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Gas exchange in apple during controlled atmosphere storage

Pome fruit, as well as all other fruit and vegetables, are a source of vital food constituents such as proteins, vitamins, polysaccharides, phenolics and minerals. In 2018, the trade of apple and pear at the auctions of the Association of Belgian Horticultural Cooperatives (VBT) amounted to 100 000 and 180 000 tons, respectively, corresponding to a collective turnover of 130 million EUR.
Since fruit start to deteriorate after harvest due to respiration and associated metabolic processes, they are commonly stored under conditions of low temperature, decreased O2 partial pressure and slightly increased CO2 partial pressure (controlled atmosphere storage, CA) to optimally maintain their quality. When the O2 partial pressure during storage becomes too low, however, the fermentation pathway becomes dominant which leads to off-flavors and the development of storage disorders. For this reason, O2 and CO2 levels during CA are kept at safe and steady, but suboptimal setpoints that may be above or below the critical levels. The application of CA storage, however, causes an important challenge towards fruit batches from different origins and seasons. Respiration rate, fruit size and tissue structure may be different between fruit and batches, affecting internal gas gradients. In general, more ripe, larger and more dense fruit are more sensitive to low O2 partial pressures compared to less ripe, smaller and porous fruit. Computer models are available to study the effect of these parameters on the physiology of fruit stored under CA and to optimize the storage protocol. These models assume material properties such as gas diffusivities that are homogenous throughout the fruit. However, the microstructure of fruit tissue is quite heterogeneous and this may very well affect gas transport considerably. So far, however, no model is available that takes into account the complete heterogeneity of tissue microstructure. The objective of this dissertation was, therefore, to quantify the heterogeneity of apple tissue and to incorporate it into existing models for gas transport in apple.
Existing models to describe gas transport in pome fruit use a single phase formulation. This formulation uses an effective diffusivity, either obtained from experiments or from a microscale simulation of gas transfer through cells and pores in small tissue samples. This single phase formulation in essence also assumes equilibrium between pores and cells everywhere in the apple. Still, the rate of gas transport is significantly different in these two phases and, therefore, a two-phase multicomponent multiscale model may be more appropriate, solving gas transport in the pores and cells with separate equations, and interphase transport between them. In a first step towards achieving such a model, a two-phase formulation was proposed in Chapter 3. The two-phase formulation contains two effective diffusivity values per gas species, one for pores and one for cells, and an additional interphase transfer term that is a function of microstructural parameters obtained from micro-CT analysis. It was assumed that the effective diffusivity values, like in the single phase model, can be obtained from 3D microscale simulations on the tissue geometries of the micro-CT scans. This model in principle allows also non-equilibrium conditions between the two phases inside apples. Previously developed models for similar applications showed that, besides the interphase transfer term, additional terms occur in the two-phase model that describe diffusion phenomena at the interface of pores and cells. Based on what was suggested in literature, these additional terms were lumped into the effective diffusivities of the separate phases. The proposed two-phase multiscale model was evaluated for steady-state conditions using in silico experiments and was found sufficiently accurate for the purposes of this dissertation. O2 diffusion in ‘Jonagold’ apples stored in CA conditions was evaluated with the two-phase model and results showed that equilibrium conditions were satisfied. Afterwards, the sensitivity of the two-phase model was checked towards: (i), open porosity in the cortex samples; (ii), tissue respiration; and (iii), interphase resistance. The former two had significant effects on the O2 distribution inside the apple. Increasing the interphase resistance significantly, on the other hand, had a small effect on the average O2 distribution, but resulted in non-equilibrium O2 concentrations between the pore and cell phase.
In Chapter 4, X-ray CT scans of intact ‘Braeburn’ apples with unprecedented resolution were made to visualize the internal microstructure of an entire apple fruit. A high microstructural variability was observed, both between different apples as well as within a single fruit. To optimize controlled atmosphere storage conditions, the effect of this heterogeneity on transport of metabolic gasses (O2, CO2) needed to be clarified. ‘Braeburn’ apples, who are highly susceptible to internal browning during storage, were characterized in terms of porosity distribution throughout the whole apple. These scans were used to identify different tissue compartments inside the apples. Based on the 3D connectivity of the pores, the cortex tissue was divided into a region with high porosity (HPC) and low porosity (LPC). The HPC had a porosity of 30.4 ± 2.1 % and featured relatively larger pores compared to the LPC, which had a porosity of 13.2 ± 3.3 %. On the internal boundary of the HPC and LPC, around a relative radius of 0.4, the porosity reached minimal values. A barrier to gas transport was identified at this position, where the exocarp and the main vascular tissue of the apple are situated. Furthermore, results showed that in two out of four tested apples the ovary was connected to the environment due to incomplete growth.
Chapter 5 incorporated these compartments in the developed two-phase multiscale model. For each compartment, effective diffusivities were estimated and microstructural parameters were calculated. The relationship between gas diffusivity and microstructural parameters was studied and results showed that the gas diffusivity of a tissue sample did not always scale with the porosity of said sample, but rather with the ratio of the open porosity over the tortuosity. Macroscale simulations in commercial CA storage conditions showed that the O2 concentration profiles were highly variable between and within ‘Braeburn’ apples. Internal microstructure, and, most importantly, the open porosity, seems to vary between the apples in order to provide sufficient O2 throughout the apple. Furthermore, the results of the macroscale simulations with multiple compartments also showed that minimal O2 concentrations are not necessarily reached in the center of the ‘Braeburn’ apple, which can potentially have a relationship with the position of internal brown development in the cortex tissue.
To further include structural heterogeneity in the modelling of gas transport, an alternative modeling approach to the two phase model was required. Hereto, a network modelling approach was studied in Chapter 6. The network model translated the pore and cell phase of the apple into individual cells and pores, represented as nodes, which made up a nodal network of the apple tissue. Compared to the multicompartment multiscale model, more detailed results towards O2 concentration profiles were found: a larger concentration gradient was found in the radial direction of ‘Braeburn’ apples when using the network model. Network modelling, therefore, provided a good and computationally efficient alternative to multiscale modelling to further investigate the transport of gasses in heterogeneous porous fruit such as apples.
Concerning future prospects, a relatively new approach for storing fruit based on a dynamically controlled atmosphere (DCA) has triggered significant interest from the horticultural sector in Flanders and beyond. Instead of a constant setpoint for O2, the lowest O2 setpoint below which fermentation occurs is continuously searched based on the stress response of the fruit. With DCA, the occurrence of disorders and quality losses are further minimized compared to CA. The dynamic nature of the method, however, means that gas concentrations are time-dependent and can be continuously changing. In this respect, the models presented in this thesis will need to be elaborated and evaluated for transient conditions. Future work should focus on a comparison of the proposed two-phase model with the more elaborate two-phase model formulations in literature, and experimental results obtained with needle oxygen sensors or gas scattering spectroscopy measurements.

Date:12 Sep 2015 →  16 Oct 2020
Keywords:X-ray CT, Appel, dynamisch gecontroleerde atmosfeer
Disciplines:Agriculture, land and farm management, Biotechnology for agriculture, forestry, fisheries and allied sciences, Plant biology, Agricultural plant production, Horticultural production
Project type:PhD project