Bart M. Nicolaı̈

Light penetration properties of NIR radiation in fruit with respect to non-destructive quality assessment

Abstract Some issues related to the non-destructive measurement of apple quality attributes by means of NIR reflectance spectroscopy are addressed. A comparison was made between two optical configurations, which can be used to perform NIR-spectroscopic measurements: the bifurcated and the 0°/45° optical configuration. A relationship was established between the reflectance spectra (880–1650 nm) and the soluble solids content by means of the partial least squares technique. Depending on the data pre-processing method, correlation coefficients between 79 and 91% were obtained. The results obtained with the bifurcated fibre were only marginally better than those obtained with the 0°/45° configuration. The apple skin reflectance and skin transmission properties with regard to NIR radiation were also investigated. The intensity of the light source was high enough to penetrate through the apple skin and gather information about the apple parenchyma tissue. A method was developed to calculate the light penetration depth for each wavelength in the range from 500 to 1900 nm. This method was applied to measure the light penetration depth in ‘Jonagold’ apple fruit tissue. The penetration depth is wavelength dependent: up to 4 mm in the 700–900 nm range and between 2 and 3 mm in the 900–1900 nm range.

Prediction of the optimal picking date of different apple cultivars by means of VIS/NIR-spectroscopy

Abstract The use of visible/near infrared (VIS/NIR) spectroscopy was evaluated to determine the internal quality and the optimal harvest dates of apples non-destructively. Calibration models were constructed with data from eight cultivars, three orchards and 2 years, in order to make the models as robust as possible for future use. The prediction of the maturity, defined as the number of days before commercial harvest, was reasonably accurate. The most robust model predicted the maturity with a validation correlation of 0.90 (SEP=7.4 days). The prediction of maturity, according to the Streif index, showed a validation correlation of 0.84 (SEP=0.18 kg/% brix×starch index) for one orchard. Maturity was orchard-dependent, however, and as a consequence, a combined prediction equation was not accurate. Individual quality characteristics (soluble solids, Streif index, acidity and firmness) were well predicted. The calibration model for soluble solids content resulted in a validation correlation of 0.84 (SEP=0.73% brix) for the results over 2 years from one orchard, but like the Streif index, was orchard-dependent and appeared to account largely for the orchard dependence of the latter. Acidity and firmness were predicted with a validation correlation of 0.80 and 0.78 and SEPs of 2.07 ml NaOH and 1.13 kg for, respectively, two and three orchards over the 2 years.

Effect of biological variability on the robustness of NIR models for soluble solids content of apples

A statistical analysis was performed on a large spectral data set to analyse the effect of orchard, season and cultivar. Season and cultivar were responsible for a major amount of the spectral variability, whereas the influence of the orchard was low and only appeared for certain cultivars during specific seasons. The robustness of the calibration models for soluble solids content with respect to the three factors was tested based on external validations. It was found that the accuracy of the models increased considerably when including more variability in the calibration set. Further, overfitting of the calibration model was avoided. On the other hand, adding more data to the calibration set increased the chance of adding atypical data, which resulted in reduced model accuracy. It is, therefore, important to construct the calibration data set in such a way that it is representative for future measurements. When the effect of a certain factor is known a priori, e.g. cultivar, it is recommended to use specific calibration models.

Combinations of pulsed white light and UV-C or mild heat treatment to inactivate conidia of Botrytis cinerea and Monilia fructigena.

