Analysis of apple (Malus domestica Borkh.) wax by means of chromatographic techniques and confocal microscopy

The wax of three economically important Belgian apple cultivars, 'Jonagold', 'Jonagored' and 'Elstar', was analysed chemically and structurally before and after 4 and 8 months of optimal ultra low oxygen (ULO) storage and during shelf life. Macroscopically and microscopically the surface properties of 'Elstar' were clearly different from the two other cultivars. These properties changed similarly for all cultivars during storage and shelf life. Chromatographic patterns of the three cultivars were dominated by the carbohydrate fraction with nonacosane (C29) as the major component. Again 'Elstar' differed in composition compared to 'Jonagold' and 'Jonagored'. For the non-destructive structural analysis confocal microscopy was used, and this led to the development of a new technique for the determination of the wax layer thickness. This technique was compared with the chemical technique known for determination of wax layer thickness. With the confocal technique 'Elstar', 'Jonagold' and 'Jonagored' could be significantly (p=0.05) discriminated based on wax-layer thickness, showing values of 4.14 mum, 2.58 mum and 3.41 mum, respectively

Prediction of postharvest water loss across the cuticle of apple (Malus domestica Borkh.) by means of finite element modelling

The cuticle of apple basically consists of a cutin and wax layer in parallel, each with different structures and diffusion properties. In order to obtain an accurate description of moisture transport through this cuticular membrane, a complex finite element model was developed. Model geometries of cracks and lenticels were based on confocal laser scanning microscopy (CLSM) images of the cuticle. In all geometries separate diffusion coefficients were attributed to the different surface layers. These diffusion properties were experimentally determined by means of a gravimetrical set-up. In a first series of simulations with the developed model, actual diffusion coefficients were obtained for cutin, tissue and wax. These actual diffusion coefficients differed from the experimentally determined apparent coefficients by taking into account the specific contribution of cuticular features such as cracks, open and closed lenticels. Next, the 'actual' diffusion coefficients were used in the model to predict moisture loss during long-term storage. After six months the predicted moisture loss was 4.1% for 'Elstar' and 3.5% for 'Jonagold', which accorded well to the data retained from practice. In addition, the model was evaluated in three case studies to describe the effect of different surface structures, relative humidity and blocked lenticels on moisture loss of fruit in long-term storage

Determination of the wax layer ultrastructure of Jonagold with confocal and raster electron microscopy

Until now, techniques for measuring wax layer thickness and analysing wax layer ultrastructure were always destructive and could alter the original structure of the wax. In this research confocal microscopy and raster electron microscopy were evaluated and compared to conventional scanning electron microscopy. Both new techniques have the advantage that minimal sample preparation is needed compared to conventional SEM. From the images obtained by means of confocal microscopy, no semicrystalline, flaked structure as described in literature could be observed. In contrary, a porous structure with little irregular channels was observed. At the outer surface, cracks could be observed which corresponded to the boundary edges of the epidermal cells beneath the cuticula. This was confirmed by REM. On several confocal microscopy images small droplets were observed at the surface of the wax layer. The exact origine, formation and composition of these droplets was not yet revealed. They were very unstable for they disappeared more or less when more severe techniques as SEM or REM were applied. The thickness of the wax layer could also be determined from the images, and was found to be typically between 10 to 60 mu m

Fruit Microstructure Evaluation Using Synchrotron X-Ray Computed Tomography

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