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Pears --- Controlled atmosphere storage --- Postharvest physiology --- Cell respiration --- Gas exchange

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Pears are commonly stored at a low temperature (typically around -1°C) and under controlled atmosphere conditions to minimise their respiration rate, and, hence, extend their storage life. However, the optimal gas composition is critical, as too low an O2 partial pressure in combination with too high a CO2 partial pressure induces a fermentative metabolism in the fruit. Conference pears ( Pyrus communis L. cv. Conference) are particularly sensitive to suboptimal gas conditions as they are susceptible to core breakdown. This storage disorder is characterised by the development of brown tissue which will further develop into cavities so that the fruit can no longer be commercialised. While the exact mechanism of the development of this disorder is still unknown, it has been shown to be related to the O2 and CO2 exchange of the fruits with their environment. A better understanding of the metabolic gas transport processes inside the fruit may help to improve the storage procedures so that the occurrence of core breakdown may be reduced in the future. A new method was developed to measure gas transport properties of pear tissue using a diffusion chamber with optical probes for O2 and CO2. Experimentally measured values of the CO2 diffusivity were considerably higher than those of the O2 diffusivity. The gas diffusivities along the axial direction were higher than along the equatorial radial direction while the values were lowest at the skin. The effect of temperature on diffusivity was small compared to that of biological variability. Picking date had no effect on the gas diffusivity of the tissue. The measurement setup was adapted for gas permeation measurements. Permeation coefficients of the skin and tissue along the radial direction were more or less equal while permeability along the axial direction was higher than along the radial direction. A macroscopic permeation-diffusion-reaction model was developed to study gas exchange of intact pear. The model accounted for both diffusion and pressure driven exchange of these gasses and incorporated respiration kinetics. It was discretised using the finite element method and validated successfully under steady and transient conditions at 1°C; however, there was an increasing deviation for the CO2 profile with increasing temperature. Based on an in silico study, it was shown that the higher values of the Michaelis-Menten parameters which were measured for the respiration of intact fruit compared to those of cortex tissue could be attributed to the gas exchange barrier properties of the fruit tissue. Environmental conditions such as temperature and gas composition had a large effect on the internal distribution of O2 and CO2 in fruit. A microscale gas transport model was constructed to evaluate the effect of microstructure on gas transport. The geometrical model of the fruit microstructure was based on light microscopy images (2-D). Gas transport was modeled using diffusion laws, irreversible thermodynamics and enzyme kinetics. The model equations were numerically solved using the finite element method. In silico analysis revealed that the local O2 and CO2 concentration profiles and fluxes were very different. O2 exchange occurs mainly through the intercellular space, the cell wall network and less through the intracellular liquid whereas CO2 exchange occurs at similar rates through each of these phases. The biological variation of the apparent diffusivity of gasses in tissue was related to the natural random distribution of cells and pores in the cortex tissue. The O2 transport model was extended to 3-D based on synchrotron tomography images of the cortex and epidermis tissue. A discrepancy between the measured and simulated O2 diffusion was found: the simulated O2 diffusivity was larger than the measured one. A multiscale gas transport model was constructed to describe the transport of O2 and CO2 in Conference pear at different spatial scales. Hereto the micro- and macroscale model were coupled through a homogenisation procedure in which the macroscale model parameters were calculated from the microscale model. The multiscale model was used to analyse the effect of different external air conditions at the storage temperature on gas transport inside the fruit. An in silico study revealed that the lowest O2 concentration of optimally picked pear stored at typical controlled atmosphere conditions (2.5 kPa O2, 0.7 kPa CO2 at -1°C) was higher than the Michaelis Menten constant for cytochrome c oxidase, the rate limiting enzyme of the respiration pathways. In contrast to small pears, in large pears and under extremely low O2 storage conditions the O2 concentration may decrease well below the Michaelis Menten constant for cytochrome c oxidase. This most probably leads to fermentation and physiological disorders which have been observed under such conditions. Ripe fruit have more risk of developing core breakdown since the increased respiration rate may result in anoxia in the center region of the fruit even at commercial controlled atmosphere conditions. This multiscale approach provides a much better insight in the mechanism of gas transport in pear fruit and tissue in general, and a quantitative explanation of the development of physiological disorders such as core breakdown.