The use of pulses of intense white light to inactivate conidia of the fungi Botrytis cinerea and Monilia fructigena, responsible for important economical losses during postharvest storage and transport of strawberries and sweet cherries, was investigated in this study. In the first stage, a light treatment applying pulses of 30 micros at a frequency of 15 Hz was investigated, resulting in a treatment duration varying from 1 to 250 s. The conidia of both fungi showed similar behaviour to pulsed light, with a maximal inactivation of 3 and 4 log units for B. cinerea and M. fructigena, respectively. The inactivation of the conidia increased with increasing treatment intensity, but no complete inactivation was achieved. The sigmoidal inactivation pattern obtained by the pulsed light treatment was described using a modification of the model of Geeraerd et al. [Int. J. Food Microbiol. 59 (2000) 185]. Hereto, the shoulder length was incorporated explicitly and relative values for the microbial populations were used. In the second stage, combinations of light pulses and ultraviolet-C or heat were applied. The UV light used in the experiments is the short-wave band or UV-C, running from 180 to 280 nm with a peak at 254 nm (UV-B runs from 280 to 320 nm and UV-A from 320 to 380 nm). The UV-C doses were 0.025, 0.05 and 0.10 J/cm(2), and the temperatures for the thermal treatment ranged from 35 to 45 degrees C during 3-15 min. When combining UV-C and light pulses, there was an increase in inactivation for both B. cinerea and M. fructigena, and synergism was observed. There was no effect of the order of the treatments. For the heat-light pulses combination, there was a difference between both fungi. The order of the treatments was highly significant for B. cinerea, but not for M. fructigena. Combining heat and light treatments improved the inactivation, and synergism between both methods was again observed. Complete inactivation of M. fructigena conidia was obtained after, e.g., a 40-s pulsed light treatment and 15 min at 41 degrees C, or after an 80-s light treatment and 10 min at 41 degrees C.

Analysis of the air flow in a cold store by means of computational fluid dynamics

Abstract Airflow inside a cold store is investigated using computational fluid dynamics. The airflow model is based on the steady state incompressible, Reynolds-averaged Navier–Stokes equations. The turbulence is taken into account using a k−e model. The standard as well as the Renormalisation-Group (RNG) version of the k−e model is investigated. The forced-circulation air cooler unit is modelled with an appropriate body force and resistance, corresponding to the characteristics of the fan and the tube-bank evaporator. The finite volume method of discretisation is used. The validation of the model has been performed by a comparison of the calculated time-averaged velocity magnitudes with the mean velocities measured by means of a hot-film type omni-directional velocity sensor. A relative error on the calculated air velocities of 26% was observed. The RNG k−e model does not help to improve the prediction of the recirculation. Both a finer grid and enhanced turbulence models are needed to improve the predictions.

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.

Computational fluid dynamics modelling and validation of the temperature distribution in a forced convection oven

Abstract This paper discusses the validation of a Computational Fluid Dynamics (CFD) model to calculate the heat transfer in an industrial electrical forced-convection oven. The CFD model consists of the continuity, momentum and energy equation with the standard k–e approach to model the flow turbulence. Density effects are accounted for through a weakly compressible formulation. Time-dependent boundary conditions and source terms are derived from a simplified lumped model, which results in a good qualitative agreement of the calculated oven temperatures and the measured temperature distribution. The average oven temperature difference between measurements and predictions is 4.6°C for a set point of 200°C. The heating uniformity of PVC bricks in different configurations was calculated with the CFD model, but the wall functions in the k–e model limit the accuracy to a qualitative agreement. A correlation was established between the calculated flow field variables and measured surface heat transfer coefficients.

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.

Computational fluid dynamics modelling and validation of the isothermal airflow in a forced convection oven

Abstract This article discusses the application of computational fluid dynamics (CFD) to calculate the three-dimensional isothermal airflow in an industrial electrical forced-convection oven. The governing fluid flow equations were expanded with a fan model and a turbulence model. The standard and the renormalisation group (RNG) version of the k – e turbulence model produced comparable results. The performance of the CFD model was assessed by means of point measurements of the velocity with a directionally calibrated hot-film velocity sensor. From the validation it was found that important aspects of the model are the fan head-capacity relationship, the fan swirl and the oven geometry. The calculation error was on an average 22% of the actual velocity, caused by the limitations in turbulence modelling and numerical grid density.

Temperature compensation for near infrared reflectance measurement of apple fruit soluble solids contents

Near infrared reflectance (NIR) spectroscopy has been used successfully to measure soluble solids in apple fruit. However, for practical implementation, the technique needs to be able to compensate for fruit temperature fluctuations, as it was observed that the sample temperature affects the near infrared reflectance spectrum in a non-linear way. Temperature fluctuations may occur in practice because of varying weather conditions or improper conditioning of the fruit immediately after harvest. Two techniques were found well suited to control the accuracy of the calibration models for soluble solids with respect to temperature fluctuations. The first, and most practical one, consisted of developing a global robust calibration model to cover the temperature range expected in the future. The second method involved the development of a range of temperature dedicated calibration models. The drawback of the latter approach is that the required data collection is very large. When no precautions are taken, the error on the soluble solids content reading may be as large as 4%brix.