Fruit Microstructure Evaluation Using Synchrotron X-Ray Computed Tomography

Abstract

Apple and pear are stored under controlled atmosphere conditions. Too low internal oxygen (O₂) or too high carbon dioxide (CO₂) concentrations may lead to storage disorders such as browning. The internal gas concentration is mainly determined by the fruit’s gas exchange properties, which depend on the structural arrangement of fruit cells and tissues. We used (for the first time) submicron synchrotron X-ray computed tomography (CT) to investigate how pome fruit tissues are spatially organized to facilitate or impede gas exchange. The experiments were conducted on beamline ID19 at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), i.e., on a long (150 m) imaging beamline where the spatial coherence of the beam is particularly large (transverse coherence length in the order of 100 μm). The method allows for imaging in phase contrast, which, as opposed to absorption contrast, is a powerful method to distinguish, in absorbing materials, phases with very similar X-ray attenuation but different electron densities. In this study, it is efficiently used for edge detection at cell-cell interfaces where absorption images have insufficient contrast. We visualized 3-D networks of gas-filled intercellular spaces in two fruits, apple (cv. Jonagold) and pear (cv. Conference), that provide the main routes for exchange of O₂ and CO₂ with the environment, using absorption as well as phase contrast synchrotron X-ray CT at a pixel resolution ranging from 0.7 to 5.0 μm. The differences in void dimensions and connectivity between tissues and fruits helped explain imbalances in gas exchange that may result in internal disorders and structural degradation. We also showed that tomography with synchrotron radiation operated in phase-contrast mode and is able to visualize the 3-D geometry of voids, cells, and cell walls of biological tissues with high water content at submicron voxel resolution. In terms of facilitating gas exchange, the network pattern of the voids indicated a large size and volume fraction difference with the unconnected void structure found in apple. The partial breakdown of such networks would quickly lead to an internal gas imbalance leading to internal disorders. The achievement of high-resolution 3-D microstructural properties of cells and tissues is an important breakthrough for the study of gas exchange mechanisms in fruits stored under controlled atmosphere conditions. In addition to gas exchange, the results will benefit the study of water relations and mechanics of foods.