Blueberries, some grapes, peaches…: These fruits actually only contain red pigments. Researchers have now explained why they still appear blue. Their optical analyzes show that the color is due to the properties of thin wax layers found on the fruit's peel. Some nanoparticle structures result in reflection of blue and ultraviolet wavelengths. The researchers say the findings could help develop biocompatible dyes and coatings for a variety of applications.
They stand out strikingly against the green foliage: numerous fruits and berries sparkle with bright colors to arouse attention and appetite. Because they are often eaten by birds and other animals, including the grains they contain. These seeds are then secreted intact elsewhere. Color encourages plants to spread. The red and yellow tones of many fruits are based on special color pigments. However, as is known, there are also some fruits and berries that appear blue. However, very few exotic species not native to this country actually produce blue pigments. In contrast to other dyes, its composition is very complex. It has long been known that blueberries etc. actually contain red pigments only in the pulp and peel, which is why their juice also contains this colour. But how does the blue shade come about?
Wax structures with visual impact
It was already clear that the color effect depends on the properties of the fine waxy layers covered by these fruits. However, until now, these materials have been the focus of research due to their function as protective barriers against moisture and the spread of pests. Now, for the first time, a team led by Rox Middleton from the University of Bristol has taken a closer look at the importance of fruity wax as a coloring ingredient. To do this, scientists subjected the outer layers of various fruits and blueberries to microscopic and optical analysis methods. In addition to blueberries, the focus was on plums, certain types of grapes, juniper blueberries, and the mahonia shrub, 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 blue fruits. In contrast to color pigments, shades of blue rely on the effects of scattered light, which are created by special crystalline structures formed by wax on the dark berry peels. The nanoparticle structures differed between the wax types of the different fruits examined. But the optical effect is very similar and results in reflection of blue and ultraviolet wavelengths, spectroscopic analyzes have shown. The researchers explained that the glow of fruit in the ultraviolet range could also be beneficial for plants because many species of birds that eat fruit can detect these wavelengths.
The harvested wax turns into a blue paint
In the next step, the team devoted itself to the task of obtaining the wax in its pure form for experimental purposes. To do this, they dipped the berries of the mahonia bush in a wax melt. Then they were able to extract it in its pure form. As it turned out, the material initially has a white color. But that changed when scientists thinly spread mahonia berry wax onto glass plates and then allowed it to “bloom.” Microscopic examinations revealed the formation of crystalline structures similar to those found on the surfaces of berries.
The effect of the contrasting color was also evident: the thin layer of wax gave the dark glass panels the same blue color as the dark mulberry peels.
“It was really exciting to learn that there is an unknown color mechanism in what are actually famous fruits right under our noses,” Middleton says. “But even more exciting was being able to reproduce that color by harvesting the wax to create a blue layer that no one had ever seen before.” As she and her colleagues point out, this discovery could now have potentially versatile uses. “Such coatings could potentially be used as a coloring agent in cosmetics or as a safe alternative to plastic coatings found on foods, as well as for sustainable and environmentally friendly protection from radiation,” says Middleton. The team now plans to extract the wax more effectively or recreate it for special purposes.
Source: University of Bristol Specialized article: Scientific Progress, doi: 10.1126/sciadv.adk4219
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