Time- and spatially-resolved spectroscopy to determine the bulk optical properties of ‘Braeburn’ apples after ripening in shelf life
Maristella Vanoli, Robbe Van Beers, Nadja Sadar, Anna Rizzolo, Marina Buccheri, Maurizio Grassi, Fabio Lovati, Bart Nicolaï, Ben Aernouts, Rodrigo Watté, Alessandro Torricelli, Lorenzo Spinelli, Wouter Saeys and Angelo Zanella
Postharvest Biology and Technology; Volume 168, October 2020, 111233
2020
บทคัดย่อ
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 (540−1064โ€ฏnm) and SRS (550−1000โ€ฏnm). 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 980โ€ฏnm (water) and the second at 670โ€ฏnm (chlorophyll), while in SRS spectra the main peak was measured at 550โ€ฏnm (anthocyanins) followed by that at 670โ€ฏnm. 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.