Pieter Verboven

Three-Dimensional Gas Exchange Pathways in Pome Fruit Characterized by Synchrotron X-Ray Computed Tomography

Our understanding of the gas exchange mechanisms in plant organs critically depends on insights in the three-dimensional (3-D) structural arrangement of cells and voids. Using synchrotron radiation x-ray tomography, we obtained for the first time high-contrast 3-D absorption images of in vivo fruit tissues of high moisture content at 1.4-μm resolution and 3-D phase contrast images of cell assemblies at a resolution as low as 0.7 μm, enabling visualization of individual cell morphology, cell walls, and entire void networks that were previously unknown. Intercellular spaces were always clear of water. The apple (Malus domestica) cortex contains considerably larger parenchyma cells and voids than pear (Pyrus communis) parenchyma. Voids in apple often are larger than the surrounding cells and some cells are not connected to void spaces. The main voids in apple stretch hundreds of micrometers but are disconnected. Voids in pear cortex tissue are always smaller than parenchyma cells, but each cell is surrounded by a tight and continuous network of voids, except near brachyssclereid groups. Vascular and dermal tissues were also measured. The visualized network architecture was consistent over different picking dates and shelf life. The differences in void fraction (5.1% for pear cortex and 23.0% for apple cortex) and in gas network architecture helps explain the ability of tissues to facilitate or impede gas exchange. Structural changes and anisotropy of tissues may eventually lead to physiological disorders. A combined tomography and internal gas analysis during growth are needed to make progress on the understanding of void formation in fruit.

A Three-Dimensional Multiscale Model for Gas Exchange in Fruit

Respiration of bulky plant organs such as roots, tubers, stems, seeds, and fruit depends very much on oxygen (O2) availability and often follows a Michaelis-Menten-like response. A multiscale model is presented to calculate gas exchange in plants using the microscale geometry of the tissue, or vice versa, local concentrations in the cells from macroscopic gas concentration profiles. This approach provides a computationally feasible and accurate analysis of cell metabolism in any plant organ during hypoxia and anoxia. The predicted O2 and carbon dioxide (CO2) partial pressure profiles compared very well with experimental data, thereby validating the multiscale model. The important microscale geometrical features are the shape, size, and three-dimensional connectivity of cells and air spaces. It was demonstrated that the gas-exchange properties of the cell wall and cell membrane have little effect on the cellular gas exchange of apple (Malus × domestica) parenchyma tissue. The analysis clearly confirmed that cells are an additional route for CO2 transport, while for O2 the intercellular spaces are the main diffusion route. The simulation results also showed that the local gas concentration gradients were steeper in the cells than in the surrounding air spaces. Therefore, to analyze the cellular metabolism under hypoxic and anoxic conditions, the microscale model is required to calculate the correct intracellular concentrations. Understanding the O2 response of plants and plant organs thus not only requires knowledge of external conditions, dimensions, gas-exchange properties of the tissues, and cellular respiration kinetics but also of microstructure.

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.

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.

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.

Pesticide-laden dust emission and drift from treated seeds during seed drilling: a review

Dressing seeds with pesticides to control pests is a widespread practice with important advantages. Recent incidents of bee losses, however, have directed attention to the emission of abraded pesticide-coated seed particles to the environment during sowing. This phenomenon of drift of pesticide dust can lead to pesticide contamination of air, water and other natural resources in crop-growing areas. This review article presents the state of the art of the phenomenon of dust emission and drift from pesticide seed dressing during sowing and its consequences. Firstly, pesticide seed treatment is defined and its pros and cons are set out, with the focus on dust, dust emission and dust drift from pesticide-coated seed. The factors affecting emission of pesticide dust (e.g. seed treatment quality, seed drilling technology and environmental conditions) are considered, along with its possible effects. The measuring techniques and protocols and models currently in use for calculating the behaviour of dust are reviewed, together with their features and limitations. Finally, possible mitigation measures are discussed, such as improving the seed quality and the use of modified seed drilling technology, and an overview of regulations and stewardship activities is given.

