Optical properties of fruits and vegetables 871 Absorption and tissue components

Typical absorption spectra of different fruits (apple Starking Delicious, yellow peach, tomato and kiwifruit) are reported in Fig. 8.7(a). The absorption spectrum of the apple is dominated by the water peak, centred around 970 nm, with an absolute value of about 0.4 cm-1. Minor absorption features of water are usually

Fruit Chlorophyll-a(mM) Water(%)


(Starking Delicious) 0.96 82.6

Peach 0.49 93.8

Tomato 0.52 95.0

Kiwifruit 6.91 98.8

detected around 740 and 835 nm, where the absorption coefficient is low (0.05cm-1). A significant absorption peak (0.12-0.18cm-1) at 675nm, corresponding to chlorophyll-a, is found. Boththelineshapeandtheabsolutevalue of the absorption spectra of peach and tomatoarequitesimilar tothoseofapples. However, for kiwifruit, as expected from the visual appearance of its flesh, chlorophyll-a absorption is considerable, with a maximum value up to 2 or 3 times the water maximumintheinfrared.

Information on the water content canbeobtainedbyconsidering theabsolute values of the absorption at 970 nm. In agreementwiththedifferentwater/fibres ratio in distinct species, a higher absorption was detected in tomatoes (~0.5 cm-1), than in peaches and kiwifruits (~0.45 cm-1),andinapples(~0.4 cm-1).The absorption at 675 nm provides information on thechlorophyll-acontent andpreliminary data obtained from apples suggest that this could be a useful parameter to test the ripening stage. A series of measurements performed on the same fruits showed a progressive decrease in red absorption, in agreement with the gradual reduction in the chlorophyll content withpost-harvestripening.14

To quantify the percentage volume of water and the chlorophyll-a content in the bulk of the intact fruits, a best fit oftheabsorptionspectrumwiththeline shape of water15 and of chlorophyll-a16 was performed. To account for the presence of other chromophores of fruits, such as carotenoids and anthocyanins, which exhibit characteristic peaks at shorter wavelengths than 650nm, a flat background spectrum of arbitrary amplitude was used as a free parameter in the fit.

Figure 8.7(b) shows a typical example of fit for the absorption spectrum of a Starking Delicious apple to the line shape of water and chlorophyll-a. Table 8.1 reports the chlorophyll-a and water content in different fruits. In all cases a 0.02-0.03 cm-1 contribution was added by the flat background spectrum.

8.7.2 Scattering and tissue structure

The scattering properties for all the species considered showed no particular spectral features. The value of the transport scattering coefficient decreased progressively with increasing wavelength. Typical examples are shown in Fig. 8.8(a) for a Starking Delicious apple, a peach, a tomato and a kiwifruit. The transport scattering spectrum of the kiwifruit was noisier than the spectrum


■ ■ i ■■ ■■ i ■■ ■■ i ■■ ■■ i ■■ ■■ i ■■ ■■ i ■■ ■■ i

650 700 750 800 850 900 950 1000

Wavelength (nm)

0 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 650 700 750 800 850 900 950 1000

Wavelength (nm)

Fig. 8.8 (a) Scattering spectra of apple, peach, tomato and kiwifruit. (b) Best fit of Mie theory to the scattering spectrum of a Starking apple.

of other fruits, particularly in the 675 nm region where the high absorption of chlorophyll reduced the accuracy of the evaluation of transport scattering by TRS measurements.

Even though marked variations in the absolute values were noticed depending on variety and ripeness, kiwifruits and tomatoes are usually characterised by a lower scattering than other species.


a (cm

1) b (cm-1)

r (mm)

Apple (Starking Delicious)
















Further information could be obtainedbyinterpretingthetransportscattering spectra with Mie theory. For a homogeneoussphereofradius r,Mietheorypre-dicts the wavelength dependence of the scattering and the relation between scattering and sphere size. Under the hypothesisthatthescatteringcentresare homogeneous spheres behaving individually, the relationship between |m's and wavelength (l) can be empiricallydescribedasfollows:17

where the size parameter x is defined as x = 2rcram1-1, with the refraction index of the medium nm chosen to be 1.35, and a and b arefreeparameters.Inpartic-ular, a is proportional to the density of thescatteringcentresand b dependson their size. Moreover, b can be empirically expressed as a third order polynomial function of r, therefore the estimate of b canyieldthesphereradius r.18

Figure 8.8(b) shows a typical transport scattering spectrum of a Starking D elicious apple and the best fit to Mie theory. The estimated average size of scattering centres in different fruits is shown in Table 8.2. It was observed that a and b varied in the range 2.9-17.4cm-1 and 0.12-0.95, respectively. This suggests that different fruits have different density and average dimensions of scattering centres (the range for r is 0.15-0.78 |mm). It is worth noting that, as the tissues are a complex distribution of cells and fibres, these parameters do not assess the real size of scattering centres in the tissue, rather they are average equivalent parameters, which could eventually be related to physical or chemical fruit characteristics such as firmness or sugar content.

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