Characterising kiwifruit (Actinidia sp.) near skin cellular structures using optical coherence tomography

Abstract

Pre-harvest growing conditions during the development of fruit have impacts on fruit quality at the time of harvest and during storage. These impacts may result in differences in the skin properties of the fruit and some sub-surface cellular structural changes, which have potential effects on postharvest fruit quality and storability. Optical coherence tomography (OCT) is a non-destructive imaging method that enables acquisition of three-dimensional (3D) images of sub-surface structures of semi-transparent and turbid objects. This technology is a potential tool to provide new information about the structural nature of horticulture products. This work used five commercial kiwifruit cultivars, ‘G3’, ‘G9’, ‘G14’, ‘Hort16A’ and ‘Hayward’, where ‘Hayward’ was also manipulated with different orchard management practices (crop load and girdling), to investigate the properties of the layers of structures immediately underneath the periderm. Fruit samples were harvested at commercial maturity and then 3D OCT images of the skins were captured prior to fruit quality assessment. An automated image processing protocol was established to enable visualisation and characterisation of larger near skin cellular structures in the outer parenchyma tissue. There was no obvious differentiation between the observed large cells and the stone cells that are present in Actinidia chinensis cultivars. Significant differences in the microstructures of the sub-surface cells were found amongst five commercial kiwifruit cultivars. In general ‘G3’, ‘G14’ and ‘Hayward’ had larger and greater numbers of large sub-surface cells than ‘G9’ and ‘Hort16A’. The volume fraction of large cells in the near skin tissue was lower for the yellow-fleshed cultivars (‘G3’, ‘G9’ and ‘Hort16A’) in comparison to the green-fleshed cultivars (‘G14’ and ‘Hayward’). Low crop load increased maximum large cell length. However, no other effects of crop load and girdling were observed on the microstructures of the large cells in ‘Hayward’. Overall, OCT was an efficient non-destructive technique that can be utilised to reveal the structures of skin and sub-surface cellular layers with high resolution. The developed image processing protocol was suitable for the processing of large data sets with acceptable accuracy. However, more work should be carried out to overcome image artefacts caused by surface lenticels and trichomes. Limitations caused by low penetration depth should also be improved to allow better representation of the whole fruit structure.