Els Veraverbeke

Electronic nose as a non-destructive tool to evaluate the optimal harvest date of apples

Abstract An electronic nose (E-nose) has been evaluated for use as a tool to predict the optimal harvest date of apples ( Malus domestica Borkh.). The volatiles of ‘Jonagold’ and ‘Braeburn’ apples were assessed during the preclimacteric stage for two consecutive harvest years by means of an E-nose. A principal component data analysis indicated the presence of both a year and cultivar effect. Partial least square (PLS) models were constructed based on data of both harvest years. The cultivar effect made it difficult to build accurate and robust models for the two cultivars together. As a consequence, calibration models were constructed based on data of 2 years, but for each cultivar separately. The prediction of maturity, according to the Streif Index, showed a cross-validation correlation of 0.89 and 0.92 for ‘Jonagold’ and ‘Braeburn’ fruit, respectively. The calibration models for the prediction of the maturity, defined as the number of days before commercial harvest had a validation correlation of 0.91 for ‘Jonagold’ and 0.84 for ‘Braeburn’ fruit. Individual quality characteristics (soluble solids, acidity, starch and firmness) were predicted reasonably well. The calibration model for soluble solids content resulted in a consistent validation correlation over the results over 2 years (0.76 and 0.77). The starch and firmness were predicted with a validation correlation between 0.72 and 0.80. The prediction of the total acidity was poor (validation correlation of 0.66 and 0.69). It was also demonstrated that the type of validation influences the model prediction performance. Care should be taken when interpreting and using the models to predict the optimal harvest date for other years and cultivars.

Prediction of moisture loss across the cuticle of apple (Malus sylvestris subsp mitis (Wallr.)) during storage Part 1. Model development and determination of diffusion coefficients

Abstract ‘Actual’ diffusion coefficients of tissue, cutin, and wax were estimated for the apple cultivars ‘Jonagold’ and ‘Elstar’. This was done using a modelling approach with finite elements using geometrical models based on microscopic images of the cuticle. Contrary to the ‘apparent’ diffusion coefficients, which were previously determined experimentally, the ‘actual’ diffusion coefficients account for the typical structure of the cuticle and the presence of lenticels and cracks. They were, therefore, always smaller than the apparent values. The actual diffusion coefficients of wax were 0.98×10 −15 m 2 s −1 for ‘Jonagold’ and 1.79×10 −14 m 2 s −1 for ‘Elstar’ fruit. The actual diffusivities of cutin were 4.5×10 −14 m 2 s −1 for ‘Jonagold’ and 6.65×10 −14 m 2 s −1 for ‘Elstar’ and for tissue the values were 1.12×10 −11 and 4.33×10 −12 m 2 s −1 for ‘Jonagold’ and ‘Elstar’, respectively. These diffusion coefficients and the separate geometrical features can be integrated into an overall model, which describes and predicts moisture loss of whole apples during long-term storage.

Changes in chemical wax composition of three different apple (Malus domestica Borkh.) cultivars during storage

Abstract The effects of year, picking date and storage conditions on the chemical composition of the wax layer of three apple cultivars (‘Jonagold’, ‘Jonagored’ and ‘Elstar’) were investigated by means of GCMS and multivariate statistical techniques. Wax of apples with different surface characteristics also differed in chemical composition. Controlled atmosphere (CA) storage and subsequent shelf life affected wax properties and caused changes in the chemical composition, especially during shelf life. The changes in wax composition of ‘Elstar’, however, were smaller than those in ‘Jonagold’ and ‘Jonagored’, possibly associated with lower diffusion resistance and more limited structural changes of ‘Elstar’ wax. Longer CA storage periods accelerated the changes in wax composition during subsequent shelf life. The components responsible for the changes were mainly the alkane and ester fractions. The secondary alcohol nonacosan-10-ol was particularly important in explaining the changes in wax composition of ‘Jonagold’ during shelf life and its presence could be linked with greater development of greasiness.

