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Optical Techniques for Assessing the Fruit Maturity Stage

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  026041,  2002 ASAE Annual Meeting . (doi: 10.13031/2013.10585) @2002
Authors:   Luigi Bodria, Marco Fiala, Riccardo Guidetti, Roberto Oberti
Keywords:   Fruit quality, Chlorophyll content, Chlorophyll fluorescence, Nondestructive measurements, Red pigmented fruit

Among the several changes that a fruit undergoes during ripening, chlorophyll degradation is responsible for degreening of ground color, that is a well established maturity indicator for several species. In red pigmented cultivars of apples and peaches, of high interest for the European market, this process is not visible, being masked by a uniform layer of anthocyanins.

Two different systems were developed to evaluate non-destructively the chlorophyll content in these fruits, basing on their optical properties, in order to assess their maturity stage allowing for an optimal, quality oriented harvest and post-harvest management.

A fluorescence imaging system equipped with a blue actinic light, allowed to obtain fruit's fluorescence images in which the gray level of pixels resulted well correlated with firmness of fresh apples (R2=0.81) and permitted to follow the post-harvest evolution of firmness and sugar content of stored apples, even in absence of significant skin colour changes. The system equipped with a red actinic light provided results fairly good correlated with firmness of fresh peaches and nectarines, even though the fluorescence signal resulted quite low due to the low chlorophyll content in the considered cultivars.

A dual-band, laser-diode based, punctual reflectance probe was developed and tested on fresh peaches and stored apples. The index R/IR, defined as the ratio of the signal measured in red and near-infrared band, was found to correlate the chlorophyll content of the fruits with R2=0.66, regardless the differences due to the species. Moreover, R/IR resulted a fairly good estimator of conventional quality indices (correlations R2=0.4-0.5 with sugars and firmness), and allowed to track the post-harvest ripening process, showing different evolution patterns for fresh peaches at different maturity stages.

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