MicroCT of a Grape and Raisin


Did you know famed MRI technologist, Andy Ellison, prepares his CT machinery for human bodies by scanning images of fruits and vegetables? (Nuwer, 2014) Taking a deeper look into common fruit, its layers unfold; and they become motional works of art. In addition to artistic contributions, capturing CT scans of fruits provides enlightening insight into their biochemical and physical properties. We decided to scan a grape and a raisin and compare the fruit to its dehydrated counterpart. We have used our 1272 micro-CT system to show you their frontal and cross-sectional comparative images.


Grape vs. Raisin: Nutritional Content

Recently, there has been increased debate about which is healthier, the grape or the raisin. Human studies have shown raisins to lower postprandial insulin response, modulate sugar absorption and promote satiety. In our frontal images of the grape and raisin, sugar molecules are visible as white specs throughout the structure of the grape and raisin. Grapes consist of largely glucose and fructose sugars. As the grape structure dehydrates, the sugar-to-weight ratio markedly increases, hence attributing raisins to higher sugar content when compared to grapes. On the other hand, the antioxidant, potassium, iron and other micronutrient weight concentration is increased in raisins as well. It is important to consider if the specific diet of in the individual requires low-sugar intake or if high antioxidants is a priority (Gary Williamson, 2010) (Amerine, 1958).

Grape Dehydration

The dehydration process is an integral component in the nutritional content retention of the raisin. Primarily two specific dehydration methods are employed when producing raisins; sun drying and utilization of a dehydration tunnel. Sun-drying can take up to 3 weeks and can result in the reduction of nutritional value. Physical properties of the grape and environmental conditions govern the length of drying time. (Christensen) In their natural state, the grape consists of an outer layer of epicuticular wax, cuticle and cellular wall structures that provide water repellence and vapor loss resistance. As the grape dries, the water in the grape berry will move through the cells to the cuticle and pass as vapor through the waxy exterior and evaporate from the outer surface.

Raisin drying can be accelerated by utilizing an emulsion cold dip consisting of potassium carbonate and ethyl esters of fatty acids. This treatment can increase water loss rate from the grape by threefold. In ancient times, cold dips for fruit dehydration were a combination of wood ash and olive oil. Fatty acids function to modify the outer wax layer to reduce their surface tension; they interact with soluble waxes and establish a hydrophilic connection between the water containing parenchyma cells. This causes the waxy layers of the grape to swell and tear apart, facilitating the flow of water towards the exterior.

Hot dips for grapes are used with tunnel dehydration. Grapes are immersed in a caustic soda solution for 8-15 seconds and heated to 180°F followed by a cool water rinse. This produces cracks in the skin, opens the epidermis and encourages transpiration. This method also dissolves portions of the waxy cuticle and quickens the water loss. After this, grapes are arranged on trays and stacked in rail cars for tunnel drying, taking approximately 30 hours at 150°F. Browning of the grape during dehydration is caused by natural reactions, especially when the drying time is extensive. If golden colored raisins are desired, the grapes are further treated with a sulfur dioxide solution before they are sent into the tunnel. Antioxidant activity is significantly higher in golden raisins treated with sulfur dioxide. However, sulfur dioxide is shown to cause sensitivity to some individuals. Therefore, new raisin grape cultivars, as well as sulfur dioxide alternatives are being studied.

Consumption of grapes and raisins can be tracked to prehistoric times. It is thought that hunter gatherers noted the edible dried appearance of grapes after they fell off the vine and naturally sun-dried. The early Phoenicians and Egyptians were credited for popularizing and spreading raisins throughout the western world (Breska III, Takeoka, Hidalgo, Vilches, Vasse, & Ramming, 2010). Grapes and raisins have proved to be endless in their uses; creation of wines and oils, health benefits in glycemic index management, vasodilators, and more. That’s why when our friend the raisin wined he couldn’t achieve grapeness, we told him we think he is pretty great!

If you have an interesting item that you would like showcased as the next image of the month or if you would like to try our 3D printer, feel free to email: Brandon@microphotonics.com. We would love to include your work!

June 2015 – “Grape Expectations” written by:
Kaamna C. Mirchandani, MS

Works Cited

1. Amerine, M. a. (1958). The Glucose-Fructose Ratio of California Grapes. Vitis, Department of Viticulture and Enology, University of California , 224-229.
2. Breska III, A. P., Takeoka, G. R., Hidalgo, M. B., Vilches, A., Vasse, J., & Ramming, D. W. (2010). Antioxidant activity and phenolic content of 16 raisin grape (Vitis vinifera L.) cultivars and selections. Food Chemistry , 121 (3), 740-745.
3. Christensen, L. P. The Raisin Drying Process, Harvesting and Drying Raisin Grapes, The Raisin Production Manual (Vol. Chapter 27). Davis, California: University of California Davis.
4. Gary Williamson, A. C. (2010). Polyphenol content and health benefits of raisins. Nutritional Research Journal , 30 (8), 511-519.
5. Nuwer, R. (2014, April 18). Smithsonian Mag . Retrieved from Smithsonian.com: http://www.smithsonianmag.com/smart-news/these-mri-scanned-fruits-and-vegetables-unfold-alien-births-180951148/

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