Prediction of moisture loss across the cuticle of apple (Malus sylvestris subsp mitis (Wallr.)) during storage, part 2 : model simulations and practical applications

The microscopic cuticle model to describe moisture loss through the apple cuticle, which was developed in Veraverbeke et al. [Postharvest Biol. Technol. (2003b)], was applied to simulate moisture loss of different apple cultivars in realistic storage situations. The moisture loss calculated from the original microscopic cuticle model was, therefore, integrated over a whole apple with an average apple radius to obtain a macroscopic moisture loss model. The effect of the position of an apple in a pallet box during long-term storage was further simulated by introduction of specific surface boundary conditions. The predicted moisture loss with this macroscopic model for 6 months of storage at RH 97.5% was 4.1% for ‘Elstar’ and 3.5% for ‘Jonagold’ fruit, which agreed well with data obtained from practice. In addition, three practical case studies were simulated to describe the effect of different surface structures, relative humidity and, blocked lenticels on moisture loss of fruit in long-term storage.

A mathematical model for the development of mealiness in apples

Mealiness in apples (Malus domestica Borkh.) is an internal quality defect which is characterised by a dry and crumbly texture. It is related to the relative strength of the cell wall and the middle lamella. A mathematical model has been built to relate changes in the texture attributes juiciness, tensile strength and hardness, which are associated with mealiness, to the development of the turgor pressure of the tissue and the degree of hydrolysis of the middle lamella. The latter, in turn, are described in terms of properties which are meaningful from the physiological point of view, such as starch content, soluble solids content, non-hydrolysed and hydrolysed middle lamella, water in the symplast, and water in the apoplast. Biochemical reactions as well as water transfer processes are incorporated in the model. The parameter values of the model are estimated using experimental data from a storage experiment. The model fits the three texture characteristics adequately. The correlation coefficients between the parameters were below 0.96, which indicates that the model does not overfit the data.

Spectroscopic Evaluation of the Surface Quality of Apple

Different spectroscopic techniques based on infrared and Raman were used to evaluate the natural wax and related surface quality of apple fruit. Transmission near-infrared (NIR) spectroscopy was applied to solutions of single wax components and extracted apple wax. Fourier transform infrared (FTIR) spectroscopy was used for transmission measurements of wax films on NaCl crystals, diffuse reflectance spectroscopy (DRIFTS) was used to analyze wax powders, and FT-Raman spectroscopy was explored to examine intact wax layers on whole fruit. The natural wax layers of apple fruit from a maximum of three different cultivars (Jonagold, Jonagored, and Elshof) from three picking dates (early, commercial, and late), three controlled atmosphere storage durations (0, 4, and 8 months), and three shelf life periods (0, 1, and 2 weeks) within each storage duration were examined. Canonical discriminant analysis was carried out on the first derivative NIR and FTIR spectra to describe the information contained in the spectra. Discrimination between cultivars and between storage duration based on wax layer properties was achieved with reasonable accuracy from both of the techniques. Information contained in the spectra of apples from different picking dates and shelf life periods was not significant. Differences between cultivars and storage periods in this analysis mostly related to differences in the number of aliphatic chains (e.g., alkanes and esters) and the presence of alpha-farnesene. No satisfactory results were obtained by means of Raman spectroscopy and DRIFTS.

Mass Spectrometry Based Electronic Nose for Apple Aroma Profiling

The potential of the electronic nose (E-nose) and the mass spectrometer based electronic nose (MSE-nose) to monitor changes in aroma profile during shelf life was studied. Apples were stored for eight months at three different storage conditions and the aroma profile changes were followed subsequently over a period of 15 days. Gas chromatographic headspace analysis were conducted as reference measurements. A canonical variate (CV) analysis showed a clear effect of storage and shelf life both for the MSE-Nose measurements and the GC measurements. However for the E-Nose measurements only a shelf life but no storage history effect was observed. It was also found that the aroma profile changes during shelf life depended on the storage history. Partial least squares models were built to predict the apple firmness and the number of days in shelf life. It was found that the models based on the E-nose data had a worse prediction performance compared to those based on the MSE-nose data. Fusion of both sensors did not result in improved models. Both for firmness (0.95) and days of shelf life (0.98) a high cross validation correlation was observed between measured and predicted values. The SEP of firmness and shelf life were 2.38 Hz2 g2/3 × 106 and 1.02 days, respectively.

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.