3D Pore Structure Maps of Whole Apple using High Resolution X-Ray CT

‘Braeburn’ apples (Malus × domestica Borkh.) are highly susceptible to develop storage disorders when kept in long term CA storage. Due to limited gas transport through the fruit tissue, local oxygen deficiencies may occur, leading to internal brown spots (“‘Braeburn’ browning disorder”). The pore structure of the tissues has been shown to be a major contributing factor to the gas transport properties of fruit tissues. However, this was based on analysis of relatively small samples of fruit tissue. This research aimed to visualize the complete porous microstructure of whole ‘Braeburn’ apples as a means to better understand how the pore network will affect the internal gas concentrations and the development of storage defects. High resolution X-ray CT images of 5 ‘Braeburn’ apples were made at a voxel resolution of 45 micron for a non-destructive pore structure analysis. These images were segmented into cell and pore volumes and based on image analysis of the pore structure, a 3D geometrical description of the apple anatomy was formulated. It is shown that the pore space of the ovary and the hypanthium of the fruit were completely disconnected. The hypanthium was divided into a high and low porosity zone. The average porosity of the apples was 21.81 ± 2.78%. The highly porous zone of the hypanthium had an average porosity of 26.10 ± 2.46% with a well-connected pore network, while the less porous zone only had 4.30 ± 0.67% of air spaces. Furthermore, the high porosity zone of the hypanthium, which represents on average 77.90 ± 5.45% of the apple volume, contained 93.39 ± 3.12% of the pore volume. The ovary had the highest porosity with a value of 43.66 ± 4.85%, seed lobes included. Radial porosity profiles were calculated showing a highly varying porosity from skin to ovary, with minimal values in the region around the ovary. It is suggested that the low porosity of certain parts of the ‘Braeburn’ apple is causing a barrier for gas transport and is therefore directly related to the occurrence of brown spots during the long term CA storage of these apples.status: publishe