is linked to the development of the red color in the peel. Both μa670TRS and μa670SRS decreased during fruit ripening, indicating a decline in chlorophyll in the flesh and skin, respectively. During the shelf life period, apples became soft and mealy, as mechanical and acoustic parameters decreased and RISV increased. Fruit softening was accompanied by increasing values of both μ′sTRS and μ′sSRS. The μ′sTRS and μ′sSRS were positively related to each other, were positively correlated to RISV and negatively related to mechanical and acoustic parameters. Both the TRS and SRS technique were able to follow ripening processes in ‘Braeburn’ apples during the shelf life period, as absorption phenomena were related to changes in pigments present in the fruit flesh and skin, while scattering events mirrored changes in the flesh texture.