Genotype effects on internal gas gradients in apple fruit

A permeation-diffusion-reaction model was applied to study gas exchange of apple fruit (Kanzi, Jonagold, and Braeburn) as effected by morphology and respiratory metabolism. The gas exchange properties and respiration parameters of the fruit organ tissues were measured. The actual internal tissue geometry of the fruit was reconstructed from digital fruit images and the model was solved over this geometry using the finite element method. The model was validated based on measurements of internal gas concentrations and the gas flux of the fruit to its environment. Both measurements and an in silico study revealed that gradients of metabolic gases exist in apple fruit, depending on diffusion properties and respiration of the different cultivars. Macroscale simulation confirmed that Jonagold has large potential for controlled atmosphere (CA) storage while low diffusion properties of cortex tissue in Braeburn indicated a risk of storage disorder development. Kanzi had less O(2) anoxia at CA storage compared with Braeburn.

A continuum model for metabolic gas exchange in pear fruit.

Exchange of O2 and CO2 of plants with their environment is essential for metabolic processes such as photosynthesis and respiration. In some fruits such as pears, which are typically stored under a controlled atmosphere with reduced O2 and increased CO2 levels to extend their commercial storage life, anoxia may occur, eventually leading to physiological disorders. In this manuscript we have developed a mathematical model to predict the internal gas concentrations, including permeation, diffusion, and respiration and fermentation kinetics. Pear fruit has been selected as a case study. The model has been used to perform in silico experiments to evaluate the effect of, for example, fruit size or ambient gas concentration on internal O2 and CO2 levels. The model incorporates the actual shape of the fruit and was solved using fluid dynamics software. Environmental conditions such as temperature and gas composition have a large effect on the internal distribution of oxygen and carbon dioxide in fruit. Also, the fruit size has a considerable effect on local metabolic gas concentrations; hence, depending on the size, local anaerobic conditions may result, which eventually may lead to physiological disorders. The model developed in this manuscript is to our knowledge the most comprehensive model to date to simulate gas exchange in plant tissue. It can be used to evaluate the effect of environmental stresses on fruit via in silico experiments and may lead to commercial applications involving long-term storage of fruit under controlled atmospheres.

Computation of airflow effects on heat and mass transfer in a microwave oven

Abstract The magnitude of surface heat and mass transfer coefficients in microwave ovens is important to control food surface temperature and moisture and are a result of the faint airflow present in the oven cavity and of surface radiation. Magnitude and patterns of airflow inside a microwave oven and the resulting surface heat transfer coefficients were studied using a computational fluid dynamics model of the process. The governing Navier–Stokes and energy equations were solved for both natural, forced and combined convection. The magnitude and distribution of surface heat transfer coefficients on the food surface were computed for a 3-D oven cavity with one inlet and one outlet and a cylindrical food placed inside the oven. Calculated convective heat transfer coefficient values were found to be in the same range as has been used in the literature. A combined convection regime proves beneficial for heat transfer uniformity and the reduction of moisture accumulation inside the oven. Radiation heat transfer coefficients for energy exchange between food surface and oven interior were calculated and shown to be of the same order of magnitude as the convection heat transfer coefficients.

A CONTINUUM MODEL FOR AIRFLOW, HEAT AND MASS TRANSFER IN BULK OF CHICORY ROOTS

This article presents a transient 3D CFD model of heat and mass transfer in bulks of chicory roots. The model consists of the system of conservation equations of momentum, energy, and mass for the air phase, and the energy and mass for the product phase without considering the internal gradient in temperature and moisture in the product phase. The interaction between the airflow and the porous media is described by an Ergun-type equation based on experimental data. Heat of respiration is included in the model as an empirically derived function of temperature. A finite volume code is used to solve the model equations. The results show a good agreement between the model and the experiments. Differences between predicted and measured weight loss only amount to a maximum 10% after the initial cooling period. The non-ideal experimental conditions (high velocity, low relative humidity of the air), the various sizes of the products, the small scale of the porous region compared to the size of the product, and the estimation of transfer correlations contribute to the observed differences between experiment and simulation. The model can be applied to study the cooling process in an industrial cold store to find the optimal process settings to improve product quality and reduce product weight loss.