Gas exchange modeling in fruit using diffusivity maps computed from X-ray CT images

Decreased O2 and slightly increased CO2 partial pressures of controlled atmosphere (CA) storage slow down the postharvest respiration processes of pear and apple fruit and, therefore, ensure their year round availability. While the O2 partial pressures of the storage atmosphere can be precisely controlled, gas diffusion resistance of the fruit tissue and O2 consumption may create too low O2 levels inside the fruit, potentially leading to the development of internal physiological disorders. To understand the effect of hypoxic storage on the physiology of the fruit, the O2 level inside the fruit needs to be known. A modeling approach is so far the best option since the direct measurement of internal gas concentration of fruit is destructive and not practical. Available gas transport models for O2 typically assume a homogeneous gas transport parameter. However, there is now evidence that fruit tissue microstructure is quite heterogeneous over the entire fruit and this would likely result in a heterogeneous gas diffusivity. In this dissertation, the heterogeneity of the tissue microstructure is mapped based on X-ray computed tomography (CT) images and integrated into existing respiratory gas exchange models to investigate its effect on the O2 concentration inside the fruit. A method was developed to create three dimensional (3D) porosity maps. The method was based on a regression model between the grayscale intensity of low resolution CT images and the actual tissue porosity at identical locations. The latter was calculated from high resolution CT images of the same tissue in which pores and cells could be distinguished. The model was constructed for four products that have considerably different tissue microstructures; pear, apple, eggplant and turnip. Grayscale values of juice of the sample and air representing 0 and 100 % porosity, respectively, were included in the data as extreme values. Using the model, the porosity distribution could be mapped based on juice scans only. The constructed porosity maps reflect the heterogeneity of the tissue microstructure both within and between products. The porosity mapping method was then extended to create effective O2 diffusivity maps. A model for relating the effective O2 diffusivity to the total porosity was developed for the same products. The model did not predict the O2 diffusivity well in regions with low connectivity and/or high tortuosity of the pore microstructure. The relationship between the O2 diffusivity and the open porosity and tortuosity was, therefore, also explored. The O2 diffusivity was calculated based on a microscale gas transport model; the total and open porosity and tortuosity were derived from segmented high resolution images of tissues sampled along radial direction of the product. The O2 diffusivity correlated better with the open than total porosity. The addition of the tortuosity in the model did not improve the fit of the correlation. For eggplant and turnip the model based on total porosity was sufficiently accurate as these products consist of more open and less tortuous pores. On the other hand, better results were obtained with the model based on open porosity for pear and apple which have less open channels. An open porosity map was, therefore, estimated for intact apple and pear fruit using a regression model between total porosity and open porosity. Finally, maps of effective O2 diffusivity coefficients were calculated for apple, pear, eggplant and turnip based on the previously created porosity maps. The O2 diffusivity maps were then used to study gas transport in 'Conference' pear and its impact on internal browning. Late harvested pear fruit were stored under CA with browning-inducing conditions: no pre-cooling period, 0.5 kPa O2, 0.7 kPa CO2 at 1 oC. Porosity and effective O2 diffusivity distributions were mapped in pears before and after storage using the developed methods. The effective CO2 diffusivity were used in respiratory gas exchange calculations to obtain the spatial distribution of the O2 and CO2 concentration and the respiratory quotient (RQ) in the fruit. The resulting concentration contours were quite heterogeneous and the low oxygen concentrations and high RQ values found in the core region of the fruit indicated the occurrence of fermentation. Moreover, the gas concentration and RQ contours corresponded well with tissue that was affected by browning and cavities after 8-months storage. In contrast, when homogeneous O2 and CO2 diffusivities were used for computing the internal gas and RQ distributions, smooth contours were obtained that did not correspond to contours of the tissue affected by browning and cavities; nowhere the thresholds indicative for the development of browning were exceeded. In future research, the porosity mapping technique may be integrated into inline systems of fruit quality sorting based on X-ray radiography and tomography, although this will require significant further research in terms of optimizing speed and cost of the systems and development of adequate processing algorithm for inline applications. For the diffusivity mapping, more complex multiphase transport models and orthotropic diffusivity tensors should be considered to describe gas transport more appropriately, compared to a local isotropic diffusivity. Furthermore, the gas transport models should be validated using appropriate methods. The developed and validated models can then be extended to other respiration-related gasses. Finally, browning-related change of respiration, tissue microstructure and, inherently, effective tissue diffusivity during storage should be further investigated to explore the dynamic change of internal gas distribution.

Publication year:2021

Accessibility:Open