Effect of skin andpenetrationdepth 861 Skin

Measurements were performed on apples(GoldenDelicious,Granny Smithand Starking Delicious), peaches, nectarines, kiwifruit and melons. The tests proved that TRS allows the assessment of theinternalopticalpropertiesandthatthe optical properties of the skin do not preventtheassessmentof informationonthe bulk, at least for fruitswiththinskins.

For apples, no significant change in the measured optical properties (both absorption and scattering) is caused by skinremoval. Thisis provedbytheexper-imental finding that in none of the cases considered did skin removal alter the results significantly, despite the different optical properties of the skin in each distinct situation, for example a yellow-skinnedapple(Golden Delicious)com-pared with a red-skinned one (Starking Delicious),asshownin Fig.8.5.Similar outcomes were obtained for peaches and nectarines (data not shown). The peeling of the skin did not alter markedly the results, confirming that TRS is most sensitive to the internal features.

The situation is different for thick-skinned fruits. In particular, for kiwifruit where peeling led to a 20-25% increase in the absorption coefficient over the entire NIR range examined (720-840 nm). However, this effect concerns only the absolute estimate of the optical properties. The spectral line shape is not significantly altered. Consequently, even though the skin influences the results, it does not necessarily make TRS measurements inappropriate for the assessment of internal quality of thick-skinned fruits. For melons (Cantaloupe) measured in the bed region, the skin removal significantly reduces the chlorophyll absorption, while it has no significant effect on the NIR absorption. In both wavelength ranges, a 15-25% decrease is observed in the measured values of the scattering.

8.6.2 Penetration depth

In a further experiment, the penetration depth of a TRS measurement was determined. It is well known that the volume probed by a TRS measurement is a 'banana shaped' region connecting the injection and collection points.13 It is not easy to define the measurement volume, since the photon paths are more densely packed in the banana region but can be distributed in the whole medium. Attempts were made to determine the maximum depth in the pulp that can give some detectable contribution to the TRS curve. A series of measurements were performed on a Starking Delicious apple where slices of pulp were cut from opposite sides of the measurement site. Spectra were taken of the whole apple, and then slices were removed to yield a total thickness of 4.1, 2.7, 2.1 and 1.5 cm.

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Fig. 8.5 Influence of skin on TRS measurements: absorption (a) and transport scattering (b) spectra of a Starking Delicious apple before (closed symbols) and after (open symbols)

peeling.

The fitted absorption and scattering spectraareshownin Fig. 8.6.Fortheabsorp-tion measurement, |ma is unchanged downtoathicknessof2.7cm.Forthe2.1cm thick slice, |ma starts deviating from the measurement of the whole apple with a discrepancy of 25% at 680nm, while for a thickness of 1.5 cm the discrepancy increases up to 50%. The highest variations are observed on the tails of the spectrum, where the absorption is lower. The results for the scattering coefficient show

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Fig. 8.6 Absorption (a) and scattering (b) spectra of a Starking Delicious apple. Different curves correspond to measurements on the whole apple, and on slices of the same apple obtained by cutting the fruit on the opposite side of the measurement site.

a similar behaviour, with almost no changes down to a thickness of 2.7 cm, and discrepancies of 15% and 25% for a 2.1 and 1.5cm thickness, respectively. Overall, these data show that the TRS measurement is probing a depth of at least 2 cm in the pulp. Of course this is a rough estimate, yet it confirms that the TRS measurement is not confined to the surface of the fruit. Moreover, the penetration depth can be somehow dependent on the optical properties, and deeper penetration is expected in less absorbing and/or scattering fruit.

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Fig. 8.7 (a) Absorption spectra of apple, peach, tomato and kiwifruit. (b) Best fit of chlorophyll-a and water line shape to the absorption spectrum of a Starking apple.

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