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Résumé :
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Environmental effects and human health risks associated with synthetic chemicals have prompted several fruit growers to convert to integrated, and even more so to organic production systems. As a consequence the organic market is considered to b[...]
Environmental effects and human health risks associated with synthetic chemicals have prompted several fruit growers to convert to integrated, and even more so to organic production systems. As a consequence the organic market is considered to be a growing agricultural sector and organic products can now be purchased worldwide. Organic crops are believed to be healthier and tastier than non-organic ones, although only a limited amount of well-designed experiment can prove this claim. Consumers have to be supplied with fruits of outstanding quality the whole year round. As postharvest quality is affected by many preharvest factors the aim of this research is to study if organic apple can fulfil the demand for good postharvest quality. To carry out a well balanced study is even more important given the contradictory literature available on the quality of organic products. Moreover, only very few studies are available on the quality of organic crops during storage and shelf-life. This doctoral thesis aims to elaborate on the postharvest quality of organic apple as compared to its integrated grown counterpart. It also analyses how the production system might interact with variety, region, season and storage conditions.As quality loss in the postharvest chain firstly arises from texture decline, the softening process should be fully explored. Softening is the combined result of the loss in turgidity and the weakening of the cell wall (by loosening its structure). The exact mechanism of the latter is still not entirely known. In this thesis an attempt is made to improve the understanding of the enzymatic processes underlying apple softening by developing a mechanistic model describing softening as a function of the measured ß-galactosidase (ß-Gal) and polygalacturonase (PG) activities. These enzymes are chosen as they are generally considered as crucial in the enzymatic degradation of cell walls and thus in the softening of apple fruit. The developed model supports the generally accepted concept of ß-Gal acting before PG (possibly together with other pectin backbone cleaving enzymes) finally breaks down the pectin chains with the latter being the rate limiting step. Organic and integrated apples did not prove to be significantly different when compared using the developed mechanistic model. The effect of cultivar and season overruled that of the production system.As a second approach, data is also analysed longitudinally by a nonlinear mixed effects model making it possible to include biological age as a source of biological variation in the data. Differences between organic and integrated apple are shown to be inconsistent, in spite of the significant overall effect of production system. Both approaches show that differences between organic and integrated apples are not direct consequences of the production system, but arise from the different harvest maturity. The production system only indirectly affects the texture of apple during storage realised through its effect on harvest maturity. The acoustic stiffness coefficient is proved to be capable of monitoring textural changes during storage and shelf-life and is able to differentiate between certain seasons, cultivars, or apple grown in different production systems. As cell wall degradation versus water-loss derived texture changes alternately dominate in determining fruit texture, it is concluded that penetrometric measurement techniques should be used in combination with acoustic methods to monitor texture changes. Overall quality, taste or aroma parameters of organic and integrated apple are also not significantly different, though they showed significant changes during storage and shelf-life. The effect of storage conditions and shelf life is much larger than that of the production system on the flavour of apple. Storage regime has a considerable effect on the aroma of apple fruit, though these differences disappeared by the end of shelf-life thanks to successful aroma regeneration. The gas chromatography - mass spectrometry and headspace fingerprint mass spectrometry techniques both identify differences due to storage and shelf-life, but quite like texture and taste parameters, could not discriminate between different production systems or regions.In summary, it can be stated that experienced differences in postharvest quality of organic and integrated apples mostly arise from differences in harvest maturity. Thus plants receiving optimal nutrition supply and successful plant protection can bear crops with similar postharvest quality (texture and flavour) independent of the production system they come from.
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