Food scientists contribute to our bounty of fresh fruits by applying a wide range of scientific knowledge to maintain and improve our high quality, abundant food supply. Their efforts allow us to make the best use of our food resources and minimize waste. How foods such as fruit behave in cultivation, harvesting, processing, distribution, and storage is complex; botanists and food scientists are using micro-CT to study the internal structure of fruits in order to better understand the relationship between a fruit’s physical properties, perceptions of fruit quality, and post-harvesting transformations.

Micro-CT has been successfully used in many plant studies to look at the fruit cortex, the cell walls, the pore network, and the cells themselves. Other studies have looked at the shrinkage and collapse found when fruits are dried. Fruit structure ultimately does affect consumer perceptions of quality—consider how skin thickness, the texture of the fruit’s flesh, and the size and color of fruit affect the fruits we select from produce markets, and how tasty we expect the fruits to be.

Plant tissue microstructure has conventionally been visualized with methods that took a great deal of manual preparation. Many of those processes are invasive, and sectioning the fruit may lead to artifacts in the images, which are 2D images. Micro-CT offers several benefits to botanists and food scientists for studying fruit: it is faster than other methods, provides 3D images, and allows researchers to visualize microstructure of tissue samples that need no other pretreatment than selecting a sample—the whole fruit or a selected section—and securing it in the sample holder.

Micro-CT Scan of Fruit Specimens

Morphological differences in internal structures among our examples of a nectarine, kiwi, grape, and clementine are readily visualized and studied with the use of micro-CT.

In the nectarine, we see the little demarcation between the thin, soft skin and the fleshy fruit. We also see clear separation between the flesh and the pit, including the air pockets around the pit and the kernel, which is located inside the pit. Fruits such as nectarines, peaches, plums, apricots, cherries, and almonds all belong to the Prunus genus of trees and each produces similarly structured seeds. However, of these fruits we only eat almond seeds, which are bred to reduce the amygdalin content. Amygdalin undergoes hydrolysis to form hydrogen cyanide, a toxic substance.

The kiwifruit scan shows some entrapped air between the thick, hairy skin of the fruit and flesh, and clear identification of the dense seeds surrounding the fruit core. With some kiwifruits, small parenchyma cells may form, which deposit raphides, or long sharp calcium oxalate crystals.¹ Because of the greater electron density of calcium compared to the other primary elemental constituents of the fruit, such as carbon, oxygen, nitrogen, and hydrogen, if calcium oxalate crystals were present they would be readily visualized through the use of micro-CT. Calcium oxalate crystals cause throat irritation upon consumption, so commercial kiwifruit are grown under conditions designed to minimize raphide formation.¹ Since this is a commercially grown fruit, we do not see any of these dense structures in this axial image.

Similar to the nectarine, the grapes show little contrast between their thin, fragile skin and their flesh. We can also see large air pockets in the center of the flesh and the presence of some seeds. In this axial view, we can actually see vascular tissue extending out into these pockets.² In most grapes sold today, the seeds actually do not form a dense coating or do not form at all, allowing the grape to be consumed whole as seedless.

As in the kiwifruit, we observe a finely resolved difference between the porous peel and the flesh of the clementine. In examining the external surfaces of the peel more closely, we see a very fine, mostly closed pore structure compared to the side facing the flesh. This region is known as the cuticle and serves as the primary barrier between the fruit and its environment prior to preservative wax application.³ Since our clementine was purchased from a local retailer, it is likely wax was applied to the skin, further closing the pores to prolong storage suitability and control the passage of vapors into and out of the fruit. Just beyond this cuticle layer we can observe the oil glands located amongst the parenchyma cells. These glands are visible as the large porous region in the peel and contain primarily d-limonene, a powerful solvent used as a defense mechanism protecting the fruit from predators.³ Working our way deeper into the center of the fruit past the peel and pith, we find the actual flesh, neatly cordoned off into the segments with which we are all familiar. Past the flesh we see another region of low density, likely an air pocket around the pithy core, which we can visualize in the center of the axial view.

Conclusion

Micro-CT excels as a tool to nondestructively examine the internal structure of samples and continues to be successfully applied in the study of fruit growth and development. We hope you found this image of the month interesting. If you have an image of the month sample that you would like us to scan, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com

Scan Specifications

All scans completed on our high-speed SkyScan 1275 micro-CT system here at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using DataViewer and CTVox.

Works Cited

1.
1. http://plantsinaction.science.uq.edu.au/edition1/?q=content/11-2-4-kiwifruit-development-case-study

2.

2. http://articles.extension.org/pages/55719/basic-grape-berry-structure

3.

3. http://irrec.ifas.ufl.edu/flcitrus/pdfs/short_course_and_workshop/citrus_flowering_97/Petracek-Peel_Morphology_and_Fruit_Blemishes.pdf