Optical coherence tomography visualizes microstructure of apple peel

Optical coherence tomography (OCT) has emerged recently as a non-destructive technique to visualize subsurface structures of materials. The objective of this work was to investigate whether OCT is an appropriate method to non-destructively assess the peel structural properties of apple fruit and to compare OCT to confocal microscopy and micro-CT. Peel structural properties of apples from different cultivars (‘Braeburn’, ‘Arlet’, ‘Royal Gala’ and ‘Ida Red’) were measured. The effects of preharvest fertilization treatments and storage duration on calculated structural properties of apple peel from OCT images were investigated. It was found that OCT is a fast, high resolution and high field of view technique that renders 2D and 3D images of apple peel. En-face OCT allows the visualization of the surface topology including cracks in the wax, surface roughness and lenticels. Depth scans with OCT reveal the cellular structure down to a few hundreds of micrometre into the fruit, superior to confocal imaging. Spectral-domain OCT resulted in higher quality images than time-domain OCT. Cuticle, epidermis and hypodermis cells can be resolved in a superior way compared to micro-CT. It was concluded that OCT can be used to visualize peel structural differences between apples, as well as to measure structural changes that occur during storage. OCT is a fast method and allows for combining a high depth resolution with a wide lateral image size.

► An OCT system showed a higher penetration depth and field of view, but lower resolution than confocal microscopy.
► Cuticle, epidermis and hypodermis cells are resolved in a superior way by OCT compared to micro-CT.
► Considerable differences exist in cuticle and peel thickness of apples from different cultivars.
► Fertilization treatments affected cuticle and epidermis thickness.