Time- and spatially-resolved spectroscopy to determine the bulk optical properties of 'Braeburn' apples after ripening in shelf life

Bulk optical properties, in terms of absorption (mu(a)) and reduced scattering coefficients (mu(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 mu(a)580SRS and of mu(a)670SRS 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 mu a980TRS were higher than those of mu(a)980SRS, 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 mu(a)580SRS and mu(a)580TRS and between mu(a)980SRS and mu(a)980TRSbut a low positive relationship was observed between mu(a)670TRSand mu(a)670SRS. On the contrary, high correlations were found between mu(a)670SRS and the spectral index IAD (index of absorbance dif-ference) related to chlorophyll in the skin and between mu(a)580SRS and the spectral index ARI (anthocyanin re-flectance index), related to anthocyanin content in the peel, suggesting that mu(a)580SRS is linked to the develop-ment of the red color in the peel. Both mu(a)670TRS and mu(a)670SRS 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 mu(sTRS)' and mu(sSRS)'. The mu(sTRS)' and mu(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 ab-sorption phenomena were related to changes in pigments present in the fruit flesh and skin, while scattering events mirrored changes in the flesh texture.