Scanning a Grape and Raisin with a MicroCT

Andy Ellison, noted MRI technologist makes sure that his CT machinery is prepared for use on human bodies by scanning fruits and veggies and creating images of it (Nuwer, 2014). As a deeper view of everyday fruits became visible, it revealed a new world of layers that transformed into moving work of art.

Beyond the contributions to the art world, the capture of fruit CT scans offers some informative new perspective into the physical and biochemical properties of this produce. Therefore, we chose to conduct our own scans of a grape and a raising, in order to compare the full fruit to the dehydrated alternative version. To conduct the scan, we chose to use the 1272 micro-CT system, which would make it possible to display both a frontal and cross-sectional image of each for comparison.

Nutritional Content Comparison of the Grape and the Raisin

Over the last handful of years, the debate with regards to which version of the grape is more healthful – the hydrated fruit or the dehydrated one. According to some of the human studies that have been conducted, there is a reduced postprandial insulin response when raisins are eaten, they modulate the absorption of sugar, and they promote a greater feeling of fullness.

When looking at our frontal images of the two versions of the fruit, the sugar molecules can be seen in the form of white specs all through the structures. Grapes are made up mostly of the sugars fructose and glucose. As dehydration occurs in the structure of the grape, it experiences a notably higher sugar-to-weight ratio. This helps to explain why raisins are seen as having a higher sugar content when they are compared to grapes. Equally, raisins also show a rise in potassium, iron, antioxidants, and other micronutrient weight concentrations. This is an important consideration when following a diet strategy that restricts the intake of sugars or in which antioxidants are considered to be a priority ((Gary Williamson, 2010) (Amerine, 1958)).

Dehydration of the Grape

The process used to dehydrate a grape to make a raisin is a vital factor in the retention of its nutritional content. There are two main methods that are used for the dehydration of grapes in order to produce raisins. Those are: dehydration tunnel use and sun drying.

When sun drying is used, the process can take as long as three weeks to complete and the outcome can be a reduction in the nutritional value. The amount of time required to dry out the grapes is determined by the environmental conditions as well as the grape’s physical properties.

(Christensen) When grapes are in their natural and hydrated form, their external layer is made up of the structures of epicuticular wax, cuticle and cellular wall. That helps to fruit to retain its moisture and repel water. As that grape undergoes the drying process, the water within it makes its way through the external cells to the cuticle and then passes a vapor through the outer layer of wax so that it can then evaporate away.

That process of dehydration can be accelerated through the use of an emulsion cold dip. That is comprised of fatty acid ethyl esters and potassium carbonate. By treating the grapes in that way, it is possible to boost the rate of water loss from the fruit by three times. Hundreds of years ago, the process of cold dipping to dehydrate fruit involved the use of olive oil and wood ash. The fatty acids work through the modification of the outer wax layer in order to decrease the grape’s surface tension. The fatty acids affect the soluble waxes on the exterior of the grape so that a hydrophilic connection could occur among the parenchyma cells, which contain water.

This effect results in the swelling and tearing apart of the grape’s layer of wax, making it easier for the water to flow toward the fruit’s exterior.

On the other hand, tunnel dehydration methods can also be applied to grapes, using hot dips. In this case, the grapes are submerged in a caustic soda solution for a period of about 8 to 15 seconds. They are then heated to a temperature of 180ºF, before they are subjected to a rinse of cool water. The result is a cracked skin, which allows improved transpiration.

Using this technique can also dissolve some of the waxy cuticle, which allows water to be lost at a faster rate. Once this is complete, the grapes are set onto trays that are stacked within rail cars for tunnel drying. At a temperature of 150ºF, it takes about 30 hours to complete.

During the dehydration process, natural reactions occur which cause the grape’s browning. This is particularly the case when the grape undergoes an extensive drying time. When the color of the grape is meant to be more golden, then they must be subjected to a treatment of sulfur dioxide solution before they are sent into the tunnel. Sulfur dioxide-treated golden raisins have a notably higher antioxidant activity. Equally, though, that substance does cause sensitivity among some people, so many newer grape cultivars and alternatives to sulfur dioxide are currently being researched.

Both grapes and raisins have been eaten since all the way back to prehistoric times. It is believed that our hunter-gatherer ancestors observed and tested the edible dried grapes that naturally fell from the vines and that dried naturally in the sun. It was the Ancient Egyptians and Phoenicians that are believed to have first popularized raisins throughout the Western world (Breska III, Takeoka, Hidalgo, Vilches, Vasse, & Ramming, 2010). Both grapes and raisins have been shown to have a broad spectrum of different uses, from the creation of oils to wines, they have considerable healthful benefits in diet strategies that involve the management of the glycemic index, they function as vasodilators, and they come with a long list of additional advantages.

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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