Applications analysing fruit maturity and quality defects

8.8.1 Picking date experiment

To prove the applicability of the technique in real life applications, the compact prototype for TRS measurements was sent to Horticulture Research International and there tested on a picking date experiment to check the tracking of maturity stages in apples.14 Fruits of the Gala variety were harvested from the same orchard at three different picking dates (pick 1 = 15 September, pick 2 = 25 September and pick 3 = 9 October), stored under controlled atmosphere at 1.5°C for 7


Fig. 8.9 Plot of the absorption and scattering measurements of 30 apples taken from a Gala cultivar at successive harvest dates: pick 1 (black triangle), pick 2 (grey triangle), pick 3 (white triangle), and measured all together with the prototype after 7 months' storage under controlled atmosphere.

in 20

months, and then measured all together using the prototype. For each fruit, four equally spaced positions on the equatorial plane were measured and averaged. Results are presented in Fig. 8.9, where every fruit is coded by its |ma and |ms' at 672nm. The measured |ma decreases passing from pick 1 (black triangle) to pick 2 (grey triangle) and to pick 3 (white triangle), indicating a decrease in chlorophyll (CHL) content. Also the scattering coefficient is somehow related to the picking date with a general decrease for latest harvest.

Similar results were found for peaches. The technique is not only able to distinguish between different batches of fruits but can also monitor small variations due to shelf-life storage.

8.8.2 Detection of defects

Encouraging results have been obtained by applying TRS to non-invasive detection of defects in fruits. Preliminary measurements show that TRS can discriminate mealiness,19 watercore and bruise in apple, and brown heart in pears.20

Brown heart (BH) is an internal disorder sometimes shown by pears during controlled atmosphere (CA) storage. The symptoms are in no way recognisable from the outside of the fruit and are visible only after cutting the fruit. The aim of this work was to test TRS for analysing pears at risk of being affected by BH, in order to check if internal browning can be detected in the intact fruit by non-

Fig. 8.10 Absorption coefficient at 690 nm (b) and transport scattering coefficient at 720 nm (c) as a function of the position aroundtheequatorofa partiallyBHpearpicked at late harvest. Reported measurements were performed at the end of storage (black diamond) and at the end of shelf life (grey diamond). A photograph of the equatorial section of the fruit is shown in (a). Units for absorption and scattering are cm-1.

Fig. 8.10 Absorption coefficient at 690 nm (b) and transport scattering coefficient at 720 nm (c) as a function of the position aroundtheequatorofa partiallyBHpearpicked at late harvest. Reported measurements were performed at the end of storage (black diamond) and at the end of shelf life (grey diamond). A photograph of the equatorial section of the fruit is shown in (a). Units for absorption and scattering are cm-1.

destructive means. 'Conference' pear fruitsatlowrisk(earlyharvest,lowCO2 CA storage) and high risk (late harvest, highCO2 CA storage)forBHweremea-sured with TRS at 690 nm and 720nmoneightpointsaroundtheequator.BH was detected in pears by a significant increaseoftheabsorptioncoefficient ma at 720 nm. The absorption coefficient |ma at690 nmrespondedbybothincreasingin the presence of BH in affected fruits and decreasingwithripeninginsoundfruits, so it alone cannot have a unique interpretation. The decrease of the absorption coefficient |ma at 690 nm in sound fruits canbeattributedtodegradationofchloro-phyll, which has an absorption peak at 675 nm. The scattering coefficient m/ at 720 nm was influenced by translucency ofsoakedlookingtissue, asinoverripe fruits and in bruised regions. This technique allows a description of the virtual appearance of the internal tissue in the intact fruit to a depth of 2 cm, of the presence of defects and of their position inside the fruit, as it can be visually confirmed only after cutting the fruit.

An example is reported in Fig. 8.10, where the plots of the absorption coefficient at 672 nm and of the scattering coefficient at 720 nm are compared with the photograph of a partially BH pear.

8.9 Future trends

The use of the optical properties of the pulp of fruits and vegetables for the assessment of the internal quality of fruit has still to be investigated. More studies are required to correlate the measured optical properties with other chemical or physical parameters of the fruit such as soluble solids (sugar), acidity or firmness.

Since TRS permits the measurement of the absorption spectrum of the pulp independent of the scattering properties, it may be possible to detect absorbing substances such as chlorophylls and anthocyanins in the visible region or sugar and water in the NIR region. This technique might be suitable for following the ripening process pre-harvest, or for monitoring fruit changes during long-term storage. Scattering inside a fruit is mainly due to refractive index mismatches between liquids and membranes. Thus, the mean scattering coefficient could provide information on the internal structure, as suggested by a study on kiwifruits. In our work, changes in the scattering coefficient were related to the stage of maturity and to the ripening process, and could contribute to monitoring them.

Clearly, many technical aspects need still to be solved before an industrial application can take place. Most of all, the fruit characterisation in terms of pulp optical properties has to be compared to the presently accepted estimators of fruit quality, that is, sugar content, acidity and firmness.

A possible criticism of the usefulness of TRS for applications in agriculture is the cost and complexity of the instrumentation, especially whenever more than one wavelength is needed. However, rapid progress in optoelectronics, particularly in telecommunications, has led to considerable growth in instrumentation for time-resolved measurements, so that the development of a compact and low-cost time-resolved instrument is now feasible. A first prototype, working with semiconductor lasers, a compact photomultiplier and all-fibre optics that can be used as a stand alone portable instrument, was built in our laboratory. The compact prototype is characterised by ease of use and portability and a relatively low cost (about 20000 euro before assembly).

Post-harvest selection of fruit at industrial level employs automated machines for grading and sorting of fruits based on external parameters (colour, size) and weight. Typical speed for in-line analysis is 5 fruits per second. The acquisition time of TRS measurements can be as low as 500 ms per point in the wavelength range 700-800nm on most fruits. In this respect TRS measurements are not far from being applicable in on-line analysis. However, in view of a possible application of the TRS technique at industrial level, it is necessary to address several factors like acquisition time, number of measurement points, use of multichannel acquisition, and contact between fruit and optical probe. Detection of an internal disorder may in fact require mapping of the fruit to localise the defect. Moreover, in performing a non-contact measurement which could speed up the measurement time, care should be taken to reject background light and to enhance the signal. On the other hand, the TRS technique could be useful in the orchards, in the packing house or in the marketing chain as a complementary tool for nondestructive characterisation of fruits.

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