Secret of blue fruit color revealed

Blue fruits such as mahonia berries (left) have a wax layer with nanostructures (right) that are responsible for the color impression. © Rox Middleton

Blueberries, some types of grapes, plums...: These fruits actually only contain red dyes. Researchers have now explained why they still appear blue. Their optical analyzes show that the color is due to characteristics of the thin wax layers on the fruit peels. Certain nanoparticle structures lead to a reflection of blue wavelengths and UV light. The researchers say the results could help develop biocompatible dyes and coatings for a variety of applications.

They stand out strikingly against the green foliage: many fruits and berries shine in bright colors to arouse attention and appetite. Because they are often eaten by birds and other animals, including the kernels they contain. These seeds are then excreted intact elsewhere. The color encourages the plants to spread. The red and yellow tones of many fruits are based on special color pigments. However, as is well known, there are also some fruits and berries that appear blue. However, only very few exotic species that are not native to this country actually produce blue pigment dyes. In contrast to other dyes, their formation is energetically complex. It has long been known that blueberries etc. actually only have red pigments in the flesh and peel, which is why their juice also has this color. But how does the shade of blue come about?

Wax structures with an optical effect

It was already obvious that the color effect is based on characteristics of the fine layers of wax with which these fruits are covered. Until now, however, these substances have been the focus of research because of their function as protective barriers against moisture and pest infestation. Now, for the first time, the team led by Rox Middleton from the University of Bristol has taken a closer look at the importance of fruity wax as a coloring element. To do this, the scientists subjected the outer layers of various blue fruits and berries to microscopic and optical analysis methods. In addition to blueberries, the focus was on plums, certain types of grapes and the blue berries of the juniper and the Mahonia bush, which is popular as an ornamental plant.

The results documented for the first time that a structural color mechanism is responsible for the appearance of the blue fruits. In contrast to color pigments, the blue tones are based on the scattering effects of light, which are created by the special crystal structures that the wax forms on the dark berry skins. The nanoparticle structures differed between the wax types of the various fruits examined. But the optical effect is very similar and leads to the reflection of blue and ultraviolet wavelengths, the spectroscopic analyzes showed. The glow of fruits in the UV range can also be useful for plants because many fruit-eating bird species can detect these wavelengths, the researchers explain.

Harvested wax turns into blue coating

In the next step, the team devoted itself to the task of obtaining the wax in pure form for experimental purposes. To do this, they dipped mahonia bush berries into a substance in which the wax dissolved. They were then able to extract it in its pure form. As it turned out, the material initially has a whitish color. But this changed when the scientists spread the mahonia berry wax thinly on glass plates and then allowed it to “bloom”. The microscopic examinations revealed that crystal structures similar to those on the surfaces of the berries were formed.

When painted on dark glass plates, the harvested mahonia berry wax also develops a blue color effect. © Rox Middleton

And the corresponding color effect was also evident: the thin wax layer gave the dark glass plates the same blue hue as the dark berry peels.

“It was really exciting to find out that there is an unknown color mechanism in what are actually well-known fruits right under our noses. “But what was even more exciting was being able to reproduce that color by harvesting the wax to create a blue coating that no one has ever seen before,” says Middleton. As she and her colleagues point out, the discovery could now have versatile potential uses. “Such coatings could potentially be used as a coloring substance in cosmetics or as a safe replacement for plastic coatings on food, as well as as sustainable and environmentally friendly UV protection,” says Middleton. The team is now planning to extract the wax even more effectively or to recreate it for special purposes.

Source: University of Bristol, specialist article: Science Advances, doi: 10.1126/sciadv.adk4219

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