Bulk optical properties, in terms of absorption (µ a ) 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 µ a 580 SRS and of µ a 670 SRS 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 µ a 980 TRS were higher than those of µ a 980 SRS , 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 µ a 580 SRS and µ a 580 TRS and between µ a 980 SRS and µ a 980 TRS but a low positive relationship was observed between µ a 670 TRS and µ a 670 SRS . On the contrary, high correlations were found between µ a 670 SRS and the spectral index I AD (index of absorbance difference) related to chlorophyll in the skin and between µ a 580 SRS and the spectral index ARI (anthocyanin reflectance index), related to anthocyanin content in the peel, suggesting that µ a 580 SRS is linked to the development of the red color in the peel. Both µ a 670 TRS and µ a 670 SRS 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.

The decline of relative chlorophyll contents during fruit ripening is considered to be an important indicator of fruit physiological condition. The recent availability of low-cost portable visible spectrum (VIS) spectrometers has spurred research interest towards optical sensing of chlorophyll changes in intact fruit, with many scientists attempting to link the shifts in optical signals, attributed to chlorophyll changes, to different maturity and quality parameters. One of the widely available portable devices for non-destructive estimation of relative chlorophyll contents is the DA meter, which provides a maturity index that is calculated as a difference between absorption at 670 nm (near the chlorophyll-a absorption peak) and 720 nm (background of the spectrum), abbreviated as IAD. In the present study, the evolution of IAD and its relation to starch pattern index (SPI) and fruit flesh firmness (FFF) was monitored in fruit of two cv. 'Gala' clones during maturation and storage, aiming to identify a potential existence of a usable IAD range for the assessment and prediction of the optimal harvest window and storage potential. In both clones, canopy positions, fruit sides, and seasons IAD, SPI, and FFF generally changed in a linear fashion over time, but with partially very different slopes, i.e., they were changing at different rates. What all of these parameters had in common was the presence of a very high biological variability, which is typical of apple fruit. Significantly powerful estimations of SPI (r 2 > 0.7, p < 0.005) and pre-and post-storage FFF (r 2 > 0.6, p < 0.005) were achieved. However, the very large biological variability could not be neutralized, which means that the predictions always included large confidence intervals of up to 0.46-0.59 units for SPI and 0.82-1.1 kgF FFF, which ultimately makes them unusable for practical applications. Experiments done under real-life conditions in a commercial fruit storage facility on several different fruit batches confirmed that IAD measured at harvest cannot be used indiscriminately for predicting post-storage FFF of cv. 'Gala' originating from different orchards. Nevertheless, mean IAD values that were obtained at optimal maturity from samples of the same orchards remained stable over seasons (0.8-1.2), which strongly suggests that, provided that the calibrations and validations are not only cultivar, but also orchard-specific, IAD has a potential for estimating maturity and storability of apple fruit. In this case, IAD could replace standard maturity indices, otherwise it would be suited for use as a supplementary index for determining fruits physiological maturity status.Agronomy 2019, 9, 642 2 of 16 they are based on destructive analyses, with the obtained data presenting low repeatability and high vulnerability to interpretation errors [1,2].In recent years, various methods for quality and ripeness evaluation of intact agricultural products have been developed [3][4][5][6][7]. A potential technique for assessing fruits physiological stage is to m...

Consumer expectations of apple fruit quality have been steadily increasing over the last couple of decades. One of the main quality attributes affecting product price is colour, determined primarily by the relative amounts of pigments chlorophylls, carotenoids, and anthocyanins, which can be measured non-destructively by visible spectrophotometry. This paper provides a brief overview of studies dealing with the optical pigment-determination of intact apple fruits in visible part of electromagnetic spectrum. Firstly ripening-associated pigment changes in apples as well as the basics of light interaction and measurements in turbid materials are explained. Subsequently the two most appropriate methods for pigment determination of intact apple fruit are presented along with examples. This is followed by a discussion of the various reflectance and remittance indices for the qualification, as well as quantification, of the main apple pigments. Results are reported regarding studies on the relationships between spectral data and derived indices with various destructive quality and ripening parameters. In the conclusion some of the main findings reviewed in this paper are outlined and discussed briefly.

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